One document matched: draft-ietf-tls-tls13-04.xml
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<rfc ipr="pre5378Trust200902" docName="draft-ietf-tls-tls13-04" category="std" obsoletes="3268, 4346, 4366, 5246" updates="4492">
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<front>
<title abbrev="TLS">The Transport Layer Security (TLS) Protocol Version 1.3</title>
<author initials="T." surname="Dierks" fullname="Tim Dierks">
<organization>Independent</organization>
<address>
<email>tim@dierks.org</email>
</address>
</author>
<author initials="E." surname="Rescorla" fullname="Eric Rescorla">
<organization>RTFM, Inc.</organization>
<address>
<email>ekr@rtfm.com</email>
</address>
</author>
<date year="2015" month="January" day="03"/>
<area>General</area>
<keyword>Internet-Draft</keyword>
<abstract>
<t>This document specifies Version 1.3 of the Transport Layer Security
(TLS) protocol. The TLS protocol provides communications security
over the Internet. The protocol allows client/server applications to
communicate in a way that is designed to prevent eavesdropping,
tampering, or message forgery.</t>
</abstract>
</front>
<middle>
<section anchor="introduction" title="Introduction">
<t>DISCLAIMER: This is a WIP draft of TLS 1.3 and has not yet seen significant security analysis.</t>
<t>RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH
The source for this draft is maintained in GitHub. Suggested changes
should be submitted as pull requests at
https://github.com/tlswg/tls13-spec. Instructions are on that page as
well. Editorial changes can be managed in GitHub, but any substantive
change should be discussed on the TLS mailing list.</t>
<t>The primary goal of the TLS protocol is to provide privacy and data integrity
between two communicating applications. The protocol is composed of two layers:
the TLS Record Protocol and the TLS Handshake Protocol. At the lowest level,
layered on top of some reliable transport protocol (e.g., TCP <xref target="RFC0793"/>), is
the TLS Record Protocol. The TLS Record Protocol provides connection security
that has two basic properties:</t>
<t><list style="symbols">
<t>The connection is private. Symmetric cryptography is used for
data encryption (e.g., AES <xref target="AES"/>, etc.). The keys for
this symmetric encryption are generated uniquely for each
connection and are based on a secret negotiated by another
protocol (such as the TLS Handshake Protocol). The Record
Protocol can also be used without encryption, i.e., in integrity-only
modes.</t>
<t>The connection is reliable. Messages include an authentication
tag which protects them against modification.</t>
<t>The Record Protocol can operate in an insecure mode but is generally
only used in this mode while another protocol is using the Record
Protocol as a transport for negotiating security parameters.</t>
</list></t>
<t>The TLS Record Protocol is used for encapsulation of various higher- level
protocols. One such encapsulated protocol, the TLS Handshake Protocol, allows
the server and client to authenticate each other and to negotiate an encryption
algorithm and cryptographic keys before the application protocol transmits or
receives its first byte of data. The TLS Handshake Protocol provides connection
security that has three basic properties:</t>
<t><list style="symbols">
<t>The peer’s identity can be authenticated using asymmetric, or
public key, cryptography (e.g., RSA <xref target="RSA"/>, DSA <xref target="DSS"/>, etc.). This
authentication can be made optional, but is generally required for
at least one of the peers.</t>
<t>The negotiation of a shared secret is secure: the negotiated
secret is unavailable to eavesdroppers, and for any authenticated
connection the secret cannot be obtained, even by an attacker who
can place himself in the middle of the connection.</t>
<t>The negotiation is reliable: no attacker can modify the
negotiation communication without being detected by the parties to
the communication.</t>
</list></t>
<t>One advantage of TLS is that it is application protocol independent.
Higher-level protocols can layer on top of the TLS protocol transparently. The
TLS standard, however, does not specify how protocols add security with TLS;
the decisions on how to initiate TLS handshaking and how to interpret the
authentication certificates exchanged are left to the judgment of the designers
and implementors of protocols that run on top of TLS.</t>
<section anchor="requirements-terminology" title="Requirements Terminology">
<t>The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”,
“SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be
interpreted as described in RFC 2119 <xref target="RFC2119"/>.</t>
</section>
<section anchor="major-differences-from-tls-12" title="Major Differences from TLS 1.2">
<t>draft-04</t>
<t><list style="symbols">
<t>Modify key computations to include session hash.</t>
<t>Remove ChangeCipherSpec</t>
<t>Renumber the new handshake messages to be somewhat more
consistent with existing convention and to remove a duplicate
registration.</t>
<t>Remove renegotiation.</t>
<t>Update format of signatures with context.</t>
<t>Remove point format negotiation.</t>
</list></t>
<t>draft-03</t>
<t><list style="symbols">
<t>Remove GMT time.</t>
<t>Merge in support for ECC from RFC 4492 but without explicit
curves.</t>
<t>Remove the unnecessary length field from the AD input to AEAD
ciphers.</t>
<t>Rename {Client,Server}KeyExchange to {Client,Server}KeyShare</t>
<t>Add an explicit HelloRetryRequest to reject the client’s</t>
</list></t>
<t>draft-02</t>
<t><list style="symbols">
<t>Increment version number.</t>
<t>Reworked handshake to provide 1-RTT mode.</t>
<t>Remove custom DHE groups.</t>
<t>Removed support for compression.</t>
<t>Removed support for static RSA and DH key exchange.</t>
<t>Removed support for non-AEAD ciphers</t>
</list></t>
</section>
</section>
<section anchor="goals" title="Goals">
<t>The goals of the TLS protocol, in order of priority, are as follows:</t>
<t><list style="numbers">
<t>Cryptographic security: TLS should be used to establish a secure connection
between two parties.</t>
<t>Interoperability: Independent programmers should be able to develop
applications utilizing TLS that can successfully exchange cryptographic
parameters without knowledge of one another’s code.</t>
<t>Extensibility: TLS seeks to provide a framework into which new public key
and record protection methods can be incorporated as necessary. This will also
accomplish two sub-goals: preventing the need to create a new protocol (and
risking the introduction of possible new weaknesses) and avoiding the need to
implement an entire new security library.</t>
<t>Relative efficiency: Cryptographic operations tend to be highly CPU
intensive, particularly public key operations. For this reason, the TLS
protocol has incorporated an optional session caching scheme to reduce the
number of connections that need to be established from scratch. Additionally,
care has been taken to reduce network activity.</t>
</list></t>
</section>
<section anchor="goals-of-this-document" title="Goals of This Document">
<t>This document and the TLS protocol itself are based on the SSL 3.0 Protocol
Specification as published by Netscape. The differences between this protocol
and SSL 3.0 are not dramatic, but they are significant enough that the various
versions of TLS and SSL 3.0 do not interoperate (although each protocol
incorporates a mechanism by which an implementation can back down to prior
versions). This document is intended primarily for readers who will be
implementing the protocol and for those doing cryptographic analysis of it. The
specification has been written with this in mind, and it is intended to reflect
the needs of those two groups. For that reason, many of the algorithm-dependent
data structures and rules are included in the body of the text (as opposed to
in an appendix), providing easier access to them.</t>
<t>This document is not intended to supply any details of service definition or of
interface definition, although it does cover select areas of policy as they are
required for the maintenance of solid security.</t>
</section>
<section anchor="presentation-language" title="Presentation Language">
<t>This document deals with the formatting of data in an external representation.
The following very basic and somewhat casually defined presentation syntax will
be used. The syntax draws from several sources in its structure. Although it
resembles the programming language “C” in its syntax and XDR <xref target="RFC4506"/> in
both its syntax and intent, it would be risky to draw too many parallels. The
purpose of this presentation language is to document TLS only; it has no
general application beyond that particular goal.</t>
<section anchor="basic_block_size" title="Basic Block Size">
<t>The representation of all data items is explicitly specified. The basic data
block size is one byte (i.e., 8 bits). Multiple byte data items are
concatenations of bytes, from left to right, from top to bottom. From the byte
stream, a multi-byte item (a numeric in the example) is formed (using C
notation) by:</t>
<figure><artwork><![CDATA[
value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) |
... | byte[n-1];
]]></artwork></figure>
<t>This byte ordering for multi-byte values is the commonplace network byte order
or big-endian format.</t>
</section>
<section anchor="miscellaneous" title="Miscellaneous">
<t>Comments begin with “/*” and end with “*/”.</t>
<t>Optional components are denoted by enclosing them in “[[ ]]” double
brackets.</t>
<t>Single-byte entities containing uninterpreted data are of type
opaque.</t>
</section>
<section anchor="vectors" title="Vectors">
<t>A vector (single-dimensioned array) is a stream of homogeneous data elements.
The size of the vector may be specified at documentation time or left
unspecified until runtime. In either case, the length declares the number of
bytes, not the number of elements, in the vector. The syntax for specifying a
new type, T’, that is a fixed- length vector of type T is</t>
<figure><artwork><![CDATA[
T T'[n];
]]></artwork></figure>
<t>Here, T’ occupies n bytes in the data stream, where n is a multiple of the size
of T. The length of the vector is not included in the encoded stream.</t>
<t>In the following example, Datum is defined to be three consecutive bytes that
the protocol does not interpret, while Data is three consecutive Datum,
consuming a total of nine bytes.</t>
<figure><artwork><![CDATA[
opaque Datum[3]; /* three uninterpreted bytes */
Datum Data[9]; /* 3 consecutive 3 byte vectors */
]]></artwork></figure>
<t>Variable-length vectors are defined by specifying a subrange of legal lengths,
inclusively, using the notation <floor..ceiling>. When these are encoded, the
actual length precedes the vector’s contents in the byte stream. The length
will be in the form of a number consuming as many bytes as required to hold the
vector’s specified maximum (ceiling) length. A variable-length vector with an
actual length field of zero is referred to as an empty vector.</t>
<figure><artwork><![CDATA[
T T'<floor..ceiling>;
]]></artwork></figure>
<t>In the following example, mandatory is a vector that must contain between 300
and 400 bytes of type opaque. It can never be empty. The actual length field
consumes two bytes, a uint16, which is sufficient to represent the value 400
(see <xref target="numbers"/>). On the other hand, longer can represent up to 800 bytes of
data, or 400 uint16 elements, and it may be empty. Its encoding will include a
two-byte actual length field prepended to the vector. The length of an encoded
vector must be an even multiple of the length of a single element (for example,
a 17-byte vector of uint16 would be illegal).</t>
<figure><artwork><![CDATA[
opaque mandatory<300..400>;
/* length field is 2 bytes, cannot be empty */
uint16 longer<0..800>;
/* zero to 400 16-bit unsigned integers */
]]></artwork></figure>
</section>
<section anchor="numbers" title="Numbers">
<t>The basic numeric data type is an unsigned byte (uint8). All larger numeric
data types are formed from fixed-length series of bytes concatenated as
described in <xref target="basic_block_size"/> and are also unsigned. The following numeric
types are predefined.</t>
<figure><artwork><![CDATA[
uint8 uint16[2];
uint8 uint24[3];
uint8 uint32[4];
uint8 uint64[8];
]]></artwork></figure>
<t>All values, here and elsewhere in the specification, are stored in network byte
(big-endian) order; the uint32 represented by the hex bytes 01 02 03 04 is
equivalent to the decimal value 16909060.</t>
<t>Note that in some cases (e.g., DH parameters) it is necessary to represent
integers as opaque vectors. In such cases, they are represented as unsigned
integers (i.e., leading zero octets are not required even if the most
significant bit is set).</t>
</section>
<section anchor="enumerateds" title="Enumerateds">
<t>An additional sparse data type is available called enum. A field of type enum
can only assume the values declared in the definition. Each definition is a
different type. Only enumerateds of the same type may be assigned or compared.
Every element of an enumerated must be assigned a value, as demonstrated in the
following example. Since the elements of the enumerated are not ordered, they
can be assigned any unique value, in any order.</t>
<figure><artwork><![CDATA[
enum { e1(v1), e2(v2), ... , en(vn) [[, (n)]] } Te;
]]></artwork></figure>
<t>An enumerated occupies as much space in the byte stream as would its maximal
defined ordinal value. The following definition would cause one byte to be used
to carry fields of type Color.</t>
<figure><artwork><![CDATA[
enum { red(3), blue(5), white(7) } Color;
]]></artwork></figure>
<t>One may optionally specify a value without its associated tag to force the
width definition without defining a superfluous element.</t>
<t>In the following example, Taste will consume two bytes in the data stream but
can only assume the values 1, 2, or 4.</t>
<figure><artwork><![CDATA[
enum { sweet(1), sour(2), bitter(4), (32000) } Taste;
]]></artwork></figure>
<t>The names of the elements of an enumeration are scoped within the defined type.
In the first example, a fully qualified reference to the second element of the
enumeration would be Color.blue. Such qualification is not required if the
target of the assignment is well specified.</t>
<figure><artwork><![CDATA[
Color color = Color.blue; /* overspecified, legal */
Color color = blue; /* correct, type implicit */
]]></artwork></figure>
<t>For enumerateds that are never converted to external representation, the
numerical information may be omitted.</t>
<figure><artwork><![CDATA[
enum { low, medium, high } Amount;
]]></artwork></figure>
</section>
<section anchor="constructed-types" title="Constructed Types">
<t>Structure types may be constructed from primitive types for convenience. Each
specification declares a new, unique type. The syntax for definition is much
like that of C.</t>
<figure><artwork><![CDATA[
struct {
T1 f1;
T2 f2;
...
Tn fn;
} [[T]];
]]></artwork></figure>
<t>The fields within a structure may be qualified using the type’s name, with a
syntax much like that available for enumerateds. For example, T.f2 refers to
the second field of the previous declaration. Structure definitions may be
embedded.</t>
<section anchor="variants" title="Variants">
<t>Defined structures may have variants based on some knowledge that is available
within the environment. The selector must be an enumerated type that defines
the possible variants the structure defines. There must be a case arm for every
element of the enumeration declared in the select. Case arms have limited
fall-through: if two case arms follow in immediate succession with no fields in
between, then they both contain the same fields. Thus, in the example below,
“orange” and “banana” both contain V2. Note that this is a new piece of syntax
in TLS 1.2.</t>
<t>The body of the variant structure may be given a label for reference. The
mechanism by which the variant is selected at runtime is not prescribed by the
presentation language.</t>
<figure><artwork><![CDATA[
struct {
T1 f1;
T2 f2;
....
Tn fn;
select (E) {
case e1: Te1;
case e2: Te2;
case e3: case e4: Te3;
....
case en: Ten;
} [[fv]];
} [[Tv]];
]]></artwork></figure>
<t>For example:</t>
<figure><artwork><![CDATA[
enum { apple, orange, banana } VariantTag;
struct {
uint16 number;
opaque string<0..10>; /* variable length */
} V1;
struct {
uint32 number;
opaque string[10]; /* fixed length */
} V2;
struct {
select (VariantTag) { /* value of selector is implicit */
case apple:
V1; /* VariantBody, tag = apple */
case orange:
case banana:
V2; /* VariantBody, tag = orange or banana */
} variant_body; /* optional label on variant */
} VariantRecord;
]]></artwork></figure>
</section>
</section>
<section anchor="cryptographic-attributes" title="Cryptographic Attributes">
<t>The two cryptographic operations — digital signing, and authenticated
encryption with additional data (AEAD) — are designated digitally-signed,
and aead-ciphered, respectively. A field’s cryptographic processing
is specified by prepending an appropriate key word designation before
the field’s type specification. Cryptographic keys are implied by the
current session state (see <xref target="connection-states"/>).</t>
<t>A digitally-signed element is encoded as a struct DigitallySigned:</t>
<figure><artwork><![CDATA[
struct {
SignatureAndHashAlgorithm algorithm;
opaque signature<0..2^16-1>;
} DigitallySigned;
]]></artwork></figure>
<t>The algorithm field specifies the algorithm used (see <xref target="signature-algorithms"/>
for the definition of this field). Note that the algorithm
field was introduced in TLS 1.2, and is not in earlier versions. The signature is a digital signature
using those algorithms over the contents of the element. The contents
themselves do not appear on the wire but are simply calculated. The length of
the signature is specified by the signing algorithm and key.</t>
<t>In previous versions of TLS, the ServerKeyExchange format meant that attackers
can obtain a signature of a message with a chosen, 32-byte prefix. Because TLS
1.3 servers are likely to also implement prior versions, the contents of the
element always start with 64 bytes of octet 32 in order to clear that
chosen-prefix.</t>
<t>Following that padding is a NUL-terminated context string in order to
disambiguate signatures for different purposes. The context string will be
specified whenever a digitally-signed element is used.</t>
<t>Finally, the specified contents of the digitally-signed structure follow the
NUL at the end of the context string. (See the example at the end of this
section.)</t>
<t>In RSA signing, the opaque vector contains the signature generated using the
RSASSA-PKCS1-v1_5 signature scheme defined in <xref target="RFC3447"/>. As discussed in
<xref target="RFC3447"/>, the DigestInfo MUST be DER-encoded <xref target="X680"/> <xref target="X690"/>. For hash
algorithms without parameters (which includes SHA-1), the
DigestInfo.AlgorithmIdentifier.parameters field MUST be NULL, but
implementations MUST accept both without parameters and with NULL parameters.
Note that earlier versions of TLS used a different RSA signature scheme that
did not include a DigestInfo encoding.</t>
<t>In DSA, the 20 bytes of the SHA-1 hash are run directly through the Digital
Signing Algorithm with no additional hashing. This produces two values, r and
s. The DSA signature is an opaque vector, as above, the contents of which are
the DER encoding of:</t>
<figure><artwork><![CDATA[
Dss-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER
}
]]></artwork></figure>
<t>Note: In current terminology, DSA refers to the Digital Signature Algorithm and
DSS refers to the NIST standard. In the original SSL and TLS specs, “DSS” was
used universally. This document uses “DSA” to refer to the algorithm, “DSS” to
refer to the standard, and it uses “DSS” in the code point definitions for
historical continuity.</t>
<t>All ECDSA computations MUST be performed according to ANSI X9.62 <xref target="X962"/>
or its successors. Data to be signed/verified is hashed, and the
result run directly through the ECDSA algorithm with no additional
hashing. The default hash function is SHA-1 <xref target="SHS"/>. However, an
alternative hash function, such as one of the new SHA hash functions
specified in FIPS 180-2 may be used instead if the certificate
containing the EC public key explicitly requires use of another hash
function. (The mechanism for specifying the required hash function
has not been standardized, but this provision anticipates such
standardization and obviates the need to update this document in
response. Future PKIX RFCs may choose, for example, to specify the
hash function to be used with a public key in the parameters field of
subjectPublicKeyInfo.) [[OPEN ISSUE: This needs updating per 4492-bis
https://github.com/tlswg/tls13-spec/issues/59]]</t>
<t>In AEAD encryption, the plaintext is simultaneously encrypted and integrity
protected. The input may be of any length, and aead-ciphered output is
generally larger than the input in order to accommodate the integrity check
value.</t>
<t>In the following example</t>
<figure><artwork><![CDATA[
struct {
uint8 field1;
uint8 field2;
digitally-signed opaque {
uint8 field3<0..255>;
uint8 field4;
};
} UserType;
]]></artwork></figure>
<t>Assume that the context string for the signature was specified as “Example”.
The input for the signature/hash algorithm would be:</t>
<figure><artwork><![CDATA[
2020202020202020202020202020202020202020202020202020202020202020
2020202020202020202020202020202020202020202020202020202020202020
4578616d706c6500
]]></artwork></figure>
<t>followed by the encoding of the inner struct (field3 and field4).</t>
<t>The length of the structure, in bytes, would be equal to two
bytes for field1 and field2, plus two bytes for the signature and hash
algorithm, plus two bytes for the length of the signature, plus the length of
the output of the signing algorithm. The length of the signature is known
because the algorithm and key used for the signing are known prior to encoding
or decoding this structure.</t>
</section>
<section anchor="constants" title="Constants">
<t>Typed constants can be defined for purposes of specification by declaring a
symbol of the desired type and assigning values to it.</t>
<t>Under-specified types (opaque, variable-length vectors, and structures that
contain opaque) cannot be assigned values. No fields of a multi-element
structure or vector may be elided.</t>
<t>For example:</t>
<figure><artwork><![CDATA[
struct {
uint8 f1;
uint8 f2;
} Example1;
Example1 ex1 = {1, 4}; /* assigns f1 = 1, f2 = 4 */
]]></artwork></figure>
</section>
</section>
<section anchor="HMAC" title="The Pseudorandom Function">
<t>A construction is required to do expansion of secrets into blocks
of data for the purposes of key generation or validation. This pseudorandom
function (PRF) takes as input a secret, a seed, and an identifying label and
produces an output of arbitrary length.</t>
<t>In this section, we define one PRF, based on HMAC <xref target="RFC2104"/>. This PRF with the SHA-256
hash function is used for all cipher suites defined in this document and in TLS
documents published prior to this document when TLS 1.2 is negotiated. New
cipher suites MUST explicitly specify a PRF and, in general, SHOULD use the TLS
PRF with SHA-256 or a stronger standard hash function.</t>
<t>First, we define a data expansion function, P_hash(secret, data), that uses a
single hash function to expand a secret and seed into an arbitrary quantity of
output:</t>
<figure><artwork><![CDATA[
P_hash(secret, seed) = HMAC_hash(secret, A(1) + seed) +
HMAC_hash(secret, A(2) + seed) +
HMAC_hash(secret, A(3) + seed) + ...
]]></artwork></figure>
<t>where + indicates concatenation.</t>
<t>A() is defined as:</t>
<figure><artwork><![CDATA[
A(0) = seed
A(i) = HMAC_hash(secret, A(i-1))
]]></artwork></figure>
<t>P_hash can be iterated as many times as necessary to produce the required
quantity of data. For example, if P_SHA256 is being used to create 80 bytes of
data, it will have to be iterated three times (through A(3)), creating 96 bytes
of output data; the last 16 bytes of the final iteration will then be
discarded, leaving 80 bytes of output data.</t>
<t>TLS’s PRF is created by applying P_hash to the secret as:</t>
<figure><artwork><![CDATA[
PRF(secret, label, seed) = P_<hash>(secret, label + seed)
]]></artwork></figure>
<t>The label is an ASCII string. It should be included in the exact
form it is given without a length byte or trailing null character.
For example, the label “slithy toves” would be processed by hashing
the following bytes:</t>
<figure><artwork><![CDATA[
73 6C 69 74 68 79 20 74 6F 76 65 73
]]></artwork></figure>
</section>
<section anchor="the-tls-record-protocol" title="The TLS Record Protocol">
<t>The TLS Record Protocol is a layered protocol. At each layer, messages
may include fields for length, description, and content. The Record
Protocol takes messages to be transmitted, fragments the data into
manageable blocks, protects the records, and transmits the
result. Received data is decrypted and verified, reassembled, and then
delivered to higher-level clients.</t>
<t>Four protocols that use the record protocol are described in this document: the
handshake protocol, the alert protocol, the change cipher spec protocol, and
the application data protocol. In order to allow extension of the TLS protocol,
additional record content types can be supported by the record protocol. New
record content type values are assigned by IANA in the TLS Content Type
Registry as described in <xref target="iana-considerations"/>.</t>
<t>Implementations MUST NOT send record types not defined in this document unless
negotiated by some extension. If a TLS implementation receives an unexpected
record type, it MUST send an unexpected_message alert.</t>
<t>Any protocol designed for use over TLS must be carefully designed to deal with
all possible attacks against it. As a practical matter, this means that the
protocol designer must be aware of what security properties TLS does and does
not provide and cannot safely rely on the latter.</t>
<t>Note in particular that type and length of a record are not protected by
encryption. If this information is itself sensitive, application designers may
wish to take steps (padding, cover traffic) to minimize information leakage.</t>
<section anchor="connection-states" title="Connection States">
<t>A TLS connection state is the operating environment of the TLS Record
Protocol. It specifies a record protection algorithm and its
parameters as well as the record protection keys and IVs for the
connection in both the read and the write directions. The security
parameters are be set by the TLS Handshake Protocol, which also determines
when new cryptographic keys are installed and used for record
protection.
The initial current state always specifies that records are
not protected.</t>
<t>The security parameters for a TLS Connection read and write state are set by
providing the following values:</t>
<t><list style="hanging">
<t hangText='connection end'><vspace blankLines='0'/>
Whether this entity is considered the “client” or the “server” in
this connection.</t>
<t hangText='PRF algorithm'><vspace blankLines='0'/>
An algorithm used to generate keys from the master secret (see
<xref target="HMAC"/> and <xref target="key-calculation"/>).</t>
<t hangText='record protection algorithm'><vspace blankLines='0'/>
The algorithm to be used for record protection. This algorithm must
be of the AEAD type and thus provides integrity and confidentiality
as a single primitive. It is possible to have AEAD algorithms which
do not provide any confidentiality and
<xref target="record-payload-protection"/> defines a special NULL_NULL AEAD
algorithm for use in the initial handshake). This specification
includes the key size of this algorithm and the lengths of explicit
and implicit initialization vectors (or nonces).</t>
<t hangText='handshake master secret'><vspace blankLines='0'/>
A 48-byte secret shared between the two peers in the connection and
used to generate keys for protecting the handshake.</t>
<t hangText='master secret'><vspace blankLines='0'/>
A 48-byte secret shared between the two peers in the connection
and used to generate keys for protecting application data.</t>
<t hangText='client random'><vspace blankLines='0'/>
A 32-byte value provided by the client.</t>
<t hangText='server random'><vspace blankLines='0'/>
A 32-byte value provided by the server.</t>
</list></t>
<t>These parameters are defined in the presentation language as:</t>
<figure><artwork><![CDATA[
enum { server, client } ConnectionEnd;
enum { tls_prf_sha256 } PRFAlgorithm;
enum { aes_gcm } RecordProtAlgorithm;
/* The algorithms specified in PRFAlgorithm and
RecordProtAlgorithm may be added to. */
struct {
ConnectionEnd entity;
PRFAlgorithm prf_algorithm;
RecordProtAlgorithm record_prot_algorithm;
uint8 enc_key_length;
uint8 block_length;
uint8 fixed_iv_length;
uint8 record_iv_length;
opaque hs_master_secret[48];
opaque master_secret[48];
opaque client_random[32];
opaque server_random[32];
} SecurityParameters;
]]></artwork></figure>
<t>The record layer will use the security parameters to generate the following four
items (some of which are not required by all ciphers, and are thus empty):</t>
<figure><artwork><![CDATA[
client write key
server write key
client write IV
server write IV
]]></artwork></figure>
<t>The client write parameters are used by the server when receiving and
processing records and vice versa. The algorithm used for generating these
items from the security parameters is described in <xref target="key-calculation"/></t>
<t>Once the security parameters have been set and the keys have been generated,
the connection states can be instantiated by making them the current states.
These current states MUST be updated for each record processed. Each connection
state includes the following elements:</t>
<t><list style="hanging">
<t hangText='cipher state'><vspace blankLines='0'/>
The current state of the encryption algorithm. This will consist
of the scheduled key for that connection.</t>
<t hangText='sequence number'><vspace blankLines='0'/>
Each connection state contains a sequence number, which is
maintained separately for read and write states. The sequence
number MUST be set to zero whenever a connection state is made the
active state. Sequence numbers are of type uint64 and may not
exceed 2^64-1. Sequence numbers do not wrap. If a TLS
implementation would need to wrap a sequence number, it must
terminate the connection. A sequence number is incremented after each
record: specifically, the first record transmitted under a
particular connection state MUST use sequence number 0.</t>
</list></t>
</section>
<section anchor="record-layer" title="Record Layer">
<t>The TLS record layer receives uninterpreted data from higher layers in
non-empty blocks of arbitrary size.</t>
<section anchor="fragmentation" title="Fragmentation">
<t>The record layer fragments information blocks into TLSPlaintext records
carrying data in chunks of 2^14 bytes or less. Client message boundaries are
not preserved in the record layer (i.e., multiple client messages of the same
ContentType MAY be coalesced into a single TLSPlaintext record, or a single
message MAY be fragmented across several records).</t>
<figure><artwork><![CDATA[
struct {
uint8 major;
uint8 minor;
} ProtocolVersion;
enum {
reserved(20), alert(21), handshake(22),
application_data(23), (255)
} ContentType;
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
opaque fragment[TLSPlaintext.length];
} TLSPlaintext;
]]></artwork></figure>
<t><list style="hanging">
<t hangText='type'><vspace blankLines='0'/>
The higher-level protocol used to process the enclosed fragment.</t>
<t hangText='version'><vspace blankLines='0'/>
The version of the protocol being employed. This document
describes TLS Version 1.3, which uses the version { 3, 4 }. The
version value 3.4 is historical, deriving from the use of {3, 1}
for TLS 1.0. (See <xref target="record-layer-1"/>.) Note that a client that
supports multiple versions of TLS may not know what version will
be employed before it receives the ServerHello. See
<xref target="backward-compatibility"/> for discussion about what record layer
version number should be employed for ClientHello.</t>
<t hangText='length'><vspace blankLines='0'/>
The length (in bytes) of the following TLSPlaintext.fragment. The
length MUST NOT exceed 2^14.</t>
<t hangText='fragment'><vspace blankLines='0'/>
The application data. This data is transparent and treated as an
independent block to be dealt with by the higher-level protocol
specified by the type field.</t>
</list></t>
<t>Implementations MUST NOT send zero-length fragments of Handshake or Alert
types. Zero-length fragments of Application data MAY
be sent as they are potentially useful as a traffic analysis countermeasure.</t>
</section>
<section anchor="record-payload-protection" title="Record Payload Protection">
<t>The record protection functions translate a TLSPlaintext structure into a
TLSCiphertext. The deprotection functions reverse the process. In TLS 1.3
as opposed to previous versions of TLS, all ciphers are modelled as
“Authenticated Encryption with Additional Data” (AEAD) <xref target="RFC5116"/>.
AEAD functions provide a unified encryption and authentication
operation which turns plaintext into authenticated ciphertext and
back again.</t>
<t>AEAD ciphers take as input a single key, a nonce, a plaintext, and “additional
data” to be included in the authentication check, as described in Section 2.1
of <xref target="RFC5116"/>. The key is either the client_write_key or the server_write_key.</t>
<figure><artwork><![CDATA[
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
opaque nonce_explicit[SecurityParameters.record_iv_length];
aead-ciphered struct {
opaque content[TLSPlaintext.length];
} fragment;
} TLSCiphertext;
]]></artwork></figure>
<t><list style="hanging">
<t hangText='type'><vspace blankLines='0'/>
The type field is identical to TLSPlaintext.type.</t>
<t hangText='version'><vspace blankLines='0'/>
The version field is identical to TLSPlaintext.version.</t>
<t hangText='length'><vspace blankLines='0'/>
The length (in bytes) of the following TLSCiphertext.fragment.
The length MUST NOT exceed 2^14 + 2048.</t>
<t hangText='fragment'><vspace blankLines='0'/>
The AEAD encrypted form of TLSPlaintext.fragment.</t>
</list></t>
<t>Each AEAD cipher suite MUST specify how the nonce supplied to the AEAD
operation is constructed, and what is the length of the
TLSCiphertext.nonce_explicit part. In many cases, it is appropriate to use
the partially implicit nonce technique described in Section 3.2.1 of
<xref target="RFC5116"/>; with record_iv_length being the length of the explicit part. In
this case, the implicit part SHOULD be derived from key_block as
client_write_iv and server_write_iv (as described in <xref target="key-calculation"/>), and
the explicit part is included in GenericAEAEDCipher.nonce_explicit.</t>
<t>The plaintext is the TLSPlaintext.fragment.</t>
<t>The additional authenticated data, which we denote as additional_data, is
defined as follows:</t>
<figure><artwork><![CDATA[
additional_data = seq_num + TLSPlaintext.type +
TLSPlaintext.version
]]></artwork></figure>
<t>where “+” denotes concatenation.</t>
<t>Note: In versions of TLS prior to 1.3, the additional_data included a
length field. This presents a problem for cipher constructions with
data-dependent padding (such as CBC). TLS 1.3 removes the length
field and relies on the AEAD cipher to provide integrity for the
length of the data.</t>
<t>The AEAD output consists of the ciphertext output by the AEAD encryption
operation. The length will generally be larger than TLSPlaintext.length, but
by an amount that varies with the AEAD cipher. Since the ciphers might
incorporate padding, the amount of overhead could vary with different
TLSPlaintext.length values. Each AEAD cipher MUST NOT produce an expansion of
greater than 1024 bytes. Symbolically,</t>
<figure><artwork><![CDATA[
AEADEncrypted = AEAD-Encrypt(write_key, nonce, plaintext,
additional_data)
]]></artwork></figure>
<t>[[OPEN ISSUE: Reduce these values?
https://github.com/tlswg/tls13-spec/issues/55]]</t>
<t>In order to decrypt and verify, the cipher takes as input the key, nonce, the
“additional_data”, and the AEADEncrypted value. The output is either the
plaintext or an error indicating that the decryption failed. There is no
separate integrity check. That is:</t>
<figure><artwork><![CDATA[
TLSPlaintext.fragment = AEAD-Decrypt(write_key, nonce,
AEADEncrypted,
additional_data)
]]></artwork></figure>
<t>If the decryption fails, a fatal bad_record_mac alert MUST be generated.</t>
<t>As a special case, we define the NULL_NULL AEAD cipher which is simply
the identity operation and thus provides no security. This cipher
MUST ONLY be used with the initial TLS_NULL_WITH_NULL_NULL cipher suite.</t>
</section>
</section>
<section anchor="key-calculation" title="Key Calculation">
<t>[[OPEN ISSUE: This needs to be revised. See https://github.com/tlswg/tls13-spec/issues/5]]
The Record Protocol requires an algorithm to generate keys required by the
current connection state (see <xref target="the-security-parameters"/>) from the security
parameters provided by the handshake protocol.</t>
<t>The master secret is expanded into a sequence of secure bytes, which
is then split to a client write encryption key and a server write
encryption key. Each of these is generated from the byte sequence in
that order. Unused values are empty. Some ciphers may additionally
require a client write IV and a server write IV.</t>
<t>When keys are generated, the then current master secret (MS) is used
as an entropy source. For handshake records, this means the
hs_master_secret. For application data records, this means the
regular master_secret.</t>
<t>To generate the key material, compute</t>
<figure><artwork><![CDATA[
key_block = PRF(MS,
"key expansion",
SecurityParameters.server_random +
SecurityParameters.client_random);
]]></artwork></figure>
<t>where MS is the relevant master secret. The PRF is computed enough
times to generate the necessary amount of data for the key_block,
which is then partitioned as follows:</t>
<figure><artwork><![CDATA[
client_write_key[SecurityParameters.enc_key_length]
server_write_key[SecurityParameters.enc_key_length]
client_write_IV[SecurityParameters.fixed_iv_length]
server_write_IV[SecurityParameters.fixed_iv_length]
]]></artwork></figure>
<t>Currently, the client_write_IV and server_write_IV are only generated for
implicit nonce techniques as described in Section 3.2.1 of <xref target="RFC5116"/>.</t>
</section>
</section>
<section anchor="the-tls-handshaking-protocols" title="The TLS Handshaking Protocols">
<t>TLS has three subprotocols that are used to allow peers to agree upon security
parameters for the record layer, to authenticate themselves, to instantiate
negotiated security parameters, and to report error conditions to each other.</t>
<t>The Handshake Protocol is responsible for negotiating a session, which consists
of the following items:</t>
<t><list style="hanging">
<t hangText='session identifier'><vspace blankLines='0'/>
An arbitrary byte sequence chosen by the server to identify an
active or resumable session state.</t>
<t hangText='peer certificate'><vspace blankLines='0'/>
X509v3 <xref target="RFC3280"/> certificate of the peer. This element of the state
may be null.</t>
<t hangText='cipher spec'><vspace blankLines='0'/>
Specifies the authentication and key establishment algorithms,
the pseudorandom function (PRF) used to generate keying
material, and the record protection algorithm (See
<xref target="the-security-parameters"/> for formal definition.)</t>
<t hangText='resumption premaster secret'><vspace blankLines='0'/>
48-byte secret shared between the client and server.</t>
<t hangText='is resumable'><vspace blankLines='0'/>
A flag indicating whether the session can be used to initiate new
connections.</t>
</list></t>
<t>These items are then used to create security parameters for use by the record
layer when protecting application data. Many connections can be instantiated
using the same session through the resumption feature of the TLS Handshake
Protocol.</t>
<section anchor="alert-protocol" title="Alert Protocol">
<t>One of the content types supported by the TLS record layer is the alert type.
Alert messages convey the severity of the message (warning or fatal) and a
description of the alert. Alert messages with a level of fatal result in the
immediate termination of the connection. In this case, other connections
corresponding to the session may continue, but the session identifier MUST be
invalidated, preventing the failed session from being used to establish new
connections. Like other messages, alert messages are encrypted
as specified by the current connection state.</t>
<figure><artwork><![CDATA[
enum { warning(1), fatal(2), (255) } AlertLevel;
enum {
close_notify(0),
unexpected_message(10),
bad_record_mac(20),
decryption_failed_RESERVED(21),
record_overflow(22),
decompression_failure_RESERVED(30),
handshake_failure(40),
no_certificate_RESERVED(41),
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45),
certificate_unknown(46),
illegal_parameter(47),
unknown_ca(48),
access_denied(49),
decode_error(50),
decrypt_error(51),
export_restriction_RESERVED(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
user_canceled(90),
no_renegotiation(100),
unsupported_extension(110),
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
]]></artwork></figure>
<section anchor="closure-alerts" title="Closure Alerts">
<t>The client and the server must share knowledge that the connection is ending in
order to avoid a truncation attack. Either party may initiate the exchange of
closing messages.</t>
<t><list style="hanging">
<t hangText='close_notify'><vspace blankLines='0'/>
This message notifies the recipient that the sender will not send
any more messages on this connection. Note that as of TLS 1.1,
failure to properly close a connection no longer requires that a
session not be resumed. This is a change from TLS 1.0 to conform
with widespread implementation practice.</t>
</list></t>
<t>Either party may initiate a close by sending a close_notify alert. Any data
received after a closure alert is ignored.</t>
<t>Unless some other fatal alert has been transmitted, each party is required to
send a close_notify alert before closing the write side of the connection. The
other party MUST respond with a close_notify alert of its own and close down
the connection immediately, discarding any pending writes. It is not required
for the initiator of the close to wait for the responding close_notify alert
before closing the read side of the connection.</t>
<t>If the application protocol using TLS provides that any data may be carried
over the underlying transport after the TLS connection is closed, the TLS
implementation must receive the responding close_notify alert before indicating
to the application layer that the TLS connection has ended. If the application
protocol will not transfer any additional data, but will only close the
underlying transport connection, then the implementation MAY choose to close
the transport without waiting for the responding close_notify. No part of this
standard should be taken to dictate the manner in which a usage profile for TLS
manages its data transport, including when connections are opened or closed.</t>
<t>Note: It is assumed that closing a connection reliably delivers pending data
before destroying the transport.</t>
</section>
<section anchor="error-alerts" title="Error Alerts">
<t>Error handling in the TLS Handshake protocol is very simple. When an error is
detected, the detecting party sends a message to the other party. Upon
transmission or receipt of a fatal alert message, both parties immediately
close the connection. Servers and clients MUST forget any session-identifiers,
keys, and secrets associated with a failed connection. Thus, any connection
terminated with a fatal alert MUST NOT be resumed.</t>
<t>Whenever an implementation encounters a condition which is defined as a fatal
alert, it MUST send the appropriate alert prior to closing the connection. For
all errors where an alert level is not explicitly specified, the sending party
MAY determine at its discretion whether to treat this as a fatal error or not.
If the implementation chooses to send an alert but intends to close the
connection immediately afterwards, it MUST send that alert at the fatal alert
level.</t>
<t>If an alert with a level of warning is sent and received, generally the
connection can continue normally. If the receiving party decides not to proceed
with the connection (e.g., after having received a no_renegotiation alert that
it is not willing to accept), it SHOULD send a fatal alert to terminate the
connection. Given this, the sending party cannot, in general, know how the
receiving party will behave. Therefore, warning alerts are not very useful when
the sending party wants to continue the connection, and thus are sometimes
omitted. For example, if a peer decides to accept an expired certificate
(perhaps after confirming this with the user) and wants to continue the
connection, it would not generally send a certificate_expired alert.</t>
<t>The following error alerts are defined:</t>
<t><list style="hanging">
<t hangText='unexpected_message'><vspace blankLines='0'/>
An inappropriate message was received. This alert is always fatal
and should never be observed in communication between proper
implementations.</t>
<t hangText='bad_record_mac'><vspace blankLines='0'/>
This alert is returned if a record is received which cannot be
deprotected. Because AEAD algorithms combine decryption and
verification, this message is used for all deprotection failures.
This message is always fatal and should never be observed in
communication between proper implementations (except when messages
were corrupted in the network).</t>
<t hangText='decryption_failed_RESERVED'><vspace blankLines='0'/>
This alert was used in some earlier versions of TLS, and may have
permitted certain attacks against the CBC mode <xref target="CBCATT"/>. It MUST
NOT be sent by compliant implementations.</t>
<t hangText='record_overflow'><vspace blankLines='0'/>
A TLSCiphertext record was received that had a length more than
2^14+2048 bytes, or a record decrypted to a TLSPlaintext record
with more than 2^14 bytes. This message is always fatal and
should never be observed in communication between proper
implementations (except when messages were corrupted in the
network).</t>
<t hangText='decompression_failure'><vspace blankLines='0'/>
This alert was used in previous versions of TLS. TLS 1.3 does not
include compression and TLS 1.3 implementations MUST NOT send this
alert when in TLS 1.3 mode.</t>
<t hangText='handshake_failure'><vspace blankLines='0'/>
Reception of a handshake_failure alert message indicates that the
sender was unable to negotiate an acceptable set of security
parameters given the options available. This is a fatal error.</t>
<t hangText='no_certificate_RESERVED'><vspace blankLines='0'/>
This alert was used in SSLv3 but not any version of TLS. It MUST
NOT be sent by compliant implementations.</t>
<t hangText='bad_certificate'><vspace blankLines='0'/>
A certificate was corrupt, contained signatures that did not
verify correctly, etc.</t>
<t hangText='unsupported_certificate'><vspace blankLines='0'/>
A certificate was of an unsupported type.</t>
<t hangText='certificate_revoked'><vspace blankLines='0'/>
A certificate was revoked by its signer.</t>
<t hangText='certificate_expired'><vspace blankLines='0'/>
A certificate has expired or is not currently valid.</t>
<t hangText='certificate_unknown'><vspace blankLines='0'/>
Some other (unspecified) issue arose in processing the
certificate, rendering it unacceptable.</t>
<t hangText='illegal_parameter'><vspace blankLines='0'/>
A field in the handshake was out of range or inconsistent with
other fields. This message is always fatal.</t>
<t hangText='unknown_ca'><vspace blankLines='0'/>
A valid certificate chain or partial chain was received, but the
certificate was not accepted because the CA certificate could not
be located or couldn’t be matched with a known, trusted CA. This
message is always fatal.</t>
<t hangText='access_denied'><vspace blankLines='0'/>
A valid certificate was received, but when access control was
applied, the sender decided not to proceed with negotiation. This
message is always fatal.</t>
<t hangText='decode_error'><vspace blankLines='0'/>
A message could not be decoded because some field was out of the
specified range or the length of the message was incorrect. This
message is always fatal and should never be observed in
communication between proper implementations (except when messages
were corrupted in the network).</t>
<t hangText='decrypt_error'><vspace blankLines='0'/>
A handshake cryptographic operation failed, including being unable
to correctly verify a signature or validate a Finished message.
This message is always fatal.</t>
<t hangText='export_restriction_RESERVED'><vspace blankLines='0'/>
This alert was used in some earlier versions of TLS. It MUST NOT
be sent by compliant implementations.</t>
<t hangText='protocol_version'><vspace blankLines='0'/>
The protocol version the client has attempted to negotiate is
recognized but not supported. (For example, old protocol versions
might be avoided for security reasons.) This message is always
fatal.</t>
<t hangText='insufficient_security'><vspace blankLines='0'/>
Returned instead of handshake_failure when a negotiation has
failed specifically because the server requires ciphers more
secure than those supported by the client. This message is always
fatal.</t>
<t hangText='internal_error'><vspace blankLines='0'/>
An internal error unrelated to the peer or the correctness of the
protocol (such as a memory allocation failure) makes it impossible
to continue. This message is always fatal.</t>
<t hangText='user_canceled'><vspace blankLines='0'/>
This handshake is being canceled for some reason unrelated to a
protocol failure. If the user cancels an operation after the
handshake is complete, just closing the connection by sending a
close_notify is more appropriate. This alert should be followed
by a close_notify. This message is generally a warning.</t>
<t hangText='no_renegotiation'><vspace blankLines='0'/>
Sent by the client in response to a hello request or by the server
in response to a client hello after initial handshaking. Versions
of TLS prior to TLS 1.3 supported renegotiation of a previously
established connection; TLS 1.3 removes this feature. This
message is always fatal.</t>
<t hangText='unsupported_extension'><vspace blankLines='0'/>
sent by clients that receive an extended server hello containing
an extension that they did not put in the corresponding client
hello. This message is always fatal.</t>
</list></t>
<t>New Alert values are assigned by IANA as described in <xref target="iana-considerations"/>.</t>
</section>
</section>
<section anchor="handshake-protocol-overview" title="Handshake Protocol Overview">
<t>The cryptographic parameters of the session state are produced by the TLS
Handshake Protocol, which operates on top of the TLS record layer. When a TLS
client and server first start communicating, they agree on a protocol version,
select cryptographic algorithms, optionally authenticate each other, and use
public-key encryption techniques to generate shared secrets.</t>
<t>The TLS Handshake Protocol involves the following steps:</t>
<t><list style="symbols">
<t>Exchange hello messages to agree on a protocol version,
algorithms, exchange random values, and check for session resumption.</t>
<t>Exchange the necessary cryptographic parameters to allow the
client and server to agree on a premaster secret.</t>
<t>Exchange certificates and cryptographic information to allow the
client and server to authenticate themselves.</t>
<t>Generate a master secret from the premaster secret and exchanged
random values.</t>
<t>Provide security parameters to the record layer.</t>
<t>Allow the client and server to verify that their peer has
calculated the same security parameters and that the handshake
occurred without tampering by an attacker.</t>
</list></t>
<t>Note that higher layers should not be overly reliant on whether TLS always
negotiates the strongest possible connection between two peers. There are a
number of ways in which a man-in-the-middle attacker can attempt to make two
entities drop down to the least secure method they support. The protocol has
been designed to minimize this risk, but there are still attacks available: for
example, an attacker could block access to the port a secure service runs on,
or attempt to get the peers to negotiate an unauthenticated connection. The
fundamental rule is that higher levels must be cognizant of what their security
requirements are and never transmit information over a channel less secure than
what they require. The TLS protocol is secure in that any cipher suite offers
its promised level of security: if you negotiate AES-GCM <xref target="GCM"/> with
a 1024-bit DHE key exchange with a host whose certificate you have
verified, you can expect to be that secure.</t>
<t>These goals are achieved by the handshake protocol, which can be
summarized as follows: The client sends a ClientHello message which
contains a random nonce (ClientHello.random), its preferences for
Protocol Version, Cipher Suite, and a variety of extensions. In
the same flight, it sends a ClientKeyShare message which contains its
share of the parameters for key agreement for some set of expected
server parameters (DHE/ECDHE groups, etc.).</t>
<t>If the client has provided a ClientKeyShare with an appropriate set of
keying material, the server responds to the ClientHello with a ServerHello
message. The ServerHello contains the server’s nonce
(ServerHello.random), the server’s choice of the Protocol Version,
Session ID and Cipher Suite, and the server’s response to the
extensions the client offered.</t>
<t>The server can then generate its own keying material share and send a
ServerKeyShare message which contains its share of the parameters for
the key agreement. The server can now compute the shared secret (the
premaster secret). At this point, the server starts encrypting all
remaining handshake traffic with the negotiated cipher suite using a key
derived from the premaster secret (via the “handshake master secret”).
The remainder of the server’s
handshake messages will be encrypted using that key.</t>
<t>Following these messages, the server will send an EncryptedExtensions
message which contains a response to any client’s extensions which are
not necessary to establish the Cipher Suite. The server will then send
its certificate in a Certificate message if it is to be authenticated.
The server may optionally request a certificate from the client by
sending a CertificateRequest message at this point.
Finally, if the server is authenticated, it will send a CertificateVerify
message which provides a signature over the entire handshake up to
this point. This serves both to authenticate the server and to establish
the integrity of the negotiation. Finally, the server sends a Finished
message which includes an integrity check over the handshake keyed
by the shared secret and demonstrates that the server and client have
agreed upon the same keys.
[[TODO: If the server is not requesting client authentication,
it MAY start sending application data following the Finished, though
the server has no way of knowing who will be receiving the data. Add this.]]</t>
<t>Once the client receives the ServerKeyShare, it can also compute the
premaster secret and decrypt the server’s remaining handshake messages.
The client generates its own sending keys based on the premaster secret
and will encrypt the remainder of its handshake messages using those keys
and the newly established cipher suite. If the server has sent a
CertificateRequest message, the client MUST send the Certificate
message, though it may contain zero certificates. If the client has
sent a certificate, a digitally-signed CertificateVerify message is
sent to explicitly verify possession of the private key in the
certificate. Finally, the client sends the Finished message.</t>
<t>At this point, the handshake is complete, and the
client and server may exchange application layer data, which is
protected using a new set of keys derived from both the premaster
secret and the handshake transcript (see <xref target="I-D.ietf-tls-session-hash"/>
for the security rationale for this.)</t>
<t>Application data MUST NOT be sent prior to the Finished message.
[[TODO: can we make this clearer and more clearly match the text above
about server-side False Start.]]
Client Server</t>
<figure><artwork><![CDATA[
ClientHello
ClientKeyShare -------->
ServerHello
ServerKeyShare
{EncryptedExtensions*}
{Certificate*}
{CertificateRequest*}
{CertificateVerify*}
<-------- {Finished}
{Certificate*}
{CertificateVerify*}
{Finished} -------->
[Application Data] <-------> [Application Data]
Figure 1. Message flow for a full handshake
]]></artwork></figure>
<t>* Indicates optional or situation-dependent messages that are not always sent.</t>
<t>{} Indicates messages protected using keys derived from the handshake master
secret.</t>
<t>[] Indicates messages protected using keys derived from the master secret.</t>
<t>If the client has not provided an appropriate ClientKeyShare (e.g. it
includes only DHE or ECDHE groups unacceptable or unsupported by the
server), the server corrects the mismatch with a HelloRetryRequest and
the client will need to restart the handshake with an appropriate
ClientKeyShare, as shown in Figure 2:</t>
<figure><artwork><![CDATA[
Client Server
ClientHello
ClientKeyShare -------->
<-------- HelloRetryRequest
ClientHello
ClientKeyShare -------->
ServerHello
ServerKeyShare
{EncryptedExtensions*}
{Certificate*}
{CertificateRequest*}
{CertificateVerify*}
<-------- {Finished}
{Certificate*}
{CertificateVerify*}
{Finished} -------->
[Application Data] <-------> [Application Data]
]]></artwork></figure>
<t>Figure 2. Message flow for a full handshake with mismatched parameters</t>
<t>[[OPEN ISSUE: Should we restart the handshake hash?
https://github.com/tlswg/tls13-spec/issues/104.]]
[[OPEN ISSUE: We need to make sure that this flow doesn’t introduce
downgrade issues. Potential options include continuing the handshake
hashes (as long as clients don’t change their opinion of the server’s
capabilities with aborted handshakes) and requiring the client to send
the same ClientHello (as is currently done) and then checking you get
the same negotiated parameters.]]</t>
<t>If no common cryptographic parameters can be negotiated, the server
will send a fatal alert.</t>
<t>When the client and server decide to resume a previous session or duplicate an
existing session (instead of negotiating new security parameters), the message
flow is as follows:</t>
<t>The client sends a ClientHello using the Session ID of the session to
be resumed. The server then checks its session cache for a match. If a
match is found, and the server is willing to re-establish the
connection under the specified session state, it will send a
ServerHello with the same Session ID value. At this point, both client
and server MUST proceed directly to sending Finished messages, which
are protected using handshake keys as described above, computed using
resumption premaster secret created in the first handshake as the
premaster secret. Once the
re-establishment is complete, the client and server MAY begin to
exchange application layer data, which is protected using the
application secrets (See flow chart below.) If a Session ID match is
not found, the server generates a new session ID, and the TLS client
and server perform a full handshake.</t>
<figure><artwork><![CDATA[
Client Server
ClientHello
ClientKeyExhange -------->
ServerHello
<-------- {Finished}
{Finished} -------->
[Application Data] <-------> [Application Data]
Figure 3. Message flow for an abbreviated handshake
]]></artwork></figure>
<t>The contents and significance of each message will be presented in detail in
the following sections.</t>
</section>
<section anchor="handshake-protocol" title="Handshake Protocol">
<t>The TLS Handshake Protocol is one of the defined higher-level clients of the
TLS Record Protocol. This protocol is used to negotiate the secure attributes
of a session. Handshake messages are supplied to the TLS record layer, where
they are encapsulated within one or more TLSPlaintext structures, which are
processed and transmitted as specified by the current active session state.</t>
<figure><artwork><![CDATA[
enum {
reserved(0), client_hello(1), server_hello(2),
client_key_share(5), hello_retry_request(6),
server_key_share(7), certificate(11), reserved(12),
certificate_request(13), certificate_verify(15),
reserved(16), finished(20), (255)
} HandshakeType;
struct {
HandshakeType msg_type; /* handshake type */
uint24 length; /* bytes in message */
select (HandshakeType) {
case client_hello: ClientHello;
case client_key_share: ClientKeyShare;
case server_hello: ServerHello;
case hello_retry_request: HelloRetryRequest;
case server_key_share: ServerKeyShare;
case certificate: Certificate;
case certificate_request: CertificateRequest;
case certificate_verify: CertificateVerify;
case finished: Finished;
} body;
} Handshake;
]]></artwork></figure>
<t>The handshake protocol messages are presented below in the order they
MUST be sent; sending handshake messages in an unexpected order
results in a fatal error. Unneeded handshake messages can be omitted,
however.</t>
<t>New handshake message types are assigned by IANA as described in
<xref target="iana-considerations"/>.</t>
<section anchor="hello-messages" title="Hello Messages">
<t>The hello phase messages are used to exchange security enhancement capabilities
between the client and server. When a new session begins, the record layer’s
connection state AEAD algorithm is initialized to NULL_NULL.
The current connection state is used for renegotiation messages.</t>
<section anchor="client-hello" title="Client Hello">
<t>When this message will be sent:</t>
<t><list style='empty'>
<t>When a client first connects to a server, it is required to send the
ClientHello as its first message. The client will also send a
ClientHello when the server has responded to its ClientHello with a
ServerHello that selects cryptographic parameters that don’t match the
client’s ClientKeyShare. In that case, the client MUST send the same
ClientHello (without modification) along with the new ClientKeyShare.
If a server receives a ClientHello at any other time, it MUST send
a fatal no_renegotiation alert.</t>
</list></t>
<t>Structure of this message:</t>
<t><list style='empty'>
<t>The ClientHello message includes a random structure, which is used later in
the protocol.</t>
</list></t>
<figure><artwork><![CDATA[
struct {
opaque random_bytes[32];
} Random;
]]></artwork></figure>
<t><list style="hanging">
<t hangText='random_bytes'><vspace blankLines='0'/>
32 bytes generated by a secure random number generator.</t>
</list></t>
<t>Note: Versions of TLS prior to TLS 1.3 used the top 32 bits of
the Random value to encode the time since the UNIX epoch.</t>
<t>Note:
The ClientHello message includes a variable-length session identifier. If not
empty, the value identifies a session between the same client and server whose
security parameters the client wishes to reuse. The session identifier MAY be
from an earlier connection, this connection, or from another currently active
connection. The second option is useful if the client only wishes to update the
random structures and derived values of a connection, and the third option
makes it possible to establish several independent secure connections without
repeating the full handshake protocol. These independent connections may occur
sequentially or simultaneously; a SessionID becomes valid when the handshake
negotiating it completes with the exchange of Finished messages and persists
until it is removed due to aging or because a fatal error was encountered on a
connection associated with the session. The actual contents of the SessionID
are defined by the server.</t>
<figure><artwork><![CDATA[
opaque SessionID<0..32>;
]]></artwork></figure>
<t>Warning: Because the SessionID is transmitted without confidentiality or
integrity protection, servers MUST NOT place confidential information in session
identifiers or let the contents of fake session identifiers cause any breach of
security. (Note that the content of the handshake as a whole, including the
SessionID, is protected by the Finished messages exchanged at the end of the
handshake.)</t>
<t>The cipher suite list, passed from the client to the server in the ClientHello
message, contains the combinations of cryptographic algorithms supported by the
client in order of the client’s preference (favorite choice first). Each cipher
suite defines a key exchange algorithm, a record protection algorithm (including
secret key length) and a PRF. The server will select a cipher
suite or, if no acceptable choices are presented, return a handshake failure
alert and close the connection. If the list contains cipher suites the server
does not recognize, support, or wish to use, the server MUST ignore those
cipher suites, and process the remaining ones as usual.</t>
<figure><artwork><![CDATA[
uint8 CipherSuite[2]; /* Cryptographic suite selector */
enum { null(0), (255) } CompressionMethod;
struct {
ProtocolVersion client_version;
Random random;
SessionID session_id;
CipherSuite cipher_suites<2..2^16-2>;
CompressionMethod compression_methods<1..2^8-1>;
select (extensions_present) {
case false:
struct {};
case true:
Extension extensions<0..2^16-1>;
};
} ClientHello;
]]></artwork></figure>
<t>TLS allows extensions to follow the compression_methods field in an extensions
block. The presence of extensions can be detected by determining whether there
are bytes following the compression_methods at the end of the ClientHello. Note
that this method of detecting optional data differs from the normal TLS method
of having a variable-length field, but it is used for compatibility with TLS
before extensions were defined.</t>
<t><list style="hanging">
<t hangText='client_version'><vspace blankLines='0'/>
The version of the TLS protocol by which the client wishes to
communicate during this session. This SHOULD be the latest
(highest valued) version supported by the client. For this
version of the specification, the version will be 3.4 (see
<xref target="backward-compatibility"/> for details about backward compatibility).</t>
<t hangText='random'><vspace blankLines='0'/>
A client-generated random structure.</t>
<t hangText='session_id'><vspace blankLines='0'/>
The ID of a session the client wishes to use for this connection.
This field is empty if no session_id is available, or if the
client wishes to generate new security parameters.</t>
<t hangText='cipher_suites'><vspace blankLines='0'/>
This is a list of the cryptographic options supported by the
client, with the client’s first preference first. If the
session_id field is not empty (implying a session resumption
request), this vector MUST include at least the cipher_suite from
that session. Values are defined in <xref target="the-cipher-suite"/>.</t>
<t hangText='compression_methods'><vspace blankLines='0'/>
Versions of TLS before 1.3 supported compression and the list of
compression methods was supplied in this field. For any TLS 1.3
ClientHello, this field MUST contain only the “null” compression
method with the code point of 0. If a TLS 1.3 ClientHello is
received with any other value in this field, the server MUST
generate a fatal “illegal_parameter” alert. Note that TLS 1.3
servers may receive TLS 1.2 or prior ClientHellos which contain
other compression methods and MUST follow the procedures for
the appropriate prior version of TLS.</t>
<t hangText='extensions'><vspace blankLines='0'/>
Clients MAY request extended functionality from servers by sending
data in the extensions field. The actual “Extension” format is
defined in <xref target="hello-extensions"/>.</t>
</list></t>
<t>In the event that a client requests additional functionality using extensions,
and this functionality is not supplied by the server, the client MAY abort the
handshake. A server MUST accept ClientHello messages both with and without the
extensions field, and (as for all other messages) it MUST check that the amount
of data in the message precisely matches one of these formats; if not, then it
MUST send a fatal “decode_error” alert.</t>
<t>After sending the ClientHello message, the client waits for a ServerHello
or HelloRetryRequest message.</t>
</section>
</section>
<section anchor="client-key-share-message" title="Client Key Share Message">
<t>When this message will be sent:</t>
<t><list style='empty'>
<t>This message is always sent by the client. It MUST immediately follow the
ClientHello message. In backward compatibility mode (see Section XXX)
it will be included in the EarlyData extension (<xref target="early-data-extension"/>)
in the ClientHello.</t>
</list></t>
<t>Meaning of this message:</t>
<t><list style='empty'>
<t>This message contains the client’s cryptographic parameters
for zero or more key establishment methods.</t>
</list></t>
<t>Structure of this message:</t>
<figure><artwork><![CDATA[
struct {
NamedGroup group;
opaque key_exchange<1..2^16-1>;
} ClientKeyShareOffer;
]]></artwork></figure>
<t><list style="hanging">
<t hangText='group'>
The named group for the key share offer. This identifies the
specific key exchange method that the ClientKeyShareOffer describes.
Finite Field Diffie-Hellman parameters are described in
<xref target="ffdhe-param"/>; Elliptic Curve Diffie-Hellman parameters are
described in <xref target="ecdhe-param"/>.</t>
<t hangText='key_exchange'>
Key exchange information. The contents of this field are
determined by the value of NamedGroup entry and its corresponding
definition.</t>
</list></t>
<figure><artwork><![CDATA[
struct {
ClientKeyShareOffer offers<0..2^16-1>;
} ClientKeyShare;
]]></artwork></figure>
<t><list style="hanging">
<t hangText='offers'><vspace blankLines='0'/>
A list of ClientKeyShareOffer values.</t>
</list></t>
<t>Clients may offer an arbitrary number of ClientKeyShareOffer
values, each representing a single set of key agreement parameters;
for instance a client might offer shares for several elliptic curves
or multiple integer DH groups. The shares for each ClientKeyShareOffer
MUST by generated independently. Clients MUST NOT offer multiple
ClientKeyShareOffers for the same parameters. It is explicitly
permitted to send an empty ClientKeyShare message, as this is used
to elicit the server’s parameters if the client has no useful
information.
[TODO: Recommendation about what the client offers. Presumably which integer
DH groups and which curves.]
[TODO: Work out how this interacts with PSK and SRP.]</t>
<section anchor="ffdhe-param" title="Diffie-Hellman Parameters">
<t>Diffie-Hellman parameters for both clients and servers are encoded in
the opaque key_exchange field of the ClientKeyShareOffer or
ServerKeyShare structures. The opaque value contains the
Diffie-Hellman public value (dh_Y = g^X mod p),
encoded as a big-endian integer.</t>
<figure><artwork><![CDATA[
opaque dh_Y<1..2^16-1>;
]]></artwork></figure>
</section>
<section anchor="ecdhe-param" title="ECHDE Parameters">
<t>ECDHE parameters for both clients and servers are encoded in the
opaque key_exchange field of the ClientKeyShareOffer or
ServerKeyShare structures. The opaque value conveys the Elliptic
Curve Diffie-Hellman public value (ecdh_Y) represented as a byte
string ECPoint.point.</t>
<figure><artwork><![CDATA[
opaque point <1..2^8-1>;
]]></artwork></figure>
<t><list style="hanging">
<t hangText='point'><vspace blankLines='0'/>
This is the byte string representation of an elliptic curve
point following the conversion routine in Section 4.3.6 of ANSI
X9.62 {{X962}.</t>
</list></t>
<t>Although X9.62 supports multiple point formats, any given curve
MUST specify only a single point format. All curves currently
specified in this document MUST only be used with the uncompressed
point format.</t>
<t>Note: Versions of TLS prior to 1.3 permitted point negotiation;
TLS 1.3 removes this feature in favor of a single point format
for each curve.</t>
<t>[[OPEN ISSUE: We will need to adjust the compressed/uncompressed point issue
if we have new curves that don’t need point compression. This depends
on the CFRG’s recommendations. The expectation is that future curves will
come with defined point formats and that existing curves conform to
X9.62.]]</t>
</section>
<section anchor="server-hello" title="Server Hello">
<t>When this message will be sent:</t>
<t><list style='empty'>
<t>The server will send this message in response to a ClientHello message when
it was able to find an acceptable set of algorithms and the client’s
ClientKeyShare message was acceptable. If the client proposed groups are not
acceptable by the server, it will respond with an insufficient_security fatal alert.</t>
</list></t>
<t>Structure of this message:</t>
<figure><artwork><![CDATA[
struct {
ProtocolVersion server_version;
Random random;
SessionID session_id;
CipherSuite cipher_suite;
select (extensions_present) {
case false:
struct {};
case true:
Extension extensions<0..2^16-1>;
};
} ServerHello;
]]></artwork></figure>
<t>The presence of extensions can be detected by determining whether there are
bytes following the cipher_suite field at the end of the ServerHello.</t>
<t><list style="hanging">
<t hangText='server_version'><vspace blankLines='0'/>
This field will contain the lower of that suggested by the client
in the client hello and the highest supported by the server. For
this version of the specification, the version is 3.4. (See
<xref target="backward-compatibility"/> for details about backward compatibility.)</t>
<t hangText='random'><vspace blankLines='0'/>
This structure is generated by the server and MUST be
generated independently of the ClientHello.random.</t>
<t hangText='session_id'><vspace blankLines='0'/>
This is the identity of the session corresponding to this
connection. If the ClientHello.session_id was non-empty, the
server will look in its session cache for a match. If a match is
found and the server is willing to establish the new connection
using the specified session state, the server will respond with
the same value as was supplied by the client. This indicates a
resumed session and dictates that the parties must proceed
directly to the Finished messages. Otherwise, this field will
contain a different value identifying the new session. The server
may return an empty session_id to indicate that the session will
not be cached and therefore cannot be resumed. If a session is
resumed, it must be resumed using the same cipher suite it was
originally negotiated with. Note that there is no requirement
that the server resume any session even if it had formerly
provided a session_id. Clients MUST be prepared to do a full
negotiation — including negotiating new cipher suites — during
any handshake.</t>
<t hangText='cipher_suite'><vspace blankLines='0'/>
The single cipher suite selected by the server from the list in
ClientHello.cipher_suites. For resumed sessions, this field is
the value from the state of the session being resumed.</t>
<t hangText='extensions'><vspace blankLines='0'/>
A list of extensions. Note that only extensions offered by the
client can appear in the server’s list. In TLS 1.3 as opposed to
previous versions of TLS, the server’s extensions are split between
the ServerHello and the EncryptedExtensions <xref target="encrypted-extensions"/>
message. The ServerHello
MUST only include extensions which are required to establish
the cryptographic context.</t>
</list></t>
</section>
<section anchor="helloretryrequest" title="HelloRetryRequest">
<t>When this message will be sent:</t>
<t><list style='empty'>
<t>The server will send this message in response to a ClientHello
message when it was able to find an acceptable set of algorithms but
the client’s ClientKeyShare message did not contain an acceptable
offer. If it cannot find such a match, it will respond with a
handshake failure alert.</t>
</list></t>
<t>Structure of this message:</t>
<figure><artwork><![CDATA[
struct {
ProtocolVersion server_version;
CipherSuite cipher_suite;
NamedGroup selected_group;
Extension extensions<0..2^16-1>;
} HelloRetryRequest;
]]></artwork></figure>
<t>[[OPEN ISSUE: Merge in DTLS Cookies?]]</t>
<t><list style="hanging">
<t hangText='selected_group'><vspace blankLines='0'/>
The group which the client MUST use for its new ClientHello.</t>
</list></t>
<t>The “server_version”, “cipher_suite” and “extensions” fields have the
same meanings as their corresponding values in the ServerHello. The
server SHOULD send only the extensions necessary for the client to
generate a correct ClientHello/ClientKeyShare pair.</t>
<t>Upon receipt of a HelloRetryRequest, the client MUST send a new
ClientHello/ClientKeyShare pair to the server. The ClientKeyShare MUST
contain both the groups in the original ClientKeyShare as well as a
ClientKeyShareOffer consistent with the “selected_group” field.
I.e., it MUST be a superset of the previous ClientKeyShareOffer.</t>
<t>Upon re-sending the ClientHello/ClientKeyShare and receiving the
server’s ServerHello/ServerKeyShare, the client MUST verify that
the selected ciphersuite and NamedGroup match that supplied in
the HelloRetryRequest.</t>
</section>
<section anchor="hello-extensions" title="Hello Extensions">
<t>The extension format is:</t>
<figure><artwork><![CDATA[
struct {
ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} Extension;
enum {
signature_algorithms(13), early_data(TBD), (65535)
} ExtensionType;
]]></artwork></figure>
<t>Here:</t>
<t><list style="symbols">
<t>“extension_type” identifies the particular extension type.</t>
<t>“extension_data” contains information specific to the particular
extension type.</t>
</list></t>
<t>The initial set of extensions is defined in a companion document <xref target="TLSEXT"/>.
The list of extension types is maintained by IANA as described in
<xref target="iana-considerations"/>.</t>
<t>An extension type MUST NOT appear in the ServerHello unless the same extension
type appeared in the corresponding ClientHello. If a client receives an
extension type in ServerHello that it did not request in the associated
ClientHello, it MUST abort the handshake with an unsupported_extension fatal
alert.</t>
<t>Nonetheless, “server-oriented” extensions may be provided in the future within
this framework. Such an extension (say, of type x) would require the client to
first send an extension of type x in a ClientHello with empty extension_data to
indicate that it supports the extension type. In this case, the client is
offering the capability to understand the extension type, and the server is
taking the client up on its offer.</t>
<t>When multiple extensions of different types are present in the ClientHello or
ServerHello messages, the extensions MAY appear in any order. There MUST NOT be
more than one extension of the same type.</t>
<t>Finally, note that extensions can be sent both when starting a new session and
when requesting session resumption. Indeed, a client that requests session
resumption does not in general know whether the server will accept this
request, and therefore it SHOULD send the same extensions as it would send if
it were not attempting resumption.</t>
<t>In general, the specification of each extension type needs to describe the
effect of the extension both during full handshake and session resumption. Most
current TLS extensions are relevant only when a session is initiated: when an
older session is resumed, the server does not process these extensions in
Client Hello, and does not include them in Server Hello. However, some
extensions may specify different behavior during session resumption.</t>
<t>There are subtle (and not so subtle) interactions that may occur in this
protocol between new features and existing features which may result in a
significant reduction in overall security. The following considerations should
be taken into account when designing new extensions:</t>
<t><list style="symbols">
<t>Some cases where a server does not agree to an extension are error
conditions, and some are simply refusals to support particular features. In
general, error alerts should be used for the former, and a field in the
server extension response for the latter.</t>
<t>Extensions should, as far as possible, be designed to prevent any attack that
forces use (or non-use) of a particular feature by manipulation of handshake
messages. This principle should be followed regardless of whether the feature
is believed to cause a security problem. <vspace blankLines='1'/>
Often the fact that the extension fields are included in the inputs to the
Finished message hashes will be sufficient, but extreme care is needed when
the extension changes the meaning of messages sent in the handshake phase.
Designers and implementors should be aware of the fact that until the
handshake has been authenticated, active attackers can modify messages and
insert, remove, or replace extensions.</t>
<t>It would be technically possible to use extensions to change major aspects
of the design of TLS; for example the design of cipher suite negotiation.
This is not recommended; it would be more appropriate to define a new version
of TLS — particularly since the TLS handshake algorithms have specific
protection against version rollback attacks based on the version number, and
the possibility of version rollback should be a significant consideration in
any major design change.</t>
</list></t>
<section anchor="signature-algorithms" title="Signature Algorithms">
<t>The client uses the “signature_algorithms” extension to indicate to the server
which signature/hash algorithm pairs may be used in digital signatures. The
“extension_data” field of this extension contains a
“supported_signature_algorithms” value.</t>
<figure><artwork><![CDATA[
enum {
none(0), md5(1), sha1(2), sha224(3), sha256(4), sha384(5),
sha512(6), (255)
} HashAlgorithm;
enum { anonymous(0), rsa(1), dsa(2), ecdsa(3), (255) }
SignatureAlgorithm;
struct {
HashAlgorithm hash;
SignatureAlgorithm signature;
} SignatureAndHashAlgorithm;
SignatureAndHashAlgorithm
supported_signature_algorithms<2..2^16-2>;
]]></artwork></figure>
<t>Each SignatureAndHashAlgorithm value lists a single hash/signature pair that
the client is willing to verify. The values are indicated in descending order
of preference.</t>
<t>Note: Because not all signature algorithms and hash algorithms may be accepted
by an implementation (e.g., DSA with SHA-1, but not SHA-256), algorithms here
are listed in pairs.</t>
<t><list style="hanging">
<t hangText='hash'><vspace blankLines='0'/>
This field indicates the hash algorithm which may be used. The
values indicate support for unhashed data, MD5 <xref target="RFC1321"/>, SHA-1,
SHA-224, SHA-256, SHA-384, and SHA-512 <xref target="SHS"/>, respectively. The
“none” value is provided for future extensibility, in case of a
signature algorithm which does not require hashing before signing.</t>
<t hangText='signature'><vspace blankLines='0'/>
This field indicates the signature algorithm that may be used.
The values indicate anonymous signatures, RSASSA-PKCS1-v1_5
<xref target="RFC3447"/> and DSA <xref target="DSS"/>, and ECDSA <xref target="ECDSA"/>, respectively. The
“anonymous” value is meaningless in this context but used in
<xref target="server-key-share-message"/>. It MUST NOT appear in this extension.</t>
</list></t>
<t>The semantics of this extension are somewhat complicated because the cipher
suite indicates permissible signature algorithms but not hash algorithms.
<xref target="server-certificate"/> and <xref target="server-key-share-message"/> describe the
appropriate rules.</t>
<t>If the client supports only the default hash and signature algorithms (listed
in this section), it MAY omit the signature_algorithms extension. If the client
does not support the default algorithms, or supports other hash and signature
algorithms (and it is willing to use them for verifying messages sent by the
server, i.e., server certificates and server key share), it MUST send the
signature_algorithms extension, listing the algorithms it is willing to accept.</t>
<t>If the client does not send the signature_algorithms extension, the server MUST
do the following:</t>
<t><list style="symbols">
<t>If the negotiated key exchange algorithm is one of (DHE_RSA, ECDHE_RSA), behave as if client had sent the value
{sha1,rsa}.</t>
<t>If the negotiated key exchange algorithm is DHE_DSS, behave
as if the client had sent the value {sha1,dsa}.</t>
<t>If the negotiated key exchange algorithm is ECDHE_ECDSA,
behave as if the client had sent value {sha1,ecdsa}.</t>
</list></t>
<t>Note: this is a change from TLS 1.1 where there are no explicit rules, but as a
practical matter one can assume that the peer supports MD5 and SHA-1.</t>
<t>Note: this extension is not meaningful for TLS versions prior to 1.2. Clients
MUST NOT offer it if they are offering prior versions. However, even if clients
do offer it, the rules specified in <xref target="TLSEXT"/> require servers to ignore
extensions they do not understand.</t>
<t>Servers MUST NOT send this extension. TLS servers MUST support receiving this
extension.</t>
<t>When performing session resumption, this extension is not included in Server
Hello, and the server ignores the extension in Client Hello (if present).</t>
</section>
<section anchor="negotiated-groups" title="Negotiated Groups">
<t>When sent by the client, the “supported_groups” extension indicates
the named groups which the client supports, ordered from most
preferred to least preferred.</t>
<t>Note: In versions of TLS prior to TLS 1.3, this extension was named
“elliptic curves” and only contained elliptic curve groups. See
<xref target="RFC4492"/> and <xref target="I-D.ietf-tls-negotiated-ff-dhe"/>.</t>
<t>The “extension_data” field of this extension SHALL contain a
“NamedGroupList” value:</t>
<figure><artwork><![CDATA[
enum {
// Elliptic Curve Groups.
sect163k1 (1), sect163r1 (2), sect163r2 (3),
sect193r1 (4), sect193r2 (5), sect233k1 (6),
sect233r1 (7), sect239k1 (8), sect283k1 (9),
sect283r1 (10), sect409k1 (11), sect409r1 (12),
sect571k1 (13), sect571r1 (14), secp160k1 (15),
secp160r1 (16), secp160r2 (17), secp192k1 (18),
secp192r1 (19), secp224k1 (20), secp224r1 (21),
secp256k1 (22), secp256r1 (23), secp384r1 (24),
secp521r1 (25),
// Finite Field Groups.
ffdhe2432(256), ffdhe3072(257), ffdhe4096(258),
ffdhe6144(259), ffdhe8192(260),
// Reserved Code Points.
reserved (0xFE00..0xFEFF),
reserved(0xFF01),
reserved(0xFF02),
(0xFFFF)
} NamedGroup;
struct {
NamedGroup named_group_list<1..2^16-1>
} NamedGroupList;
]]></artwork></figure>
<t>sect163k1, etc: Indicates support of the corresponding named curve
The named curves defined here are those specified in SEC 2 [13].
Note that many of these curves are also recommended in ANSI
X9.62 <xref target="X962"/> and FIPS 186-2 <xref target="DSS"/>. Values 0xFE00 through 0xFEFF are
reserved for private use. Values 0xFF01 and 0xFF02 were used in
previous versions of TLS but MUST NOT be offered by TLS 1.3
implementations.
[[OPEN ISSUE: Triage curve list.]]</t>
<t>ffdhe2432, etc: Indicates support of the corresponding finite field
group, defined in <xref target="I-D.ietf-tls-negotiated-ff-dhe"/></t>
<t>Items in named_curve_list are ordered according to the client’s
preferences (favorite choice first).</t>
<t>As an example, a client that only supports secp192r1 (aka NIST P-192;
value 19 = 0x0013) and secp224r1 (aka NIST P-224; value 21 = 0x0015)
and prefers to use secp192r1 would include a TLS extension consisting
of the following octets. Note that the first two octets indicate the
extension type (Supported Group Extension):</t>
<figure><artwork><![CDATA[
00 0A 00 06 00 04 00 13 00 15
]]></artwork></figure>
<t>The client MUST supply a “named_groups” extension containing at
least one group for each key exchange algorithm (currently
DHE and ECDHE) for which it offers a cipher suite.
If the client does not supply a “named_groups” extension with a
compatible group, the server MUST NOT negotiate a cipher suite of the
relevant type. For instance, if a client supplies only ECDHE groups,
the server MUST NOT negotiate finite field Diffie-Hellman. If no
acceptable group can be selected across all cipher suites, then the
server MUST generate a fatal “handshake_failure” alert.</t>
<t>NOTE: A server participating in an ECDHE-ECDSA key exchange may use
different curves for (i) the ECDSA key in its certificate, and (ii)
the ephemeral ECDH key in the ServerKeyExchange message. The server
must consider the supported groups in both cases.</t>
<t>[[TODO: IANA Considerations.]]</t>
</section>
<section anchor="early-data-extension" title="Early Data Extension">
<t>TLS versions before 1.3 have a strict message ordering and do not
permit additional messages to follow the ClientHello. The EarlyData
extension allows TLS messages which would otherwise be sent as
separate records to be instead inserted in the ClientHello. The
extension simply contains the TLS records which would otherwise have
been included in the client’s first flight.</t>
<figure><artwork><![CDATA[
struct {
TLSCipherText messages<5 .. 2^24-1>;
} EarlyDataExtension;
]]></artwork></figure>
<t>Extra messages for the client’s first flight MAY either be transmitted
standalone or sent as EarlyData. However, when a client does not know
whether TLS 1.3 can be negotiated – e.g., because the server may
support a prior version of TLS or because of network intermediaries –
it SHOULD use the EarlyData extension. If the EarlyData extension
is used, then clients MUST NOT send any messages other than the
ClientHello in their initial flight.</t>
<t>Any data included in EarlyData is not integrated into the handshake
hashes directly. E.g., if the ClientKeyShare is included in
EarlyData, then the handshake hashes consist of ClientHello +
ServerHello, etc. However, because the ClientKeyShare is in a
ClientHello extension, it is still hashed transitively. This procedure
guarantees that the Finished message covers these messages even if
they are ultimately ignored by the server (e.g., because it is sent to
a TLS 1.2 server). TLS 1.3 servers MUST understand messages sent in
EarlyData, and aside from hashing them differently, MUST treat them as
if they had been sent immediately after the ClientHello.</t>
<t>Servers MUST NOT send the EarlyData extension. Negotiating TLS 1.3
serves as acknowledgement that it was processed as described above.</t>
<t>[[OPEN ISSUE: This is a fairly general mechanism which is possibly
overkill in the 1-RTT case, where it would potentially be more
attractive to just have a “ClientKeyShare” extension. However,
for the 0-RTT case we will want to send the Certificate, CertificateVerify,
and application data, so a more general extension seems appropriate
at least until we have determined we don’t need it for 0-RTT.]]</t>
</section>
</section>
</section>
<section anchor="server-key-share-message" title="Server Key Share Message">
<t>When this message will be sent:</t>
<t><list style='empty'>
<t>This message will be sent immediately after the ServerHello message if
the client has provided a ClientKeyShare message which is compatible
with the selected cipher suite and group parameters.</t>
</list></t>
<t>Meaning of this message:</t>
<t><list style='empty'>
<t>This message conveys cryptographic information to allow the client to
compute the premaster secret: a Diffie-Hellman public key with which the
client can complete a key exchange (with the result being the premaster secret)
or a public key for some other algorithm.</t>
</list></t>
<t>Structure of this message:</t>
<figure><artwork><![CDATA[
struct {
NamedGroup group;
opaque key_exchange<1..2^16-1>;
} ServerKeyShare;
]]></artwork></figure>
<t><list style="hanging">
<t hangText='group'>
The named group for the key share offer. This identifies the
selected key exchange method from the ClientKeyShare message
(<xref target="client-key-share-message"/>), identifying which value from the
ClientKeyShareOffer the server has accepted as is responding to.</t>
<t hangText='key_exchange'>
Key exchange information. The contents of this field are
determined by the value of NamedGroup entry and its corresponding
definition.</t>
</list></t>
</section>
<section anchor="encrypted-extensions" title="Encrypted Extensions">
<t>When this message will be sent:</t>
<t><list style='empty'>
<t>If this message is sent, it MUST be sent immediately after the server’s
ServerKeyShare.</t>
</list></t>
<t>Meaning of this message:</t>
<t><list style='empty'>
<t>The EncryptedExtensions message simply contains any extensions
which should be protected, i.e., any which are not needed to
establish the cryptographic context. The same extension types
MUST NOT appear in both the ServerHello and EncryptedExtensions.
If the same extension appears in both locations, the client
MUST rely only on the value in the EncryptedExtensions block.
[[OPEN ISSUE: Should we just produce a canonical list of what
goes where and have it be an error to have it in the wrong
place? That seems simpler. Perhaps have a whitelist of which
extensions can be unencrypted and everything else MUST be
encrypted.]]</t>
</list></t>
<t>Structure of this message:</t>
<figure><artwork><![CDATA[
struct {
Extension extensions<0..2^16-1>;
} EncryptedExtensions;
]]></artwork></figure>
<t><list style="hanging">
<t hangText='extensions'><vspace blankLines='0'/>
A list of extensions.</t>
</list></t>
</section>
<section anchor="server-certificate" title="Server Certificate">
<t>When this message will be sent:</t>
<t><list style='empty'>
<t>The server MUST send a Certificate message whenever the agreed-upon key
exchange method uses certificates for authentication (this includes all key
exchange methods defined in this document except DH_anon). This message will
always immediately follow either the EncryptedExtensions message if one is
sent or the ServerKeyShare message.</t>
</list></t>
<t>Meaning of this message:</t>
<t><list style='empty'>
<t>This message conveys the server’s certificate chain to the client.</t>
</list></t>
<t><list style='empty'>
<t>The certificate MUST be appropriate for the negotiated cipher suite’s key
exchange algorithm and any negotiated extensions.</t>
</list></t>
<t>Structure of this message:</t>
<figure><artwork><![CDATA[
opaque ASN1Cert<1..2^24-1>;
struct {
ASN1Cert certificate_list<0..2^24-1>;
} Certificate;
]]></artwork></figure>
<t><list style="hanging">
<t hangText='certificate_list'><vspace blankLines='0'/>
This is a sequence (chain) of certificates. The sender’s
certificate MUST come first in the list. Each following
certificate MUST directly certify the one preceding it. Because
certificate validation requires that root keys be distributed
independently, the self-signed certificate that specifies the root
certificate authority MAY be omitted from the chain, under the
assumption that the remote end must already possess it in order to
validate it in any case.</t>
</list></t>
<t>The same message type and structure will be used for the client’s response to a
certificate request message. Note that a client MAY send no certificates if it
does not have an appropriate certificate to send in response to the server’s
authentication request.</t>
<t>Note: PKCS #7 <xref target="PKCS7"/> is not used as the format for the certificate vector
because PKCS #6 <xref target="PKCS6"/> extended certificates are not used. Also, PKCS #7
defines a SET rather than a SEQUENCE, making the task of parsing the list more
difficult.</t>
<t>The following rules apply to the certificates sent by the server:</t>
<t><list style="symbols">
<t>The certificate type MUST be X.509v3, unless explicitly negotiated
otherwise (e.g., <xref target="RFC5081"/>).</t>
<t>The end entity certificate’s public key (and associated
restrictions) MUST be compatible with the selected key exchange
algorithm.</t>
</list></t>
<figure><artwork><![CDATA[
Key Exchange Alg. Certificate Key Type
DHE_RSA RSA public key; the certificate MUST allow the
ECDHE_RSA key to be used for signing (the
digitalSignature bit MUST be set if the key
usage extension is present) with the signature
scheme and hash algorithm that will be employed
in the server key exchange message.
Note: ECDHE_RSA is defined in [RFC4492].
DHE_DSS DSA public key; the certificate MUST allow the
key to be used for signing with the hash
algorithm that will be employed in the server
key exchange message.
ECDHE_ECDSA ECDSA-capable public key; the certificate MUST
allow the key to be used for signing with the
hash algorithm that will be employed in the
server key exchange message. The public key
MUST use a curve and point format supported by
the client, as described in [RFC4492].
]]></artwork></figure>
<t><list style="symbols">
<t>The “server_name” and “trusted_ca_keys” extensions <xref target="TLSEXT"/> are used to
guide certificate selection.</t>
</list></t>
<t>If the client provided a “signature_algorithms” extension, then all
certificates provided by the server MUST be signed by a hash/signature
algorithm pair that appears in that extension. Note that this implies that a
certificate containing a key for one signature algorithm MAY be signed using a
different signature algorithm (for instance, an RSA key signed with a DSA key).
This is a departure from TLS 1.1, which required that the algorithms be the
same.</t>
<t>If the server has multiple certificates, it chooses one of them based on the
above-mentioned criteria (in addition to other criteria, such as transport
layer endpoint, local configuration and preferences, etc.). If the server has a
single certificate, it SHOULD attempt to validate that it meets these criteria.</t>
<t>Note that there are certificates that use algorithms and/or algorithm
combinations that cannot be currently used with TLS. For example, a certificate
with RSASSA-PSS signature key (id-RSASSA-PSS OID in SubjectPublicKeyInfo)
cannot be used because TLS defines no corresponding signature algorithm.</t>
<t>As cipher suites that specify new key exchange methods are specified for the
TLS protocol, they will imply the certificate format and the required encoded
keying information.</t>
</section>
<section anchor="certificate-request" title="Certificate Request">
<t>When this message will be sent:</t>
<t><list style='empty'>
<t>A non-anonymous server can optionally request a certificate from the client,
if appropriate for the selected cipher suite. This message, if sent, will
immediately follow the server’s Certificate message).</t>
</list></t>
<t>Structure of this message:</t>
<figure><artwork><![CDATA[
enum {
rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
fortezza_dms_RESERVED(20), (255)
} ClientCertificateType;
opaque DistinguishedName<1..2^16-1>;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
SignatureAndHashAlgorithm
supported_signature_algorithms<2..2^16-2>;
DistinguishedName certificate_authorities<0..2^16-1>;
} CertificateRequest;
]]></artwork></figure>
<t><list style="hanging">
<t hangText='certificate_types'><vspace blankLines='0'/>
A list of the types of certificate types that the client may
offer.
<figure><artwork><![CDATA[
rsa_sign a certificate containing an RSA key
dss_sign a certificate containing a DSA key
rsa_fixed_dh a certificate containing a static DH key.
dss_fixed_dh a certificate containing a static DH key
]]></artwork></figure>
</t>
<t hangText='supported_signature_algorithms'><vspace blankLines='0'/>
A list of the hash/signature algorithm pairs that the server is
able to verify, listed in descending order of preference.</t>
<t hangText='certificate_authorities'><vspace blankLines='0'/>
A list of the distinguished names <xref target="X501"/> of acceptable
certificate_authorities, represented in DER-encoded format. These
distinguished names may specify a desired distinguished name for a
root CA or for a subordinate CA; thus, this message can be used to
describe known roots as well as a desired authorization space. If
the certificate_authorities list is empty, then the client MAY
send any certificate of the appropriate ClientCertificateType,
unless there is some external arrangement to the contrary.</t>
</list></t>
<t>The interaction of the certificate_types and
supported_signature_algorithms fields is somewhat complicated.
certificate_types has been present in TLS since SSLv3, but was
somewhat underspecified. Much of its functionality is superseded by
supported_signature_algorithms. The following rules apply:</t>
<t><list style="symbols">
<t>Any certificates provided by the client MUST be signed using a
hash/signature algorithm pair found in
supported_signature_algorithms.</t>
<t>The end-entity certificate provided by the client MUST contain a
key that is compatible with certificate_types. If the key is a
signature key, it MUST be usable with some hash/signature
algorithm pair in supported_signature_algorithms.</t>
<t>For historical reasons, the names of some client certificate types
include the algorithm used to sign the certificate. For example,
in earlier versions of TLS, rsa_fixed_dh meant a certificate
signed with RSA and containing a static DH key. In TLS 1.2, this
functionality has been obsoleted by the
supported_signature_algorithms, and the certificate type no longer
restricts the algorithm used to sign the certificate. For
example, if the server sends dss_fixed_dh certificate type and
{{sha1, dsa}, {sha1, rsa}} signature types, the client MAY reply
with a certificate containing a static DH key, signed with RSA-
SHA1.</t>
</list></t>
<t>New ClientCertificateType values are assigned by IANA as described in
<xref target="iana-considerations"/>.</t>
<t>Note: Values listed as RESERVED may not be used. They were used in SSLv3.</t>
<t>Note: It is a fatal handshake_failure alert for an anonymous server to request
client authentication.</t>
</section>
<section anchor="server-certificate-verify" title="Server Certificate Verify">
<t>When this message will be sent:</t>
<t><list style='empty'>
<t>This message is used to provide explicit proof that the server
possesses the private key corresponding to its certificate.
certificate and also provides integrity for the handshake up
to this point. This message is only sent when the server is
authenticated via a certificate. When sent, it MUST be the
last server handshake message prior to the Finished.</t>
</list></t>
<t>Structure of this message:</t>
<figure><artwork><![CDATA[
struct {
digitally-signed struct {
opaque handshake_messages_hash[hash_length];
}
} CertificateVerify;
]]></artwork></figure>
<t><list style='empty'>
<t>Here handshake_messages_hash is a digest of all handshake messages
sent or received, starting at ClientHello and up to, but not
including, this message, including the type and length fields of the
handshake messages. This is a digest of the concatenation of all the
Handshake structures (as defined in <xref target="handshake-protocol"/>) exchanged
thus far. For the PRF defined in Section 5, the digest MUST be the
Hash used as the basis for the PRF. Any cipher suite which defines a
different PRF MUST also define the Hash to use in this
computation. Note that this is the same running hash that is used in
the Finished message <xref target="server-finished"/>.</t>
</list></t>
<t><list style='empty'>
<t>The context string for the signature is “TLS 1.3, server CertificateVerify”. A
hash of the handshake messages is signed rather than the messages themselves
because the digitally-signed format requires padding and context bytes at the
beginning of the input. Thus, by signing a digest of the messages, an
implementation need only maintain one running hash per hash type for
CertificateVerify, Finished and other messages.</t>
</list></t>
<t><list style='empty'>
<t>If the client has offered the “signature_algorithms” extension, the signature
algorithm and hash algorithm MUST be a pair listed in that extension. Note that
there is a possibility for inconsistencies here. For instance, the client might
offer DHE_DSS key exchange but omit any DSA pairs from its
“signature_algorithms” extension. In order to negotiate correctly, the server
MUST check any candidate cipher suites against the “signature_algorithms”
extension before selecting them. This is somewhat inelegant but is a compromise
designed to minimize changes to the original cipher suite design.</t>
</list></t>
<t><list style='empty'>
<t>In addition, the hash and signature algorithms MUST be compatible with the key
in the server’s end-entity certificate. RSA keys MAY be used with any permitted
hash algorithm, subject to restrictions in the certificate, if any.</t>
</list></t>
<t><list style='empty'>
<t>Because DSA signatures do not contain any secure indication of hash
algorithm, there is a risk of hash substitution if multiple hashes may be used
with any key. Currently, DSA <xref target="DSS"/> may only be used with SHA-1. Future
revisions of DSS <xref target="DSS-3"/> are expected to allow the use of other digest
algorithms with DSA, as well as guidance as to which digest algorithms should
be used with each key size. In addition, future revisions of <xref target="RFC3280"/> may
specify mechanisms for certificates to indicate which digest algorithms are to
be used with DSA.
[[TODO: Update this to deal with DSS-3 and DSS-4.
https://github.com/tlswg/tls13-spec/issues/59]]</t>
</list></t>
</section>
<section anchor="server-finished" title="Server Finished">
<t>When this message will be sent:</t>
<t><list style='empty'>
<t>The Server’s Finished message is the final message sent by the server
and indicates that the key exchange and authentication processes were successful.</t>
</list></t>
<t>Meaning of this message:</t>
<t><list style='empty'>
<t>Recipients of Finished messages MUST verify that the contents are
correct. Once a side has sent its Finished message and received and
validated the Finished message from its peer, it may begin to send and
receive application data over the connection. This data will be
protected under keys derived from the hs_master_secret (see
<xref target="cryptographic-computations"/>.</t>
</list></t>
<t>Structure of this message:</t>
<figure><artwork><![CDATA[
struct {
opaque verify_data[verify_data_length];
} Finished;
verify_data
PRF(hs_master_secret, finished_label, Hash(handshake_messages))
[0..verify_data_length-1];
finished_label
For Finished messages sent by the client, the string
"client finished". For Finished messages sent by the server,
the string "server finished".
]]></artwork></figure>
<t><list style='empty'>
<t>Hash denotes a Hash of the handshake messages. For the PRF defined in
<xref target="HMAC"/>, the Hash MUST be the Hash used as the basis for the PRF. Any cipher
suite which defines a different PRF MUST also define the Hash to use in the
Finished computation.</t>
</list></t>
<t><list style='empty'>
<t>In previous versions of TLS, the verify_data was always 12 octets long. In
the current version of TLS, it depends on the cipher suite. Any cipher suite
which does not explicitly specify verify_data_length has a verify_data_length
equal to 12. This includes all existing cipher suites. Note that this
representation has the same encoding as with previous versions. Future cipher
suites MAY specify other lengths but such length MUST be at least 12 bytes.</t>
</list></t>
<t><list style="hanging">
<t hangText='handshake_messages'><vspace blankLines='0'/>
All of the data from all messages in this handshake (not
including any HelloRequest messages) up to, but not including,
this message. This is only data visible at the handshake layer
and does not include record layer headers. This is the
concatenation of all the Handshake structures as defined in
<xref target="handshake-protocol"/>, exchanged thus far.</t>
</list></t>
<t>The value handshake_messages includes all handshake messages starting at
ClientHello up to, but not including, this Finished message. This may be
different from handshake_messages in <xref target="server-certificate-verify"/> or
<xref target="client-certificate-verify"/>. Also, the handshake_messages
for the Finished message sent by the client will be different from that for the
Finished message sent by the server, because the one that is sent second will
include the prior one.</t>
<t>Note: Alerts and any other record types are not handshake messages
and are not included in the hash computations. Also, HelloRequest
messages are omitted from handshake hashes.</t>
</section>
<section anchor="client-certificate" title="Client Certificate">
<t>When this message will be sent:</t>
<t><list style='empty'>
<t>This message is the first handshake message the client can send
after receiving the server’s Finished. This message is only sent if the server requests a
certificate. If no suitable certificate is available, the client MUST send a
certificate message containing no certificates. That is, the certificate_list
structure has a length of zero. If the client does not send any certificates,
the server MAY at its discretion either continue the handshake without client
authentication, or respond with a fatal handshake_failure alert. Also, if some
aspect of the certificate chain was unacceptable (e.g., it was not signed by a
known, trusted CA), the server MAY at its discretion either continue the
handshake (considering the client unauthenticated) or send a fatal alert.</t>
</list></t>
<t><list style='empty'>
<t>Client certificates are sent using the Certificate structure defined in
<xref target="server-certificate"/>.</t>
</list></t>
<t>Meaning of this message:</t>
<t><list style='empty'>
<t>This message conveys the client’s certificate chain to the server; the server
will use it when verifying the CertificateVerify message (when the client
authentication is based on signing) or calculating the premaster secret (for
non-ephemeral Diffie- Hellman). The certificate MUST be appropriate for the
negotiated cipher suite’s key exchange algorithm, and any negotiated extensions.</t>
</list></t>
<t>In particular:</t>
<t><list style="symbols">
<t>The certificate type MUST be X.509v3, unless explicitly negotiated
otherwise (e.g., <xref target="RFC5081"/>).</t>
<t>The end-entity certificate’s public key (and associated
restrictions) has to be compatible with the certificate types
listed in CertificateRequest: <vspace blankLines='1'/>
<figure><artwork><![CDATA[
Client Cert. Type Certificate Key Type
rsa_sign RSA public key; the certificate MUST allow the
key to be used for signing with the signature
scheme and hash algorithm that will be
employed in the certificate verify message.
dss_sign DSA public key; the certificate MUST allow the
key to be used for signing with the hash
algorithm that will be employed in the
certificate verify message.
ecdsa_sign ECDSA-capable public key; the certificate MUST
allow the key to be used for signing with the
hash algorithm that will be employed in the
certificate verify message; the public key
MUST use a curve and point format supported by
the server.
rsa_fixed_dh Diffie-Hellman public key; MUST use the same
dss_fixed_dh parameters as server's key.
rsa_fixed_ecdh ECDH-capable public key; MUST use the
ecdsa_fixed_ecdh same curve as the server's key, and MUST use a
point format supported by the server.
]]></artwork></figure>
</t>
<t>If the certificate_authorities list in the certificate request
message was non-empty, one of the certificates in the certificate
chain SHOULD be issued by one of the listed CAs.</t>
<t>The certificates MUST be signed using an acceptable hash/
signature algorithm pair, as described in <xref target="certificate-request"/>. Note
that this relaxes the constraints on certificate-signing
algorithms found in prior versions of TLS.</t>
</list></t>
<t>Note that, as with the server certificate, there are certificates that use
algorithms/algorithm combinations that cannot be currently used with TLS.</t>
</section>
<section anchor="client-certificate-verify" title="Client Certificate Verify">
<t>When this message will be sent:</t>
<t><list style='empty'>
<t>This message is used to provide explicit verification of a client
certificate. This message is only sent following a client certificate that has
signing capability (i.e., all certificates except those containing fixed
Diffie-Hellman parameters). When sent, it MUST immediately follow the client’s
Certificate message. The contents of the message are computed as described
in <xref target="server-certificate-verify"/>, except that the context string is
“TLS 1.3, client CertificateVerify”.</t>
</list></t>
<t><list style='empty'>
<t>The hash and signature algorithms used in the signature MUST be one of those
present in the supported_signature_algorithms field of the CertificateRequest
message. In addition, the hash and signature algorithms MUST be compatible with
the key in the client’s end-entity certificate. RSA keys MAY be used with any
permitted hash algorithm, subject to restrictions in the certificate, if any.</t>
</list></t>
<t><list style='empty'>
<t>Because DSA signatures do not contain any secure indication of hash
algorithm, there is a risk of hash substitution if multiple hashes may be used
with any key. Currently, DSA <xref target="DSS"/> may only be used with SHA-1. Future
revisions of DSS <xref target="DSS-3"/> are expected to allow the use of other digest
algorithms with DSA, as well as guidance as to which digest algorithms should
be used with each key size. In addition, future revisions of <xref target="RFC3280"/> may
specify mechanisms for certificates to indicate which digest algorithms are to
be used with DSA.</t>
</list></t>
</section>
</section>
</section>
<section anchor="cryptographic-computations" title="Cryptographic Computations">
<t>In order to begin connection protection, the TLS Record Protocol
requires specification of a suite of algorithms, a master secret, and
the client and server random values. The authentication, key
agreement, and record protection algorithms are determined by the
cipher_suite selected by the server and revealed in the ServerHello
message. The random values are exchanged in the hello messages. All
that remains is to calculate the master secret.</t>
<section anchor="computing-the-master-secret" title="Computing the Master Secret">
<t>The pre_master_secret is used to generate a series of master secret values,
as shown in the following diagram and described below.</t>
<figure><artwork><![CDATA[
Premaster Secret <---------+
| |
PRF |
| |
v |
Handshake <-PRF- Handshake |
Traffic Keys Master Secret |
| | Via
| | Session
+----------+----------+ | Cache
| | |
PRF PRF |
| | |
v v |
Application <-PRF- Master Resumption |
Traffic Keys Secret Premaster --+
Secret
]]></artwork></figure>
<t>First, as soon as the ClientKeyShare and ServerKeyShare messages
have been exchanged, the client and server each use the
unauthenticated key shares to generate a master secret which is used
for the protection of the remaining handshake records. Specifically,
they generate:</t>
<figure><artwork><![CDATA[
hs_master_secret = PRF(pre_master_secret, "handshake master secret",
session_hash)
[0..47];
]]></artwork></figure>
<t>During resumption, the premaster secret is initialized with the
“resumption premaster secret”, rather than using the values from the
ClientKeyShare/ServerKeyShare exchange.</t>
<t>This master secret value is used to generate the record protection
keys used for the handshake, as described in <xref target="key-calculation"/>. It is
also used with TLS Exporters <xref target="RFC5705"/>.</t>
<t>Once the hs_master_secret has been computed, the premaster secret SHOULD
be deleted from memory.</t>
<t>Once the last non-Finished message has been sent, the client and
server then compute the master secret which will be used for the
remainder of the session:</t>
<figure><artwork><![CDATA[
master_secret = PRF(hs_master_secret, "extended master secret",
session_hash)
[0..47];
]]></artwork></figure>
<t>If the server does not request client authentication, the master
secret can be computed at the time that the server sends its Finished,
thus allowing the server to send traffic on its first flight (see
[TODO] for security considerations on this practice.) If the server
requests client authentication, this secret can be computed after the
client’s Certificate and CertificateVerify have been sent, or, if the
client refuses client authentication, after the client’s empty
Certificate message has been sent.</t>
<t>For full handshakes, each side also derives a new secret which will
be used as the premaster_secret for future resumptions of the
newly established session. This is computed as:</t>
<figure><artwork><![CDATA[
resumption_premaster_secret = PRF(hs_master_secret,
"resumption premaster secret",
session_hash)
[0..47];
]]></artwork></figure>
<t>The session_hash value is a running hash of the handshake as
defined in <xref target="the-session-hash"/>. Thus, the hs_master_secret
is generated using a different session_hash from the other
two secrets.</t>
<t>All master secrets are always exactly 48 bytes in length. The length
of the premaster secret will vary depending on key exchange method.</t>
<section anchor="the-session-hash" title="The Session Hash">
<t>When a handshake takes place, we define</t>
<figure><artwork><![CDATA[
session_hash = Hash(handshake_messages)
]]></artwork></figure>
<t>where “handshake_messages” refers to all handshake messages sent or
received, starting at client hello up to the present time, with the
exception of the Finished message, including the type and length
fields of the handshake messages. This is the concatenation of all the
exchanged Handshake structures.</t>
<t>For concreteness, at the point where the handshake master secret
is derived, the session hash includes the ClientHello, ClientKeyShare,
ServerHello, and ServerKeyShare, and HelloRetryRequest (if any)
(though see [https://github.com/tlswg/tls13-spec/issues/104]).
At the point where the master secret is derived, it includes every
handshake message, with the exception of the Finished messages.
Note that if client authentication is not used, then the session
hash is complete at the point when the server has sent its first
flight. Otherwise, it is only complete when the client has sent its
first flight, as it covers the client’s Certificate and CertificateVerify.</t>
</section>
<section anchor="diffie-hellman" title="Diffie-Hellman">
<t>A conventional Diffie-Hellman computation is performed. The negotiated key (Z)
is used as the pre_master_secret, and is converted into the master_secret, as
specified above. Leading bytes of Z that contain all zero bits are stripped
before it is used as the pre_master_secret.</t>
<t>Note: Diffie-Hellman parameters are specified by the server and may be either
ephemeral or contained within the server’s certificate.</t>
</section>
<section anchor="elliptic-curve-diffie-hellman" title="Elliptic Curve Diffie-Hellman">
<t>All ECDH calculations (including parameter and key generation as well
as the shared secret calculation) are performed according to [6]
using the ECKAS-DH1 scheme with the identity map as key derivation
function (KDF), so that the premaster secret is the x-coordinate of
the ECDH shared secret elliptic curve point represented as an octet
string. Note that this octet string (Z in IEEE 1363 terminology) as
output by FE2OSP, the Field Element to Octet String Conversion
Primitive, has constant length for any given field; leading zeros
found in this octet string MUST NOT be truncated.</t>
<t>(Note that this use of the identity KDF is a technicality. The
complete picture is that ECDH is employed with a non-trivial KDF
because TLS does not directly use the premaster secret for anything
other than for computing the master secret.)</t>
</section>
</section>
</section>
<section anchor="mandatory-cipher-suites" title="Mandatory Cipher Suites">
<t>In the absence of an application profile standard specifying otherwise, a
TLS-compliant application MUST implement the cipher suite <eref target="https://github.com/tlswg/tls13-spec/issues/32">TODO:Needs to be selected</eref>. (See <xref target="the-cipher-suite"/> for the definition).</t>
</section>
<section anchor="application-data-protocol" title="Application Data Protocol">
<t>Application data messages are carried by the record layer and are fragmented
and encrypted based on the current connection state. The messages
are treated as transparent data to the record layer.</t>
</section>
<section anchor="security-considerations" title="Security Considerations">
<t>Security issues are discussed throughout this memo, especially in Appendices D,
E, and F.</t>
</section>
<section anchor="iana-considerations" title="IANA Considerations">
<t>[[TODO: Update https://github.com/tlswg/tls13-spec/issues/62]]</t>
<t>This document uses several registries that were originally created in
<xref target="RFC4346"/>. IANA has updated these to reference this document. The registries
and their allocation policies (unchanged from <xref target="RFC4346"/>) are listed below.</t>
<t><list style="symbols">
<t>TLS ClientCertificateType Identifiers Registry: Future values in
the range 0-63 (decimal) inclusive are assigned via Standards
Action <xref target="RFC2434"/>. Values in the range 64-223 (decimal) inclusive
are assigned via Specification Required <xref target="RFC2434"/>. Values from
224-255 (decimal) inclusive are reserved for Private Use
<xref target="RFC2434"/>.</t>
<t>TLS Cipher Suite Registry: Future values with the first byte in
the range 0-191 (decimal) inclusive are assigned via Standards
Action <xref target="RFC2434"/>. Values with the first byte in the range 192-254
(decimal) are assigned via Specification Required <xref target="RFC2434"/>.
Values with the first byte 255 (decimal) are reserved for Private
Use <xref target="RFC2434"/>.</t>
<t>TLS ContentType Registry: Future values are allocated via
Standards Action <xref target="RFC2434"/>.</t>
<t>TLS Alert Registry: Future values are allocated via Standards
Action <xref target="RFC2434"/>.</t>
<t>TLS HandshakeType Registry: Future values are allocated via
Standards Action <xref target="RFC2434"/>.</t>
</list></t>
<t>This document also uses a registry originally created in <xref target="RFC4366"/>. IANA has
updated it to reference this document. The registry and its allocation policy
(unchanged from <xref target="RFC4366"/>) is listed below:</t>
<t><list style="symbols">
<t>TLS ExtensionType Registry: Future values are allocated via IETF
Consensus <xref target="RFC2434"/>. IANA has updated this registry to include
the signature_algorithms extension and its corresponding value
(see <xref target="hello-extensions"/>).</t>
</list></t>
<t>This document also uses two registries originally created in <xref target="RFC4492"/>. IANA
[should update/has updated] it to reference this document. The registries
and their allocation policies are listed below.</t>
<t><list style="symbols">
<t>TLS NamedCurve registry: Future values are allocated via IETF Consensus
<xref target="RFC2434"/>.</t>
<t>TLS ECPointFormat Registry: Future values are allocated via IETF Consensus
<xref target="RFC2434"/>.</t>
</list></t>
<t>In addition, this document defines two new registries to be maintained by IANA:</t>
<t><list style="symbols">
<t>TLS SignatureAlgorithm Registry: The registry has been initially
populated with the values described in <xref target="signature-algorithms"/>. Future
values in the range 0-63 (decimal) inclusive are assigned via
Standards Action <xref target="RFC2434"/>. Values in the range 64-223 (decimal)
inclusive are assigned via Specification Required <xref target="RFC2434"/>.
Values from 224-255 (decimal) inclusive are reserved for Private
Use <xref target="RFC2434"/>.</t>
<t>TLS HashAlgorithm Registry: The registry has been initially
populated with the values described in <xref target="signature-algorithms"/>. Future
values in the range 0-63 (decimal) inclusive are assigned via
Standards Action <xref target="RFC2434"/>. Values in the range 64-223 (decimal)
inclusive are assigned via Specification Required <xref target="RFC2434"/>.
Values from 224-255 (decimal) inclusive are reserved for Private
Use <xref target="RFC2434"/>.</t>
</list></t>
</section>
</middle>
<back>
<references title='Normative References'>
<reference anchor='RFC2104'>
<front>
<title abbrev='HMAC'>HMAC: Keyed-Hashing for Message Authentication</title>
<author initials='H.' surname='Krawczyk' fullname='Hugo Krawczyk'>
<organization>IBM, T.J. Watson Research Center</organization>
<address>
<postal>
<street>P.O.Box 704</street>
<city>Yorktown Heights</city>
<region>NY</region>
<code>10598</code>
<country>US</country></postal>
<email>hugo@watson.ibm.com</email></address></author>
<author initials='M.' surname='Bellare' fullname='Mihir Bellare'>
<organization>University of California at San Diego, Dept of Computer Science and Engineering</organization>
<address>
<postal>
<street>9500 Gilman Drive</street>
<street>Mail Code 0114</street>
<city>La Jolla</city>
<region>CA</region>
<code>92093</code>
<country>US</country></postal>
<email>mihir@cs.ucsd.edu</email></address></author>
<author initials='R.' surname='Canetti' fullname='Ran Canetti'>
<organization>IBM T.J. Watson Research Center</organization>
<address>
<postal>
<street>P.O.Box 704</street>
<city>Yorktown Heights</city>
<region>NY</region>
<code>10598</code>
<country>US</country></postal>
<email>canetti@watson.ibm.com</email></address></author>
<date year='1997' month='February' />
<abstract>
<t>This document describes HMAC, a mechanism for message authentication using cryptographic hash functions. HMAC can be used with any iterative cryptographic hash function, e.g., MD5, SHA-1, in combination with a secret shared key. The cryptographic strength of HMAC depends on the properties of the underlying hash function.</t></abstract></front>
<seriesInfo name='RFC' value='2104' />
<format type='TXT' octets='22297' target='http://www.rfc-editor.org/rfc/rfc2104.txt' />
</reference>
<reference anchor='RFC2119'>
<front>
<title abbrev='RFC Key Words'>Key words for use in RFCs to Indicate Requirement Levels</title>
<author initials='S.' surname='Bradner' fullname='Scott Bradner'>
<organization>Harvard University</organization>
<address>
<postal>
<street>1350 Mass. Ave.</street>
<street>Cambridge</street>
<street>MA 02138</street></postal>
<phone>- +1 617 495 3864</phone>
<email>sob@harvard.edu</email></address></author>
<date year='1997' month='March' />
<area>General</area>
<keyword>keyword</keyword>
<abstract>
<t>
In many standards track documents several words are used to signify
the requirements in the specification. These words are often
capitalized. This document defines these words as they should be
interpreted in IETF documents. Authors who follow these guidelines
should incorporate this phrase near the beginning of their document:
<list>
<t>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
RFC 2119.
</t></list></t>
<t>
Note that the force of these words is modified by the requirement
level of the document in which they are used.
</t></abstract></front>
<seriesInfo name='BCP' value='14' />
<seriesInfo name='RFC' value='2119' />
<format type='TXT' octets='4723' target='http://www.rfc-editor.org/rfc/rfc2119.txt' />
<format type='HTML' octets='17970' target='http://xml.resource.org/public/rfc/html/rfc2119.html' />
<format type='XML' octets='5777' target='http://xml.resource.org/public/rfc/xml/rfc2119.xml' />
</reference>
<reference anchor='RFC2434'>
<front>
<title abbrev='Guidelines for IANA Considerations'>Guidelines for Writing an IANA Considerations Section in RFCs</title>
<author initials='T.' surname='Narten' fullname='Thomas Narten'>
<organization>IBM Corporation</organization>
<address>
<postal>
<street>3039 Cornwallis Ave.</street>
<street>PO Box 12195 - BRQA/502</street>
<street>Research Triangle Park</street>
<street>NC 27709-2195</street></postal>
<phone>919-254-7798</phone>
<email>narten@raleigh.ibm.com</email></address></author>
<author initials='H.T.' surname='Alvestrand' fullname='Harald Tveit Alvestrand'>
<organization>Maxware</organization>
<address>
<postal>
<street>Pirsenteret</street>
<street>N-7005 Trondheim</street>
<country>Norway</country></postal>
<phone>+47 73 54 57 97</phone>
<email>Harald@Alvestrand.no</email></address></author>
<date year='1998' month='October' />
<area>General</area>
<keyword>Internet Assigned Numbers Authority</keyword>
<keyword>IANA</keyword>
<abstract>
<t>
Many protocols make use of identifiers consisting of constants and
other well-known values. Even after a protocol has been defined and
deployment has begun, new values may need to be assigned (e.g., for a
new option type in DHCP, or a new encryption or authentication
algorithm for IPSec). To insure that such quantities have consistent
values and interpretations in different implementations, their
assignment must be administered by a central authority. For IETF
protocols, that role is provided by the Internet Assigned Numbers
Authority (IANA).
</t>
<t>
In order for the IANA to manage a given name space prudently, it
needs guidelines describing the conditions under which new values can
be assigned. If the IANA is expected to play a role in the management
of a name space, the IANA must be given clear and concise
instructions describing that role. This document discusses issues
that should be considered in formulating a policy for assigning
values to a name space and provides guidelines to document authors on
the specific text that must be included in documents that place
demands on the IANA.
</t></abstract></front>
<seriesInfo name='BCP' value='26' />
<seriesInfo name='RFC' value='2434' />
<format type='TXT' octets='25092' target='http://www.rfc-editor.org/rfc/rfc2434.txt' />
<format type='XML' octets='27060' target='http://xml.resource.org/public/rfc/xml/rfc2434.xml' />
</reference>
<reference anchor='RFC1321'>
<front>
<title abbrev='MD5 Message-Digest Algorithm'>The MD5 Message-Digest Algorithm</title>
<author initials='R.' surname='Rivest' fullname='Ronald L. Rivest'>
<organization>Massachusetts Institute of Technology, (MIT) Laboratory for Computer Science</organization>
<address>
<postal>
<street>545 Technology Square</street>
<street>NE43-324</street>
<city>Cambridge</city>
<region>MA</region>
<code>02139-1986</code>
<country>US</country></postal>
<phone>+1 617 253 5880</phone>
<email>rivest@theory.lcs.mit.edu</email></address></author>
<date year='1992' month='April' /></front>
<seriesInfo name='RFC' value='1321' />
<format type='TXT' octets='35222' target='http://www.rfc-editor.org/rfc/rfc1321.txt' />
</reference>
<reference anchor='RFC3447'>
<front>
<title>Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1</title>
<author initials='J.' surname='Jonsson' fullname='J. Jonsson'>
<organization /></author>
<author initials='B.' surname='Kaliski' fullname='B. Kaliski'>
<organization /></author>
<date year='2003' month='February' />
<abstract>
<t>This memo represents a republication of PKCS #1 v2.1 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series, and change control is retained within the PKCS process. The body of this document is taken directly from the PKCS #1 v2.1 document, with certain corrections made during the publication process. This memo provides information for the Internet community.</t></abstract></front>
<seriesInfo name='RFC' value='3447' />
<format type='TXT' octets='143173' target='http://www.rfc-editor.org/rfc/rfc3447.txt' />
</reference>
<reference anchor='RFC3280'>
<front>
<title>Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile</title>
<author initials='R.' surname='Housley' fullname='R. Housley'>
<organization /></author>
<author initials='W.' surname='Polk' fullname='W. Polk'>
<organization /></author>
<author initials='W.' surname='Ford' fullname='W. Ford'>
<organization /></author>
<author initials='D.' surname='Solo' fullname='D. Solo'>
<organization /></author>
<date year='2002' month='April' />
<abstract>
<t>This memo profiles the X.509 v3 certificate and X.509 v2 Certificate Revocation List (CRL) for use in the Internet. [STANDARDS-TRACK]</t></abstract></front>
<seriesInfo name='RFC' value='3280' />
<format type='TXT' octets='295556' target='http://www.rfc-editor.org/rfc/rfc3280.txt' />
</reference>
<reference anchor='RFC5288'>
<front>
<title>AES Galois Counter Mode (GCM) Cipher Suites for TLS</title>
<author initials='J.' surname='Salowey' fullname='J. Salowey'>
<organization /></author>
<author initials='A.' surname='Choudhury' fullname='A. Choudhury'>
<organization /></author>
<author initials='D.' surname='McGrew' fullname='D. McGrew'>
<organization /></author>
<date year='2008' month='August' />
<abstract>
<t>This memo describes the use of the Advanced Encryption Standard (AES) in Galois/Counter Mode (GCM) as a Transport Layer Security (TLS) authenticated encryption operation. GCM provides both confidentiality and data origin authentication, can be efficiently implemented in hardware for speeds of 10 gigabits per second and above, and is also well-suited to software implementations. This memo defines TLS cipher suites that use AES-GCM with RSA, DSA, and Diffie-Hellman-based key exchange mechanisms. [STANDARDS-TRACK]</t></abstract></front>
<seriesInfo name='RFC' value='5288' />
<format type='TXT' octets='16468' target='http://www.rfc-editor.org/rfc/rfc5288.txt' />
</reference>
<reference anchor="AES" >
<front>
<title>Specification for the Advanced Encryption Standard (AES)</title>
<author >
<organization>National Institute of Standards and Technology</organization>
</author>
<date year="2001" month="November" day="26"/>
</front>
<seriesInfo name="NIST" value="FIPS 197"/>
</reference>
<reference anchor="TRIPLEDES" >
<front>
<title>Recommendation for the Triple Data Encryption Algorithm (TDEA) Block Cipher</title>
<author >
<organization>National Institute of Standards and Technology</organization>
</author>
<date year="2004" month="May"/>
</front>
<seriesInfo name="NIST" value="Special Publication 800-67"/>
</reference>
<reference anchor="DSS" >
<front>
<title>Digital Signature Standard</title>
<author >
<organization>National Institute of Standards and Technology, U.S. Department of Commerce</organization>
</author>
<date year="2000"/>
</front>
<seriesInfo name="NIST" value="FIPS PUB 186-2"/>
</reference>
<reference anchor="SCH" >
<front>
<title>Applied Cryptography: Protocols, Algorithms, and Source Code in C, 2nd ed.</title>
<author initials="B." surname="Schneier" fullname="Bruce Schneier">
<organization></organization>
</author>
<date year="1996"/>
</front>
</reference>
<reference anchor="SHS" >
<front>
<title>Secure Hash Standard</title>
<author >
<organization>National Institute of Standards and Technology, U.S. Department of Commerce</organization>
</author>
<date year="2002" month="August"/>
</front>
<seriesInfo name="NIST" value="FIPS PUB 180-2"/>
</reference>
<reference anchor="X680" >
<front>
<title>Information technology - Abstract Syntax Notation One (ASN.1): Specification of basic notation</title>
<author >
<organization>ITU-T</organization>
</author>
<date year="2002"/>
</front>
<seriesInfo name="ISO/IEC" value="8824-1:2002"/>
</reference>
<reference anchor="X690" >
<front>
<title>Information technology - ASN.1 encoding Rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)</title>
<author >
<organization>ITU-T</organization>
</author>
<date year="2002"/>
</front>
<seriesInfo name="ISO/IEC" value="8825-1:2002"/>
</reference>
<reference anchor="X962" >
<front>
<title>Public Key Cryptography For The Financial Services Industry: The Elliptic Curve Digital Signature Algorithm (ECDSA)</title>
<author >
<organization>ANSI</organization>
</author>
<date year="1998"/>
</front>
<seriesInfo name="ANSI" value="X9.62"/>
</reference>
</references>
<references title='Informative References'>
<reference anchor='RFC0793'>
<front>
<title abbrev='Transmission Control Protocol'>Transmission Control Protocol</title>
<author initials='J.' surname='Postel' fullname='Jon Postel'>
<organization>University of Southern California (USC)/Information Sciences Institute</organization>
<address>
<postal>
<street>4676 Admiralty Way</street>
<city>Marina del Rey</city>
<region>CA</region>
<code>90291</code>
<country>US</country></postal></address></author>
<date year='1981' day='1' month='September' /></front>
<seriesInfo name='STD' value='7' />
<seriesInfo name='RFC' value='793' />
<format type='TXT' octets='172710' target='http://www.rfc-editor.org/rfc/rfc793.txt' />
</reference>
<reference anchor='RFC1948'>
<front>
<title abbrev='Sequence Number Attacks'>Defending Against Sequence Number Attacks</title>
<author initials='S.' surname='Bellovin' fullname='Steven M. Bellovin'>
<organization>AT&T Research</organization>
<address>
<postal>
<street>600 Mountain Avenue</street>
<city>Murray Hill</city>
<region>NJ</region>
<code>07974</code>
<country>US</country></postal>
<phone>+1 908 582 5886</phone>
<email>smb@research.att.com</email></address></author>
<date year='1996' month='May' />
<abstract>
<t>IP spoofing attacks based on sequence number spoofing have become a serious threat on the Internet (CERT Advisory CA-95:01). While ubiquitous crypgraphic authentication is the right answer, we propose a simple modification to TCP implementations that should be a very substantial block to the current wave of attacks.</t></abstract></front>
<seriesInfo name='RFC' value='1948' />
<format type='TXT' octets='13074' target='http://www.rfc-editor.org/rfc/rfc1948.txt' />
</reference>
<reference anchor='RFC2246'>
<front>
<title>The TLS Protocol Version 1.0</title>
<author initials='T.' surname='Dierks' fullname='Tim Dierks'>
<organization>Certicom</organization>
<address>
<email>tdierks@certicom.com</email></address></author>
<author initials='C.' surname='Allen' fullname='Christopher Allen'>
<organization>Certicom</organization>
<address>
<email>callen@certicom.com</email></address></author>
<date year='1999' month='January' />
<abstract>
<t>This document specifies Version 1.0 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications privacy over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery.</t></abstract></front>
<seriesInfo name='RFC' value='2246' />
<format type='TXT' octets='170401' target='http://www.rfc-editor.org/rfc/rfc2246.txt' />
</reference>
<reference anchor='RFC2785'>
<front>
<title abbrev='Methods for Avoiding Small-Subgroup Attacks'>Methods for Avoiding the "Small-Subgroup" Attacks on the Diffie-Hellman Key Agreement Method for S/MIME</title>
<author initials='R.' surname='Zuccherato' fullname='Robert Zuccherato'>
<organization>Entrust Technologies</organization>
<address>
<postal>
<street>750 Heron Road</street>
<city>Ottawa</city>
<region>Ontario</region>
<code>K1V 1A7</code>
<country>CA</country></postal>
<email>robert.zuccherato@entrust.com</email></address></author>
<date year='2000' month='March' />
<abstract>
<t>In some circumstances the use of the Diffie-Hellman key agreement scheme in a prime order subgroup of a large prime p is vulnerable to certain attacks known as "small-subgroup" attacks. Methods exist, however, to prevent these attacks. This document will describe the situations relevant to implementations of S/MIME version 3 in which protection is necessary and the methods that can be used to prevent these attacks.</t></abstract></front>
<seriesInfo name='RFC' value='2785' />
<format type='TXT' octets='24415' target='http://www.rfc-editor.org/rfc/rfc2785.txt' />
</reference>
<reference anchor='RFC3268'>
<front>
<title>Advanced Encryption Standard (AES) Ciphersuites for Transport Layer Security (TLS)</title>
<author initials='P.' surname='Chown' fullname='P. Chown'>
<organization /></author>
<date year='2002' month='June' />
<abstract>
<t>This document proposes several new ciphersuites. At present, the symmetric ciphers supported by Transport Layer Security (TLS) are RC2, RC4, International Data Encryption Algorithm (IDEA), Data Encryption Standard (DES), and triple DES. The protocol would be enhanced by the addition of Advanced Encryption Standard (AES) ciphersuites. [STANDARDS-TRACK]</t></abstract></front>
<seriesInfo name='RFC' value='3268' />
<format type='TXT' octets='13530' target='http://www.rfc-editor.org/rfc/rfc3268.txt' />
</reference>
<reference anchor='RFC3526'>
<front>
<title>More Modular Exponential (MODP) Diffie-Hellman groups for Internet Key Exchange (IKE)</title>
<author initials='T.' surname='Kivinen' fullname='T. Kivinen'>
<organization /></author>
<author initials='M.' surname='Kojo' fullname='M. Kojo'>
<organization /></author>
<date year='2003' month='May' />
<abstract>
<t>This document defines new Modular Exponential (MODP) Groups for the Internet Key Exchange (IKE) protocol. It documents the well known and used 1536 bit group 5, and also defines new 2048, 3072, 4096, 6144, and 8192 bit Diffie-Hellman groups numbered starting at 14. The selection of the primes for theses groups follows the criteria established by Richard Schroeppel. [STANDARDS-TRACK]</t></abstract></front>
<seriesInfo name='RFC' value='3526' />
<format type='TXT' octets='19166' target='http://www.rfc-editor.org/rfc/rfc3526.txt' />
</reference>
<reference anchor='RFC3766'>
<front>
<title>Determining Strengths For Public Keys Used For Exchanging Symmetric Keys</title>
<author initials='H.' surname='Orman' fullname='H. Orman'>
<organization /></author>
<author initials='P.' surname='Hoffman' fullname='P. Hoffman'>
<organization /></author>
<date year='2004' month='April' />
<abstract>
<t>Implementors of systems that use public key cryptography to exchange symmetric keys need to make the public keys resistant to some predetermined level of attack. That level of attack resistance is the strength of the system, and the symmetric keys that are exchanged must be at least as strong as the system strength requirements. The three quantities, system strength, symmetric key strength, and public key strength, must be consistently matched for any network protocol usage. While it is fairly easy to express the system strength requirements in terms of a symmetric key length and to choose a cipher that has a key length equal to or exceeding that requirement, it is harder to choose a public key that has a cryptographic strength meeting a symmetric key strength requirement. This document explains how to determine the length of an asymmetric key as a function of a symmetric key strength requirement. Some rules of thumb for estimating equivalent resistance to large-scale attacks on various algorithms are given. The document also addresses how changing the sizes of the underlying large integers (moduli, group sizes, exponents, and so on) changes the time to use the algorithms for key exchange. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t></abstract></front>
<seriesInfo name='BCP' value='86' />
<seriesInfo name='RFC' value='3766' />
<format type='TXT' octets='55939' target='http://www.rfc-editor.org/rfc/rfc3766.txt' />
</reference>
<reference anchor='RFC4086'>
<front>
<title>Randomness Requirements for Security</title>
<author initials='D.' surname='Eastlake' fullname='D. Eastlake'>
<organization /></author>
<author initials='J.' surname='Schiller' fullname='J. Schiller'>
<organization /></author>
<author initials='S.' surname='Crocker' fullname='S. Crocker'>
<organization /></author>
<date year='2005' month='June' />
<abstract>
<t>Security systems are built on strong cryptographic algorithms that foil pattern analysis attempts. However, the security of these systems is dependent on generating secret quantities for passwords, cryptographic keys, and similar quantities. The use of pseudo-random processes to generate secret quantities can result in pseudo-security. A sophisticated attacker may find it easier to reproduce the environment that produced the secret quantities and to search the resulting small set of possibilities than to locate the quantities in the whole of the potential number space.</t><t> Choosing random quantities to foil a resourceful and motivated adversary is surprisingly difficult. This document points out many pitfalls in using poor entropy sources or traditional pseudo-random number generation techniques for generating such quantities. It recommends the use of truly random hardware techniques and shows that the existing hardware on many systems can be used for this purpose. It provides suggestions to ameliorate the problem when a hardware solution is not available, and it gives examples of how large such quantities need to be for some applications. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t></abstract></front>
<seriesInfo name='BCP' value='106' />
<seriesInfo name='RFC' value='4086' />
<format type='TXT' octets='114321' target='http://www.rfc-editor.org/rfc/rfc4086.txt' />
</reference>
<reference anchor='RFC4302'>
<front>
<title>IP Authentication Header</title>
<author initials='S.' surname='Kent' fullname='S. Kent'>
<organization /></author>
<date year='2005' month='December' />
<abstract>
<t>This document describes an updated version of the IP Authentication Header (AH), which is designed to provide authentication services in IPv4 and IPv6. This document obsoletes RFC 2402 (November 1998). [STANDARDS-TRACK]</t></abstract></front>
<seriesInfo name='RFC' value='4302' />
<format type='TXT' octets='82328' target='http://www.rfc-editor.org/rfc/rfc4302.txt' />
</reference>
<reference anchor='RFC4303'>
<front>
<title>IP Encapsulating Security Payload (ESP)</title>
<author initials='S.' surname='Kent' fullname='S. Kent'>
<organization /></author>
<date year='2005' month='December' />
<abstract>
<t>This document describes an updated version of the Encapsulating Security Payload (ESP) protocol, which is designed to provide a mix of security services in IPv4 and IPv6. ESP is used to provide confidentiality, data origin authentication, connectionless integrity, an anti-replay service (a form of partial sequence integrity), and limited traffic flow confidentiality. This document obsoletes RFC 2406 (November 1998). [STANDARDS-TRACK]</t></abstract></front>
<seriesInfo name='RFC' value='4303' />
<format type='TXT' octets='114315' target='http://www.rfc-editor.org/rfc/rfc4303.txt' />
</reference>
<reference anchor='RFC4307'>
<front>
<title>Cryptographic Algorithms for Use in the Internet Key Exchange Version 2 (IKEv2)</title>
<author initials='J.' surname='Schiller' fullname='J. Schiller'>
<organization /></author>
<date year='2005' month='December' />
<abstract>
<t>The IPsec series of protocols makes use of various cryptographic algorithms in order to provide security services. The Internet Key Exchange (IKE (RFC 2409) and IKEv2) provide a mechanism to negotiate which algorithms should be used in any given association. However, to ensure interoperability between disparate implementations, it is necessary to specify a set of mandatory-to-implement algorithms to ensure that there is at least one algorithm that all implementations will have available. This document defines the current set of algorithms that are mandatory to implement as part of IKEv2, as well as algorithms that should be implemented because they may be promoted to mandatory at some future time. [STANDARDS-TRACK]</t></abstract></front>
<seriesInfo name='RFC' value='4307' />
<format type='TXT' octets='12985' target='http://www.rfc-editor.org/rfc/rfc4307.txt' />
</reference>
<reference anchor='RFC4346'>
<front>
<title>The Transport Layer Security (TLS) Protocol Version 1.1</title>
<author initials='T.' surname='Dierks' fullname='T. Dierks'>
<organization /></author>
<author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
<organization /></author>
<date year='2006' month='April' />
<abstract>
<t>This document specifies Version 1.1 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK]</t></abstract></front>
<seriesInfo name='RFC' value='4346' />
<format type='TXT' octets='187041' target='http://www.rfc-editor.org/rfc/rfc4346.txt' />
</reference>
<reference anchor='RFC4366'>
<front>
<title>Transport Layer Security (TLS) Extensions</title>
<author initials='S.' surname='Blake-Wilson' fullname='S. Blake-Wilson'>
<organization /></author>
<author initials='M.' surname='Nystrom' fullname='M. Nystrom'>
<organization /></author>
<author initials='D.' surname='Hopwood' fullname='D. Hopwood'>
<organization /></author>
<author initials='J.' surname='Mikkelsen' fullname='J. Mikkelsen'>
<organization /></author>
<author initials='T.' surname='Wright' fullname='T. Wright'>
<organization /></author>
<date year='2006' month='April' />
<abstract>
<t>This document describes extensions that may be used to add functionality to Transport Layer Security (TLS). It provides both generic extension mechanisms for the TLS handshake client and server hellos, and specific extensions using these generic mechanisms.</t><t> The extensions may be used by TLS clients and servers. The extensions are backwards compatible: communication is possible between TLS clients that support the extensions and TLS servers that do not support the extensions, and vice versa. [STANDARDS-TRACK]</t></abstract></front>
<seriesInfo name='RFC' value='4366' />
<format type='TXT' octets='66344' target='http://www.rfc-editor.org/rfc/rfc4366.txt' />
</reference>
<reference anchor='RFC4492'>
<front>
<title>Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)</title>
<author initials='S.' surname='Blake-Wilson' fullname='S. Blake-Wilson'>
<organization /></author>
<author initials='N.' surname='Bolyard' fullname='N. Bolyard'>
<organization /></author>
<author initials='V.' surname='Gupta' fullname='V. Gupta'>
<organization /></author>
<author initials='C.' surname='Hawk' fullname='C. Hawk'>
<organization /></author>
<author initials='B.' surname='Moeller' fullname='B. Moeller'>
<organization /></author>
<date year='2006' month='May' />
<abstract>
<t>This document describes new key exchange algorithms based on Elliptic Curve Cryptography (ECC) for the Transport Layer Security (TLS) protocol. In particular, it specifies the use of Elliptic Curve Diffie-Hellman (ECDH) key agreement in a TLS handshake and the use of Elliptic Curve Digital Signature Algorithm (ECDSA) as a new authentication mechanism. This memo provides information for the Internet community.</t></abstract></front>
<seriesInfo name='RFC' value='4492' />
<format type='TXT' octets='72231' target='http://www.rfc-editor.org/rfc/rfc4492.txt' />
</reference>
<reference anchor='RFC4506'>
<front>
<title>XDR: External Data Representation Standard</title>
<author initials='M.' surname='Eisler' fullname='M. Eisler'>
<organization /></author>
<date year='2006' month='May' />
<abstract>
<t>This document describes the External Data Representation Standard (XDR) protocol as it is currently deployed and accepted. This document obsoletes RFC 1832. [STANDARDS-TRACK]</t></abstract></front>
<seriesInfo name='STD' value='67' />
<seriesInfo name='RFC' value='4506' />
<format type='TXT' octets='55477' target='http://www.rfc-editor.org/rfc/rfc4506.txt' />
</reference>
<reference anchor='RFC5081'>
<front>
<title>Using OpenPGP Keys for Transport Layer Security (TLS) Authentication</title>
<author initials='N.' surname='Mavrogiannopoulos' fullname='N. Mavrogiannopoulos'>
<organization /></author>
<date year='2007' month='November' />
<abstract>
<t>This memo proposes extensions to the Transport Layer Security (TLS) protocol to support the OpenPGP key format. The extensions discussed here include a certificate type negotiation mechanism, and the required modifications to the TLS Handshake Protocol. This memo defines an Experimental Protocol for the Internet community.</t></abstract></front>
<seriesInfo name='RFC' value='5081' />
<format type='TXT' octets='15300' target='http://www.rfc-editor.org/rfc/rfc5081.txt' />
</reference>
<reference anchor='RFC5116'>
<front>
<title>An Interface and Algorithms for Authenticated Encryption</title>
<author initials='D.' surname='McGrew' fullname='D. McGrew'>
<organization /></author>
<date year='2008' month='January' />
<abstract>
<t>This document defines algorithms for Authenticated Encryption with Associated Data (AEAD), and defines a uniform interface and a registry for such algorithms. The interface and registry can be used as an application-independent set of cryptoalgorithm suites. This approach provides advantages in efficiency and security, and promotes the reuse of crypto implementations. [STANDARDS-TRACK]</t></abstract></front>
<seriesInfo name='RFC' value='5116' />
<format type='TXT' octets='50539' target='http://www.rfc-editor.org/rfc/rfc5116.txt' />
</reference>
<reference anchor='RFC5705'>
<front>
<title>Keying Material Exporters for Transport Layer Security (TLS)</title>
<author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
<organization /></author>
<date year='2010' month='March' />
<abstract>
<t>A number of protocols wish to leverage Transport Layer Security (TLS) to perform key establishment but then use some of the keying material for their own purposes. This document describes a general mechanism for allowing that. [STANDARDS-TRACK]</t></abstract></front>
<seriesInfo name='RFC' value='5705' />
<format type='TXT' octets='16346' target='http://www.rfc-editor.org/rfc/rfc5705.txt' />
</reference>
<reference anchor='I-D.ietf-tls-negotiated-ff-dhe'>
<front>
<title>Negotiated Finite Field Diffie-Hellman Ephemeral Parameters for TLS</title>
<author initials='D' surname='Gillmor' fullname='Daniel Gillmor'>
<organization />
</author>
<date month='December' day='19' year='2014' />
<abstract><t>Traditional finite-field-based Diffie-Hellman (DH) key exchange during the TLS handshake suffers from a number of security, interoperability, and efficiency shortcomings. These shortcomings arise from lack of clarity about which DH group parameters TLS servers should offer and clients should accept. This document offers a solution to these shortcomings for compatible peers by using a section of the TLS "EC Named Curve Registry" to establish common finite-field DH parameters with known structure and a mechanism for peers to negotiate support for these groups.</t></abstract>
</front>
<seriesInfo name='Internet-Draft' value='draft-ietf-tls-negotiated-ff-dhe-05' />
<format type='TXT'
target='http://www.ietf.org/internet-drafts/draft-ietf-tls-negotiated-ff-dhe-05.txt' />
</reference>
<reference anchor='I-D.ietf-tls-session-hash'>
<front>
<title>Transport Layer Security (TLS) Session Hash and Extended Master Secret Extension</title>
<author initials='K' surname='Bhargavan' fullname='Karthikeyan Bhargavan'>
<organization />
</author>
<author initials='A' surname='Delignat-Lavaud' fullname='Antoine Delignat-Lavaud'>
<organization />
</author>
<author initials='A' surname='Pironti' fullname='Alfredo Pironti'>
<organization />
</author>
<author initials='A' surname='Langley' fullname='Adam Langley'>
<organization />
</author>
<author initials='M' surname='Ray' fullname='Marsh Ray'>
<organization />
</author>
<date month='November' day='12' year='2014' />
<abstract><t>The Transport Layer Security (TLS) master secret is not cryptographically bound to important session parameters. Consequently, it is possible for an active attacker to set up two sessions, one with a client and another with a server, such that the master secrets on the two sessions are the same. Thereafter, any mechanism that relies on the master secret for authentication, including session resumption, becomes vulnerable to a man-in-the- middle attack, where the attacker can simply forward messages back and forth between the client and server. This specification defines a TLS extension that contextually binds the master secret to a log of the full handshake that computes it, thus preventing such attacks.</t></abstract>
</front>
<seriesInfo name='Internet-Draft' value='draft-ietf-tls-session-hash-03' />
<format type='TXT'
target='http://www.ietf.org/internet-drafts/draft-ietf-tls-session-hash-03.txt' />
</reference>
<reference anchor="BLEI" >
<front>
<title>Chosen Ciphertext Attacks against Protocols Based on RSA Encryption Standard PKCS</title>
<author initials="D." surname="Bleichenbacher">
<organization></organization>
</author>
<date year="1998"/>
</front>
<seriesInfo name="CRYPTO98" value="LNCS vol. 1462, pages: 1-12, 1998, Advances in Cryptology"/>
</reference>
<reference anchor="CBCATT" target="http://www.openssl.org/~bodo/tls-cbc.txt">
<front>
<title>Security of CBC Ciphersuites in SSL/TLS: Problems and Countermeasures</title>
<author initials="B." surname="Moeller">
<organization></organization>
</author>
<date year="2004" month="May" day="20"/>
</front>
</reference>
<reference anchor="CCM" target="http://csrc.nist.gov/publications/nistpubs/800-38C/SP800-38C.pdf">
<front>
<title>NIST Special Publication 800-38C: The CCM Mode for Authentication and Confidentiality</title>
<author >
<organization></organization>
</author>
<date year="2004" month="May"/>
</front>
</reference>
<reference anchor="DES" >
<front>
<title>Data Encryption Standard (DES)</title>
<author >
<organization></organization>
</author>
<date year="1999" month="October"/>
</front>
<seriesInfo name="NIST" value="FIPS PUB 46-3"/>
</reference>
<reference anchor="DSS-3" >
<front>
<title>Digital Signature Standard</title>
<author >
<organization>National Institute of Standards and Technology, U.S.</organization>
</author>
<date year="2006"/>
</front>
<seriesInfo name="NIST" value="FIPS PUB 186-3 Draft"/>
</reference>
<reference anchor="ECDSA" >
<front>
<title>Public Key Cryptography for the Financial Services Industry: The Elliptic Curve Digital Signature Algorithm (ECDSA)</title>
<author >
<organization>American National Standards Institute</organization>
</author>
<date year="2005" month="November"/>
</front>
<seriesInfo name="ANSI" value="ANS X9.62-2005"/>
</reference>
<reference anchor="ENCAUTH" >
<front>
<title>The Order of Encryption and Authentication for Protecting Communications (Or: How Secure is SSL?)</title>
<author initials="H." surname="Krawczyk">
<organization></organization>
</author>
<date year="2001"/>
</front>
</reference>
<reference anchor="FI06" target="http://www.imc.org/ietf-openpgp/mail-archive/msg14307.html">
<front>
<title>Bleichenbacher's RSA signature forgery based on implementation error</title>
<author fullname="Hal Finney">
<organization></organization>
</author>
<date year="2006" month="August" day="27"/>
</front>
</reference>
<reference anchor="GCM" >
<front>
<title>Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC</title>
<author initials="M." surname="Dworkin">
<organization></organization>
</author>
<date year="2007" month="November"/>
</front>
<seriesInfo name="NIST" value="Special Publication 800-38D"/>
</reference>
<reference anchor="PKCS6" >
<front>
<title>PKCS #6: RSA Extended Certificate Syntax Standard, version 1.5</title>
<author >
<organization>RSA Laboratories</organization>
</author>
<date year="1993" month="November"/>
</front>
</reference>
<reference anchor="PKCS7" >
<front>
<title>PKCS #7: RSA Cryptographic Message Syntax Standard, version 1.5</title>
<author >
<organization>RSA Laboratories</organization>
</author>
<date year="1993" month="November"/>
</front>
</reference>
<reference anchor="RSA" >
<front>
<title>A Method for Obtaining Digital Signatures and Public-Key Cryptosystems</title>
<author initials="R." surname="Rivest">
<organization></organization>
</author>
<author initials="A." surname="Shamir">
<organization></organization>
</author>
<author initials="L.M." surname="Adleman">
<organization></organization>
</author>
<date year="1978" month="February"/>
</front>
<seriesInfo name="Communications of the ACM" value="v. 21, n. 2, pp. 120-126."/>
</reference>
<reference anchor="SSL2" >
<front>
<title>The SSL Protocol</title>
<author fullname="Kipp Hickman">
<organization>Netscape Communications Corp.</organization>
</author>
<date year="1995" month="February" day="09"/>
</front>
</reference>
<reference anchor="SSL3" >
<front>
<title>The SSL 3.0 Protocol</title>
<author initials="A." surname="Freier">
<organization>Netscape Communications Corp.</organization>
</author>
<author initials="P." surname="Karlton">
<organization>Netscape Communications Corp.</organization>
</author>
<author initials="P." surname="Kocher">
<organization>Netscape Communications Corp.</organization>
</author>
<date year="1996" month="November" day="18"/>
</front>
</reference>
<reference anchor="TIMING" >
<front>
<title>Remote timing attacks are practical</title>
<author initials="D." surname="Boneh">
<organization></organization>
</author>
<author initials="D." surname="Brumley">
<organization></organization>
</author>
<date year="2003"/>
</front>
<seriesInfo name="USENIX" value="Security Symposium"/>
</reference>
<reference anchor="TLSEXT" >
<front>
<title>Transport Layer Security (TLS) Extensions: Extension Definitions</title>
<author initials="D." surname="Eastlake 3rd">
<organization></organization>
</author>
<date year="2008" month="February"/>
</front>
</reference>
<reference anchor="X501" >
<front>
<title>Information Technology - Open Systems Interconnection - The Directory: Models</title>
<author >
<organization></organization>
</author>
<date year="1993"/>
</front>
<seriesInfo name="ITU-T" value="X.501"/>
</reference>
</references>
<section anchor="protocol-data-structures-and-constant-values" title="Protocol Data Structures and Constant Values">
<t>This section describes protocol types and constants.</t>
<t>[[TODO: Clean this up to match the in-text description.]]</t>
<section anchor="record-layer-1" title="Record Layer">
<figure><artwork><![CDATA[
struct {
uint8 major;
uint8 minor;
} ProtocolVersion;
ProtocolVersion version = { 3, 4 }; /* TLS v1.3*/
enum {
change_cipher_spec(20), alert(21), handshake(22),
application_data(23), (255)
} ContentType;
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
opaque fragment[TLSPlaintext.length];
} TLSPlaintext;
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
opaque nonce_explicit[SecurityParameters.record_iv_length];
aead-ciphered struct {
opaque content[TLSPlaintext.length];
} fragment;
} TLSCiphertext;
]]></artwork></figure>
</section>
<section anchor="change-cipher-specs-message" title="Change Cipher Specs Message">
<figure><artwork><![CDATA[
struct {
enum { change_cipher_spec(1), (255) } type;
} ChangeCipherSpec;
]]></artwork></figure>
</section>
<section anchor="alert-messages" title="Alert Messages">
<figure><artwork><![CDATA[
enum { warning(1), fatal(2), (255) } AlertLevel;
enum {
close_notify(0),
unexpected_message(10),
bad_record_mac(20),
decryption_failed_RESERVED(21),
record_overflow(22),
decompression_failure_RESERVED(30),
handshake_failure(40),
no_certificate_RESERVED(41),
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45),
certificate_unknown(46),
illegal_parameter(47),
unknown_ca(48),
access_denied(49),
decode_error(50),
decrypt_error(51),
export_restriction_RESERVED(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
user_canceled(90),
no_renegotiation(100),
unsupported_extension(110), /* new */
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
]]></artwork></figure>
</section>
<section anchor="handshake-protocol-1" title="Handshake Protocol">
<figure><artwork><![CDATA[
enum {
reserved(0), client_hello(1), server_hello(2),
client_key_share(5), hello_retry_request(6),
server_key_share(7), certificate(11), reserved(12),
certificate_request(13), certificate_verify(15),
reserved(16), finished(20), (255)
} HandshakeType;
struct {
HandshakeType msg_type;
uint24 length;
select (HandshakeType) {
case hello_request: HelloRequest;
case client_hello: ClientHello;
case server_hello: ServerHello;
case hello_retry_request: HelloRetryRequest;
case certificate: Certificate;
case server_key_share: ServerKeyShare;
case certificate_request: CertificateRequest;
case server_hello_done: ServerHelloDone;
case certificate_verify: CertificateVerify;
case client_key_share: ClientKeyShare;
case finished: Finished;
} body;
} Handshake;
]]></artwork></figure>
<section anchor="hello-messages-1" title="Hello Messages">
<figure><artwork><![CDATA[
struct { } HelloRequest;
struct {
opaque random_bytes[32];
} Random;
opaque SessionID<0..32>;
uint8 CipherSuite[2];
enum { null(0), (255) } CompressionMethod;
struct {
ProtocolVersion client_version;
Random random;
SessionID session_id;
CipherSuite cipher_suites<2..2^16-2>;
CompressionMethod compression_methods<1..2^8-1>;
select (extensions_present) {
case false:
struct {};
case true:
Extension extensions<0..2^16-1>;
};
} ClientHello;
struct {
ProtocolVersion server_version;
Random random;
SessionID session_id;
CipherSuite cipher_suite;
select (extensions_present) {
case false:
struct {};
case true:
Extension extensions<0..2^16-1>;
};
} ServerHello;
struct {
ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} Extension;
enum {
signature_algorithms(13), (65535)
} ExtensionType;
enum{
none(0), md5(1), sha1(2), sha224(3), sha256(4), sha384(5),
sha512(6), (255)
} HashAlgorithm;
enum {
anonymous(0), rsa(1), dsa(2), ecdsa(3), (255)
} SignatureAlgorithm;
struct {
HashAlgorithm hash;
SignatureAlgorithm signature;
} SignatureAndHashAlgorithm;
SignatureAndHashAlgorithm
supported_signature_algorithms<2..2^16-2>;
]]></artwork></figure>
</section>
<section anchor="server-authentication-and-key-exchange-messages" title="Server Authentication and Key Exchange Messages">
<figure><artwork><![CDATA[
opaque ASN1Cert<2^24-1>;
struct {
ASN1Cert certificate_list<0..2^24-1>;
} Certificate;
struct {
NamedGroup group;
opaque key_exchange<1..2^16-1>;
} ServerKeyShare;
enum {
rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
fortezza_dms_RESERVED(20),
(255)
} ClientCertificateType;
opaque DistinguishedName<1..2^16-1>;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
DistinguishedName certificate_authorities<0..2^16-1>;
} CertificateRequest;
struct { } ServerHelloDone;
]]></artwork></figure>
</section>
<section anchor="client-authentication-and-key-exchange-messages" title="Client Authentication and Key Exchange Messages">
<figure><artwork><![CDATA[
struct {
ClientKeyShareOffer offers<0..2^16-1>;
} ClientKeyShare;
struct {
NamedGroup group;
opaque key_exchange<1..2^16-1>;
} ClientKeyShareOffer;
struct {
digitally-signed struct {
opaque handshake_messages_hash[hash_length];
}
} CertificateVerify;
]]></artwork></figure>
<t>The context string for the signature is “TLS 1.3, client CertificateVerify”.</t>
</section>
<section anchor="handshake-finalization-message" title="Handshake Finalization Message">
<figure><artwork><![CDATA[
struct {
opaque verify_data[verify_data_length];
} Finished;
]]></artwork></figure>
</section>
</section>
<section anchor="the-cipher-suite" title="The Cipher Suite">
<t>The following values define the cipher suite codes used in the ClientHello and
ServerHello messages.</t>
<t>A cipher suite defines a cipher specification supported in TLS Version 1.2.</t>
<t>TLS_NULL_WITH_NULL_NULL is specified and is the initial state of a TLS
connection during the first handshake on that channel, but MUST NOT be
negotiated, as it provides no more protection than an unsecured connection.</t>
<figure><artwork><![CDATA[
CipherSuite TLS_NULL_WITH_NULL_NULL = { 0x00,0x00 };
]]></artwork></figure>
<t>The following cipher suite definitions, defined in {{RFC5288}, are
used for server-authenticated (and optionally client-authenticated)
Diffie-Hellman. DHE denotes ephemeral Diffie-Hellman,
where the Diffie-Hellman parameters are signed by a signature-capable
certificate, which has been signed by the CA. The signing algorithm
used by the server is specified after the DHE component of the
CipherSuite name. The server can request any signature-capable
certificate from the client for client authentication.</t>
<figure><artwork><![CDATA[
CipherSuite TLS_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9C}
CipherSuite TLS_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9D}
CipherSuite TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9E}
CipherSuite TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9F}
CipherSuite TLS_DHE_DSS_WITH_AES_128_GCM_SHA256 = {0x00,0xA2}
CipherSuite TLS_DHE_DSS_WITH_AES_256_GCM_SHA384 = {0x00,0xA3}
]]></artwork></figure>
<t>The following cipher suite definitions, defined in {{RFC5289}, are
used for server-authenticated (and optionally client-authenticated)
Elliptic Curve Diffie-Hellman. ECDHE denotes ephemeral Diffie-Hellman,
where the Diffie-Hellman parameters are signed by a signature-capable
certificate, which has been signed by the CA. The signing algorithm
used by the server is specified after the DHE component of the
CipherSuite name. The server can request any signature-capable
certificate from the client for client authentication.</t>
<figure><artwork><![CDATA[
CipherSuite TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 = {0xC0,0x2B};
CipherSuite TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 = {0xC0,0x2C};
CipherSuite TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 = {0xC0,0x2F};
CipherSuite TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 = {0xC0,0x30};
]]></artwork></figure>
<t>The following ciphers, defined in <xref target="RFC5288"/>,
are used for completely anonymous Diffie-Hellman
communications in which neither party is authenticated. Note that this mode is
vulnerable to man-in-the-middle attacks. Using this mode therefore is of
limited use: These cipher suites MUST NOT be used by TLS 1.2 implementations
unless the application layer has specifically requested to allow anonymous key
exchange. (Anonymous key exchange may sometimes be acceptable, for example, to
support opportunistic encryption when no set-up for authentication is in place,
or when TLS is used as part of more complex security protocols that have other
means to ensure authentication.)</t>
<figure><artwork><![CDATA[
CipherSuite TLS_DH_anon_WITH_AES_128_GCM_SHA256 = {0x00,0xA6}
CipherSuite TLS_DH_anon_WITH_AES_256_GCM_SHA384 = {0x00,0xA7}
]]></artwork></figure>
<t>[[TODO: Add all the defined AEAD ciphers. This currently only lists
GCM. https://github.com/tlswg/tls13-spec/issues/53]]
Note that using non-anonymous key exchange without actually verifying the key
exchange is essentially equivalent to anonymous key exchange, and the same
precautions apply. While non-anonymous key exchange will generally involve a
higher computational and communicational cost than anonymous key exchange, it
may be in the interest of interoperability not to disable non-anonymous key
exchange when the application layer is allowing anonymous key exchange.</t>
<t>The PRFs SHALL be as follows:</t>
<t>o For cipher suites ending with _SHA256, the PRF is the TLS PRF
with SHA-256 as the hash function.</t>
<t>o For cipher suites ending with _SHA384, the PRF is the TLS PRF
with SHA-384 as the hash function.</t>
<t>New cipher suite values are been assigned by IANA as described in
<xref target="iana-considerations"/>.</t>
<t>Note: The cipher suite values { 0x00, 0x1C } and { 0x00, 0x1D } are
reserved to avoid collision with Fortezza-based cipher suites in
SSL 3.</t>
</section>
<section anchor="the-security-parameters" title="The Security Parameters">
<t>These security parameters are determined by the TLS Handshake Protocol and
provided as parameters to the TLS record layer in order to initialize a
connection state. SecurityParameters includes:</t>
<figure><artwork><![CDATA[
enum { null(0), (255) } CompressionMethod;
enum { server, client } ConnectionEnd;
enum { tls_prf_sha256 } PRFAlgorithm;
enum { aes_gcm } RecordProtAlgorithm;
/* Other values may be added to the algorithms specified in
PRFAlgorithm and RecordProtAlgorithm */
struct {
ConnectionEnd entity;
PRFAlgorithm prf_algorithm;
RecordProtAlgorithm record_prot_algorithm;
uint8 enc_key_length;
uint8 block_length;
uint8 fixed_iv_length;
uint8 record_iv_length;
opaque master_secret[48];
opaque client_random[32];
opaque server_random[32];
} SecurityParameters;
]]></artwork></figure>
</section>
<section anchor="changes-to-rfc-4492" title="Changes to RFC 4492">
<t>RFC 4492 <xref target="RFC4492"/> adds Elliptic Curve cipher suites to TLS. This document
changes some of the structures used in that document. This section details the
required changes for implementors of both RFC 4492 and TLS 1.2. Implementors of
TLS 1.2 who are not implementing RFC 4492 do not need to read this section.</t>
<t>This document adds a “signature_algorithm” field to the digitally- signed
element in order to identify the signature and digest algorithms used to create
a signature. This change applies to digital signatures formed using ECDSA as
well, thus allowing ECDSA signatures to be used with digest algorithms other
than SHA-1, provided such use is compatible with the certificate and any
restrictions imposed by future revisions of <xref target="RFC3280"/>.</t>
<t>As described in <xref target="server-certificate"/> and <xref target="client-certificate"/>, the
restrictions on the signature algorithms used to sign certificates are no
longer tied to the cipher suite (when used by the server) or the
ClientCertificateType (when used by the client). Thus, the restrictions on the
algorithm used to sign certificates specified in Sections 2 and 3 of RFC 4492
are also relaxed. As in this document, the restrictions on the keys in the
end-entity certificate remain.</t>
</section>
</section>
<section anchor="glossary" title="Glossary">
<t><list style="hanging">
<t hangText='Advanced Encryption Standard (AES)'><vspace blankLines='0'/>
AES <xref target="AES"/> is a widely used symmetric encryption algorithm. AES is
a block cipher with a 128-, 192-, or 256-bit keys and a 16-byte
block size. TLS currently only supports the 128- and 256-bit key
sizes.</t>
<t hangText='application protocol'><vspace blankLines='0'/>
An application protocol is a protocol that normally layers
directly on top of the transport layer (e.g., TCP/IP). Examples
include HTTP, TELNET, FTP, and SMTP.</t>
<t hangText='asymmetric cipher'><vspace blankLines='0'/>
See public key cryptography.</t>
<t hangText='authenticated encryption with additional data (AEAD)'><vspace blankLines='0'/>
A symmetric encryption algorithm that simultaneously provides
confidentiality and message integrity.</t>
<t hangText='authentication'><vspace blankLines='0'/>
Authentication is the ability of one entity to determine the
identity of another entity.</t>
<t hangText='certificate'><vspace blankLines='0'/>
As part of the X.509 protocol (a.k.a. ISO Authentication
framework), certificates are assigned by a trusted Certificate
Authority and provide a strong binding between a party’s identity
or some other attributes and its public key.</t>
<t hangText='client'><vspace blankLines='0'/>
The application entity that initiates a TLS connection to a
server. This may or may not imply that the client initiated the
underlying transport connection. The primary operational
difference between the server and client is that the server is
generally authenticated, while the client is only optionally
authenticated.</t>
<t hangText='client write key'><vspace blankLines='0'/>
The key used to protect data written by the client.</t>
<t hangText='connection'><vspace blankLines='0'/>
A connection is a transport (in the OSI layering model definition)
that provides a suitable type of service. For TLS, such
connections are peer-to-peer relationships. The connections are
transient. Every connection is associated with one session.</t>
<t hangText='Digital Signature Standard (DSS)'><vspace blankLines='0'/>
A standard for digital signing, including the Digital Signing
Algorithm, approved by the National Institute of Standards and
Technology, defined in NIST FIPS PUB 186-2, “Digital Signature
Standard”, published January 2000 by the U.S. Department of
Commerce <xref target="DSS"/>. A significant update <xref target="DSS-3"/> has been drafted and
was published in March 2006.</t>
<t hangText='digital signatures'><vspace blankLines='0'/>
Digital signatures utilize public key cryptography and one-way
hash functions to produce a signature of the data that can be
authenticated, and is difficult to forge or repudiate.</t>
<t hangText='handshake'><vspace blankLines='0'/>
An initial negotiation between client and server that
establishes the parameters of their transactions.</t>
<t hangText='Initialization Vector (IV)'><vspace blankLines='0'/>
Some AEAD ciphers require an initialization vector to allow
the cipher to safely protect multiple chunks of data with the
same keying material. The size of the IV depends on the cipher
suite.</t>
<t hangText='Message Authentication Code (MAC)'><vspace blankLines='0'/>
A Message Authentication Code is a one-way hash computed from a
message and some secret data. It is difficult to forge without
knowing the secret data. Its purpose is to detect if the message
has been altered.</t>
<t hangText='master secret'><vspace blankLines='0'/>
Secure secret data used for generating keys and IVs.</t>
<t hangText='MD5'><vspace blankLines='0'/>
MD5 <xref target="RFC1321"/> is a hashing function that converts an arbitrarily long
data stream into a hash of fixed size (16 bytes). Due to
significant progress in cryptanalysis, at the time of publication
of this document, MD5 no longer can be considered a ‘secure’
hashing function.</t>
<t hangText='public key cryptography'><vspace blankLines='0'/>
A class of cryptographic techniques employing two-key ciphers.
Messages encrypted with the public key can only be decrypted with
the associated private key. Conversely, messages signed with the
private key can be verified with the public key.</t>
<t hangText='one-way hash function'><vspace blankLines='0'/>
A one-way transformation that converts an arbitrary amount of data
into a fixed-length hash. It is computationally hard to reverse
the transformation or to find collisions. MD5 and SHA are
examples of one-way hash functions.</t>
<t hangText='RSA'><vspace blankLines='0'/>
A very widely used public key algorithm that can be used for
either encryption or digital signing. <xref target="RSA"/></t>
<t hangText='server'><vspace blankLines='0'/>
The server is the application entity that responds to requests for
connections from clients. See also “client”.</t>
<t hangText='session'><vspace blankLines='0'/>
A TLS session is an association between a client and a server.
Sessions are created by the handshake protocol. Sessions define a
set of cryptographic security parameters that can be shared among
multiple connections. Sessions are used to avoid the expensive
negotiation of new security parameters for each connection.</t>
<t hangText='session identifier'><vspace blankLines='0'/>
A session identifier is a value generated by a server that
identifies a particular session.</t>
<t hangText='server write key'><vspace blankLines='0'/>
The key used to protect data written by the server.</t>
<t hangText='SHA'><vspace blankLines='0'/>
The Secure Hash Algorithm <xref target="SHS"/> is defined in FIPS PUB 180-2. It
produces a 20-byte output. Note that all references to SHA
(without a numerical suffix) actually use the modified SHA-1
algorithm.</t>
<t hangText='SHA-256'><vspace blankLines='0'/>
The 256-bit Secure Hash Algorithm is defined in FIPS PUB 180-2.
It produces a 32-byte output.</t>
<t hangText='SSL'><vspace blankLines='0'/>
Netscape’s Secure Socket Layer protocol <xref target="SSL3"/>. TLS is based on
SSL Version 3.0.</t>
<t hangText='Transport Layer Security (TLS)'><vspace blankLines='0'/>
This protocol; also, the Transport Layer Security working group of
the Internet Engineering Task Force (IETF). See “Working Group
Information” at the end of this document (see page 99).</t>
</list></t>
</section>
<section anchor="cipher-suite-definitions" title="Cipher Suite Definitions">
<figure><artwork><![CDATA[
Cipher Suite Key Record
Exchange Protection PRF
TLS_NULL_WITH_NULL_NULL NULL NULL_NULL N/A
TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 DHE_RSA AES_128_GCM SHA256
TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 DHE_RSA AES_256_GCM SHA384
TLS_DHE_DSS_WITH_AES_128_GCM_SHA256 DHE_DSS AES_128_GCM SHA256
TLS_DHE_DSS_WITH_AES_256_GCM_SHA384 DHE_DSS AES_256_GCM SHA384
TLS_DH_anon_WITH_AES_128_GCM_SHA256 DH_anon AES_128_GCM SHA256
TLS_DH_anon_WITH_AES_256_GCM_SHA384 DH_anon AES_128_GCM SHA384
Key Implicit IV Explicit IV
Cipher Material Size Size
------------ -------- ---------- -----------
NULL 0 0 0
AES_128_GCM 16 4 8
AES_256_GCM 32 4 8
]]></artwork></figure>
<t><list style="hanging">
<t hangText='Key Material'><vspace blankLines='0'/>
The number of bytes from the key_block that are used for
generating the write keys.</t>
<t hangText='Implicit IV Size'><vspace blankLines='0'/>
The amount of data to be generated for the per-connection part of the
initialization vector. This is equal to SecurityParameters.fixed_iv_length).</t>
<t hangText='Explicit IV Size'><vspace blankLines='0'/>
The amount of data needed to be generated for the per-record part of the
initialization vector. This is equal to SecurityParameters.record_iv_length).</t>
</list></t>
</section>
<section anchor="implementation-notes" title="Implementation Notes">
<t>The TLS protocol cannot prevent many common security mistakes. This section
provides several recommendations to assist implementors.</t>
<section anchor="random-number-generation-and-seeding" title="Random Number Generation and Seeding">
<t>TLS requires a cryptographically secure pseudorandom number generator (PRNG).
Care must be taken in designing and seeding PRNGs. PRNGs based on secure hash
operations, most notably SHA-1, are acceptable, but cannot provide more
security than the size of the random number generator state.</t>
<t>To estimate the amount of seed material being produced, add the number of bits
of unpredictable information in each seed byte. For example, keystroke timing
values taken from a PC compatible’s 18.2 Hz timer provide 1 or 2 secure bits
each, even though the total size of the counter value is 16 bits or more.
Seeding a 128-bit PRNG would thus require approximately 100 such timer values.</t>
<t><xref target="RFC4086"/> provides guidance on the generation of random values.</t>
</section>
<section anchor="certificates-and-authentication" title="Certificates and Authentication">
<t>Implementations are responsible for verifying the integrity of certificates and
should generally support certificate revocation messages. Certificates should
always be verified to ensure proper signing by a trusted Certificate Authority
(CA). The selection and addition of trusted CAs should be done very carefully.
Users should be able to view information about the certificate and root CA.</t>
</section>
<section anchor="cipher-suites" title="Cipher Suites">
<t>TLS supports a range of key sizes and security levels, including some that
provide no or minimal security. A proper implementation will probably not
support many cipher suites. For instance, anonymous Diffie-Hellman is strongly
discouraged because it cannot prevent man- in-the-middle attacks. Applications
should also enforce minimum and maximum key sizes. For example, certificate
chains containing 512- bit RSA keys or signatures are not appropriate for
high-security applications.</t>
</section>
<section anchor="implementation-pitfalls" title="Implementation Pitfalls">
<t>Implementation experience has shown that certain parts of earlier TLS
specifications are not easy to understand, and have been a source of
interoperability and security problems. Many of these areas have been clarified
in this document, but this appendix contains a short list of the most important
things that require special attention from implementors.</t>
<t>TLS protocol issues:</t>
<t><list style="symbols">
<t>Do you correctly handle handshake messages that are fragmented to
multiple TLS records (see <xref target="fragmentation"/>)? Including corner cases
like a ClientHello that is split to several small fragments? Do
you fragment handshake messages that exceed the maximum fragment
size? In particular, the certificate and certificate request
handshake messages can be large enough to require fragmentation.</t>
<t>Do you ignore the TLS record layer version number in all TLS
records before ServerHello (see <xref target="compatibility"/>)?</t>
<t>Do you handle TLS extensions in ClientHello correctly, including
omitting the extensions field completely?</t>
<t>When the server has requested a client certificate, but no
suitable certificate is available, do you correctly send an empty
Certificate message, instead of omitting the whole message (see
<xref target="client-certificate"/>)?</t>
</list></t>
<t>Cryptographic details:</t>
<t><list style="symbols">
<t>What countermeasures do you use to prevent timing attacks against
RSA signing operations <xref target="TIMING"/>.</t>
<t>When verifying RSA signatures, do you accept both NULL and missing parameters
(see <xref target="cryptographic-attributes"/>)? Do you verify that the RSA padding
doesn’t have additional data after the hash value? <xref target="FI06"/></t>
<t>When using Diffie-Hellman key exchange, do you correctly strip
leading zero bytes from the negotiated key (see <xref target="diffie-hellman"/>)?</t>
<t>Does your TLS client check that the Diffie-Hellman parameters sent
by the server are acceptable (see
<xref target="diffie-hellman-key-exchange-with-authentication"/>)?</t>
<t>Do you use a strong and, most importantly, properly seeded random number
generator (see <xref target="random-number-generation-and-seeding"/>) Diffie-Hellman private values, the
DSA “k” parameter, and other security-critical values?</t>
</list></t>
</section>
</section>
<section anchor="backward-compatibility" title="Backward Compatibility">
<section anchor="compatibility" title="Compatibility with TLS 1.0/1.1 and SSL 3.0">
<t>[[TODO: Revise backward compatibility section for TLS 1.3.
https://github.com/tlswg/tls13-spec/issues/54]]
Since there are various versions of TLS (1.0, 1.1, 1.2, and any future
versions) and SSL (2.0 and 3.0), means are needed to negotiate the specific
protocol version to use. The TLS protocol provides a built-in mechanism for
version negotiation so as not to bother other protocol components with the
complexities of version selection.</t>
<t>TLS versions 1.0, 1.1, and 1.2, and SSL 3.0 are very similar, and use
compatible ClientHello messages; thus, supporting all of them is relatively
easy. Similarly, servers can easily handle clients trying to use future
versions of TLS as long as the ClientHello format remains compatible, and the
client supports the highest protocol version available in the server.</t>
<t>A TLS 1.3 client who wishes to negotiate with such older servers will send a
normal TLS 1.3 ClientHello, containing { 3, 4 } (TLS 1.3) in
ClientHello.client_version. If the server does not support this version, it
will respond with a ServerHello containing an older version number. If the
client agrees to use this version, the negotiation will proceed as appropriate
for the negotiated protocol.</t>
<t>If the version chosen by the server is not supported by the client (or not
acceptable), the client MUST send a “protocol_version” alert message and close
the connection.</t>
<t>If a TLS server receives a ClientHello containing a version number greater than
the highest version supported by the server, it MUST reply according to the
highest version supported by the server.</t>
<t>A TLS server can also receive a ClientHello containing a version number smaller
than the highest supported version. If the server wishes to negotiate with old
clients, it will proceed as appropriate for the highest version supported by
the server that is not greater than ClientHello.client_version. For example, if
the server supports TLS 1.0, 1.1, and 1.2, and client_version is TLS 1.0, the
server will proceed with a TLS 1.0 ServerHello. If server supports (or is
willing to use) only versions greater than client_version, it MUST send a
“protocol_version” alert message and close the connection.</t>
<t>Whenever a client already knows the highest protocol version known to a server
(for example, when resuming a session), it SHOULD initiate the connection in
that native protocol.</t>
<t>Note: some server implementations are known to implement version negotiation
incorrectly. For example, there are buggy TLS 1.0 servers that simply close the
connection when the client offers a version newer than TLS 1.0. Also, it is
known that some servers will refuse the connection if any TLS extensions are
included in ClientHello. Interoperability with such buggy servers is a complex
topic beyond the scope of this document, and may require multiple connection
attempts by the client.</t>
<t>Earlier versions of the TLS specification were not fully clear on what the
record layer version number (TLSPlaintext.version) should contain when sending
ClientHello (i.e., before it is known which version of the protocol will be
employed). Thus, TLS servers compliant with this specification MUST accept any
value {03,XX} as the record layer version number for ClientHello.</t>
<t>TLS clients that wish to negotiate with older servers MAY send any value
{03,XX} as the record layer version number. Typical values would be {03,00},
the lowest version number supported by the client, and the value of
ClientHello.client_version. No single value will guarantee interoperability
with all old servers, but this is a complex topic beyond the scope of this
document.</t>
</section>
<section anchor="compatibility-with-ssl-20" title="Compatibility with SSL 2.0">
<t>TLS 1.2 clients that wish to support SSL 2.0 servers MUST send version 2.0
CLIENT-HELLO messages defined in <xref target="SSL2"/>. The message MUST contain the same
version number as would be used for ordinary ClientHello, and MUST encode the
supported TLS cipher suites in the CIPHER-SPECS-DATA field as described below.</t>
<t>Warning: The ability to send version 2.0 CLIENT-HELLO messages will be phased
out with all due haste, since the newer ClientHello format provides better
mechanisms for moving to newer versions and negotiating extensions. TLS 1.2
clients SHOULD NOT support SSL 2.0.</t>
<t>However, even TLS servers that do not support SSL 2.0 MAY accept version 2.0
CLIENT-HELLO messages. The message is presented below in sufficient detail for
TLS server implementors; the true definition is still assumed to be <xref target="SSL2"/>.</t>
<t>For negotiation purposes, 2.0 CLIENT-HELLO is interpreted the same way as a
ClientHello with a “null” compression method and no extensions. Note that this
message MUST be sent directly on the wire, not wrapped as a TLS record. For the
purposes of calculating Finished and CertificateVerify, the msg_length field is
not considered to be a part of the handshake message.</t>
<figure><artwork><![CDATA[
uint8 V2CipherSpec[3];
struct {
uint16 msg_length;
uint8 msg_type;
Version version;
uint16 cipher_spec_length;
uint16 session_id_length;
uint16 challenge_length;
V2CipherSpec cipher_specs[V2ClientHello.cipher_spec_length];
opaque session_id[V2ClientHello.session_id_length];
opaque challenge[V2ClientHello.challenge_length;
} V2ClientHello;
]]></artwork></figure>
<t><list style="hanging">
<t hangText='msg_length'><vspace blankLines='0'/>
The highest bit MUST be 1; the remaining bits contain the length
of the following data in bytes.</t>
<t hangText='msg_type'><vspace blankLines='0'/>
This field, in conjunction with the version field, identifies a
version 2 ClientHello message. The value MUST be 1.</t>
<t hangText='version'><vspace blankLines='0'/>
Equal to ClientHello.client_version.</t>
<t hangText='cipher_spec_length'><vspace blankLines='0'/>
This field is the total length of the field cipher_specs. It
cannot be zero and MUST be a multiple of the V2CipherSpec length
(3).</t>
<t hangText='session_id_length'><vspace blankLines='0'/>
This field MUST have a value of zero for a client that claims to
support TLS 1.2.</t>
<t hangText='challenge_length'><vspace blankLines='0'/>
The length in bytes of the client’s challenge to the server to
authenticate itself. Historically, permissible values are between
16 and 32 bytes inclusive. When using the SSLv2 backward-
compatible handshake the client SHOULD use a 32-byte challenge.</t>
<t hangText='cipher_specs'><vspace blankLines='0'/>
This is a list of all CipherSpecs the client is willing and able
to use. In addition to the 2.0 cipher specs defined in <xref target="SSL2"/>,
this includes the TLS cipher suites normally sent in
ClientHello.cipher_suites, with each cipher suite prefixed by a
zero byte. For example, the TLS cipher suite {0x00,0x0A} would be
sent as {0x00,0x00,0x0A}.</t>
<t hangText='session_id'><vspace blankLines='0'/>
This field MUST be empty.</t>
<t hangText='challenge'><vspace blankLines='0'/>
Corresponds to ClientHello.random. If the challenge length is
less than 32, the TLS server will pad the data with leading (note:
not trailing) zero bytes to make it 32 bytes long.</t>
</list></t>
<t>Note: Requests to resume a TLS session MUST use a TLS client hello.</t>
</section>
<section anchor="avoiding-man-in-the-middle-version-rollback" title="Avoiding Man-in-the-Middle Version Rollback">
<t>When TLS clients fall back to Version 2.0 compatibility mode, they MUST use
special PKCS#1 block formatting. This is done so that TLS servers will reject
Version 2.0 sessions with TLS-capable clients.</t>
<t>When a client negotiates SSL 2.0 but also supports TLS, it MUST set the
right-hand (least-significant) 8 random bytes of the PKCS padding (not
including the terminal null of the padding) for the RSA encryption of the
ENCRYPTED-KEY-DATA field of the CLIENT-MASTER-KEY to 0x03 (the other padding
bytes are random).</t>
<t>When a TLS-capable server negotiates SSL 2.0 it SHOULD, after decrypting the
ENCRYPTED-KEY-DATA field, check that these 8 padding bytes are 0x03. If they
are not, the server SHOULD generate a random value for SECRET-KEY-DATA, and
continue the handshake (which will eventually fail since the keys will not
match). Note that reporting the error situation to the client could make the
server vulnerable to attacks described in <xref target="BLEI"/>.</t>
</section>
</section>
<section anchor="security-analysis" title="Security Analysis">
<t>The TLS protocol is designed to establish a secure connection between a client
and a server communicating over an insecure channel. This document makes
several traditional assumptions, including that attackers have substantial
computational resources and cannot obtain secret information from sources
outside the protocol. Attackers are assumed to have the ability to capture,
modify, delete, replay, and otherwise tamper with messages sent over the
communication channel. This appendix outlines how TLS has been designed to
resist a variety of attacks.</t>
<section anchor="handshake-protocol-2" title="Handshake Protocol">
<t>The handshake protocol is responsible for selecting a cipher spec and
generating a master secret, which together comprise the primary cryptographic
parameters associated with a secure session. The handshake protocol can also
optionally authenticate parties who have certificates signed by a trusted
certificate authority.</t>
<section anchor="authentication-and-key-exchange" title="Authentication and Key Exchange">
<t>TLS supports three authentication modes: authentication of both parties, server
authentication with an unauthenticated client, and total anonymity. Whenever
the server is authenticated, the channel is secure against man-in-the-middle
attacks, but completely anonymous sessions are inherently vulnerable to such
attacks. Anonymous servers cannot authenticate clients. If the server is
authenticated, its certificate message must provide a valid certificate chain
leading to an acceptable certificate authority. Similarly, authenticated
clients must supply an acceptable certificate to the server. Each party is
responsible for verifying that the other’s certificate is valid and has not
expired or been revoked.</t>
<t>The general goal of the key exchange process is to create a pre_master_secret
known to the communicating parties and not to attackers. The pre_master_secret
will be used to generate the master_secret (see
<xref target="computing-the-master-secret"/>). The master_secret is required to generate the
Finished messages and record protection keys (see <xref target="server-finished"/> and
<xref target="key-calculation"/>). By sending a correct Finished message, parties thus prove
that they know the correct pre_master_secret.</t>
<section anchor="anonymous-key-exchange" title="Anonymous Key Exchange">
<t>Completely anonymous sessions can be established using Diffie-Hellman for key
exchange. The server’s public parameters are contained in the server key
share message, and the client’s are sent in the client key share message.
Eavesdroppers who do not know the private values should not be able to find the
Diffie-Hellman result (i.e., the pre_master_secret).</t>
<t>Warning: Completely anonymous connections only provide protection against
passive eavesdropping. Unless an independent tamper-proof channel is used to
verify that the Finished messages were not replaced by an attacker, server
authentication is required in environments where active man-in-the-middle
attacks are a concern.</t>
</section>
<section anchor="diffie-hellman-key-exchange-with-authentication" title="Diffie-Hellman Key Exchange with Authentication">
<t>When Diffie-Hellman key exchange is used, the client and server use
the client key exchange and server key exchange messages to send
temporary Diffie-Hellman parameters. The signature in the certificate
verify message (if present) covers the entire handshake up to that
point and thus attests the certificate holder’s desire to use the
the ephemeral DHE keys.</t>
<t>Peers SHOULD validate each other’s public key Y (dh_Ys offered by
the server or DH_Yc offered by the client) by ensuring that 1 < Y <
p-1. This simple check ensures that the remote peer is properly
behaved and isn’t forcing the local system into a small subgroup.</t>
<t>Additionally, using a fresh key for each handshake provides Perfect
Forward Secrecy. Implementations SHOULD generate a new X for each
handshake when using DHE cipher suites.</t>
</section>
</section>
<section anchor="version-rollback-attacks" title="Version Rollback Attacks">
<t>Because TLS includes substantial improvements over SSL Version 2.0, attackers
may try to make TLS-capable clients and servers fall back to Version 2.0. This
attack can occur if (and only if) two TLS- capable parties use an SSL 2.0
handshake.</t>
<t>Although the solution using non-random PKCS #1 block type 2 message padding is
inelegant, it provides a reasonably secure way for Version 3.0 servers to
detect the attack. This solution is not secure against attackers who can
brute-force the key and substitute a new ENCRYPTED-KEY-DATA message containing
the same key (but with normal padding) before the application-specified wait
threshold has expired. Altering the padding of the least-significant 8 bytes of
the PKCS padding does not impact security for the size of the signed hashes and
RSA key lengths used in the protocol, since this is essentially equivalent to
increasing the input block size by 8 bytes.</t>
</section>
<section anchor="detecting-attacks-against-the-handshake-protocol" title="Detecting Attacks Against the Handshake Protocol">
<t>An attacker might try to influence the handshake exchange to make the parties
select different encryption algorithms than they would normally choose.</t>
<t>For this attack, an attacker must actively change one or more handshake
messages. If this occurs, the client and server will compute different values
for the handshake message hashes. As a result, the parties will not accept each
others’ Finished messages. Without the master_secret, the attacker cannot
repair the Finished messages, so the attack will be discovered.</t>
</section>
<section anchor="resuming-sessions" title="Resuming Sessions">
<t>When a connection is established by resuming a session, new ClientHello.random
and ServerHello.random values are hashed with the session’s master_secret.
Provided that the master_secret has not been compromised and that the secure
hash operations used to produce the record protection kayes are secure,
the connection should be secure and effectively independent from previous
connections. Attackers cannot use known keys to
compromise the master_secret without breaking the secure hash operations.</t>
<t>Sessions cannot be resumed unless both the client and server agree. If either
party suspects that the session may have been compromised, or that certificates
may have expired or been revoked, it should force a full handshake. An upper
limit of 24 hours is suggested for session ID lifetimes, since an attacker who
obtains a master_secret may be able to impersonate the compromised party until
the corresponding session ID is retired. Applications that may be run in
relatively insecure environments should not write session IDs to stable storage.</t>
</section>
</section>
<section anchor="protecting-application-data" title="Protecting Application Data">
<t>The master_secret is hashed with the ClientHello.random and ServerHello.random
to produce unique record protection secrets for each connection.</t>
<t>Outgoing data is protected using an AEAD algorithm before transmission. The
authentication data includes the sequence number, message type, message length,
and the message contents. The message type field is necessary to ensure that messages
intended for one TLS record layer client are not redirected to another. The
sequence number ensures that attempts to delete or reorder messages will be
detected. Since sequence numbers are 64 bits long, they should never overflow.
Messages from one party cannot be inserted into the other’s output, since they
use independent keys.</t>
</section>
<section anchor="denial-of-service" title="Denial of Service">
<t>TLS is susceptible to a number of denial-of-service (DoS) attacks. In
particular, an attacker who initiates a large number of TCP connections can
cause a server to consume large amounts of CPU doing asymmetric crypto
operations. However, because TLS is generally used over TCP, it is difficult for the
attacker to hide his point of origin if proper TCP SYN randomization is used
<xref target="RFC1948"/> by the TCP stack.</t>
<t>Because TLS runs over TCP, it is also susceptible to a number of DoS attacks on
individual connections. In particular, attackers can forge RSTs, thereby
terminating connections, or forge partial TLS records, thereby causing the
connection to stall. These attacks cannot in general be defended against by a
TCP-using protocol. Implementors or users who are concerned with this class of
attack should use IPsec AH <xref target="RFC4302"/> or ESP <xref target="RFC4303"/>.</t>
</section>
<section anchor="final-notes" title="Final Notes">
<t>For TLS to be able to provide a secure connection, both the client and server
systems, keys, and applications must be secure. In addition, the implementation
must be free of security errors.</t>
<t>The system is only as strong as the weakest key exchange and authentication
algorithm supported, and only trustworthy cryptographic functions should be
used. Short public keys and anonymous servers should be used with great
caution. Implementations and users must be careful when deciding which
certificates and certificate authorities are acceptable; a dishonest
certificate authority can do tremendous damage.</t>
</section>
</section>
<section anchor="working-group-information" title="Working Group Information">
<t>The discussion list for the IETF TLS working group is located at the e-mail
address <eref target="mailto:tls@ietf.org">tls@ietf.org</eref>. Information on the group and information on how to
subscribe to the list is at <eref target="https://www1.ietf.org/mailman/listinfo/tls">https://www1.ietf.org/mailman/listinfo/tls</eref></t>
<t>Archives of the list can be found at:
<eref target="http://www.ietf.org/mail-archive/web/tls/current/index.html">http://www.ietf.org/mail-archive/web/tls/current/index.html</eref></t>
</section>
<section anchor="contributors" title="Contributors">
<figure><artwork><![CDATA[
Christopher Allen (co-editor of TLS 1.0)
Alacrity Ventures
ChristopherA@AlacrityManagement.com
Martin Abadi
University of California, Santa Cruz
abadi@cs.ucsc.edu
Karthikeyan Bhargavan (co-author of [I-D.ietf-tls-session-hash])
INRIA
karthikeyan.bhargavan@inria.fr
Steven M. Bellovin
Columbia University
smb@cs.columbia.edu
Simon Blake-Wilson (co-author of RFC4492)
BCI
sblakewilson@bcisse.com
Nelson Bolyard
Sun Microsystems, Inc.
nelson@bolyard.com (co-author of RFC4492)
Ran Canetti
IBM
canetti@watson.ibm.com
Pete Chown
Skygate Technology Ltd
pc@skygate.co.uk
Antoine Delignat-Lavaud (co-author of [I-D.ietf-tls-session-hash])
INRIA
antoine.delignat-lavaud@inria.fr
Taher Elgamal
taher@securify.com
Securify
Pasi Eronen
pasi.eronen@nokia.com
Nokia
Anil Gangolli
anil@busybuddha.org
Vipul Gupta (co-author of RFC4492)
Sun Microsystems Laboratories
vipul.gupta@sun.com
Kipp Hickman
Chris Hawk (co-author of RFC4492)
Corriente Networks LLC
chris@corriente.net
Alfred Hoenes
David Hopwood
Independent Consultant
david.hopwood@blueyonder.co.uk
Daniel Kahn Gillmor
ACLU
dkg@fifthhorseman.net
Phil Karlton (co-author of SSLv3)
Paul Kocher (co-author of SSLv3)
Cryptography Research
paul@cryptography.com
Hugo Krawczyk
IBM
hugo@ee.technion.ac.il
Adam Langley (co-author of [I-D.ietf-tls-session-hash])
Google
agl@google.com
Ilari Liusvaara
ilari.liusvaara@elisanet.fi
Jan Mikkelsen
Transactionware
janm@transactionware.com
Bodo Moeller (co-author of RFC4492)
Google
bodo@openssl.org
Magnus Nystrom
RSA Security
magnus@rsasecurity.com
Alfredo Pironti (co-author of [I-D.ietf-tls-session-hash])
INRIA
alfredo.pironti@inria.fr
Marsh Ray (co-author of [I-D.ietf-tls-session-hash])
Microsoft
maray@microsoft.com
Robert Relyea
Netscape Communications
relyea@netscape.com
Jim Roskind
Netscape Communications
jar@netscape.com
Michael Sabin
Dan Simon
Microsoft, Inc.
dansimon@microsoft.com
Martin Thomson
Mozilla
mt@mozilla.com
Tom Weinstein
Tim Wright
Vodafone
timothy.wright@vodafone.com
]]></artwork></figure>
</section>
</back>
</rfc>
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