One document matched: draft-ietf-tls-cached-info-15.xml
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<rfc ipr="trust200902" category="std" docName="draft-ietf-tls-cached-info-15.txt">
<front>
<title abbrev="TLS Cached Information Extension"> Transport Layer Security (TLS) Cached Information Extension </title>
<author initials="S." surname="Santesson" fullname="Stefan Santesson">
<organization>3xA Security AB</organization>
<address>
<postal>
<street>Scheelev. 17</street>
<city>Lund</city>
<code>223 70</code>
<country>Sweden</country>
</postal>
<email>sts@aaa-sec.com</email>
</address>
</author>
<author fullname="Hannes Tschofenig" initials="H." surname="Tschofenig">
<organization>Nokia Solutions and Networks</organization>
<address>
<postal>
<street>Linnoitustie 6</street>
<city>Espoo</city>
<code>02600</code>
<country>Finland</country>
</postal>
<phone>+358 (50) 4871445</phone>
<email>Hannes.Tschofenig@gmx.net</email>
<uri>http://www.tschofenig.priv.at</uri>
</address>
</author>
<date year="2013"/>
<area>Security</area>
<workgroup>TLS</workgroup>
<keyword>Internet-Draft</keyword>
<abstract>
<t>Transport Layer Security (TLS) handshakes often include fairly
static information, such as the server
certificate and a list of trusted Certification Authorities (CAs).
This information can be of considerable size, particularly if the server
certificate is bundled with a complete certificate path (including
all intermediary certificates up to the trust anchor public key).</t>
<t>This document defines an extension that omits the exchange of already
available information. The TLS client informs a server of cached information,
for example from a previous TLS handshake, allowing the server to omit
the already available information.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>Transport Layer Security (TLS) handshakes often include fairly
static information, such as the server certificate and a list of
trusted Certification Authorities (CAs). This information can be of
considerable size, particularly if the server certificate is bundled
with a complete certificate path (including all intermediary
certificates up to the trust anchor public key).</t>
<t>Optimizing the exchange of information to a minimum helps to
improve performance in environments where devices are connected to a
network with characteristics like low bandwidth, high latency and high
loss rate. These types of networks exist, for example, when smart objects
are connected using a low power IEEE 802.15.4 radio. For more information
about the challenges with smart object deployments please see
<xref target="RFC6574"/>.</t>
<t>This specification defines a TLS extension that allows a client and a
server to exclude transmission of cached information from the TLS handshake.
</t>
<t>A typical example exchange may therefore look as follows. First, the client and the server executes the usual TLS handshake. The client may, for example, decide to cache the certificate provided by the server. When the TLS client connects to the TLS server some time in the future, without using session resumption, it then attaches the cached_information extension defined in this document to the client hello message to indicate that it had cached the certificate, and it provides the fingerprint of it. If the server's certificate had not changed then the TLS server does not need to send the full certificate to the client again. In case the information had changed, the certificate payload is transmitted to the client to allow the client to update it's state information.</t>
</section>
<!-- ******************************************************************************** -->
<section title="Terminology">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref target="RFC2119"/>.
</t>
</section>
<!-- ******************************************************************************** -->
<section anchor="cached-info" title="Cached Information Extension">
<t>
This document defines a new extension type (cached_information(TBD)), which is used in
client hello and server hello messages. The extension type
is specified as follows.
</t>
<t><figure>
<artwork>
<![CDATA[
enum {
cached_information(TBD), (65535)
} ExtensionType;
]]></artwork>
</figure></t>
<t>
The extension_data field of this extension, when included in the
client hello, MUST contain the CachedInformation structure.
</t>
<t><figure>
<artwork>
<![CDATA[
enum {
certificate_chain(1), trusted_cas(2) (255)
} CachedInformationType;
struct {
CachedInformationType type;
HashAlgorithm hash;
opaque hash_value<1..255>;
} CachedObject;
struct {
CachedObject cached_info<1..2^16-1>;
} CachedInformation;
]]></artwork>
</figure></t>
<t>
When the CachedInformationType identifies a certificate_chain, then
the hash_value field MUST include the hash calculated over the certificate_list
element of the Certificate payload provided by the TLS server in an earlier exchange, excluding the three
length bytes of the certificate_list vector.</t>
<t>When the CachedInformationType identifies a trusted_cas, then the hash_value
MUST include a hash calculated over the certificate_authorities
element of the CertificateRequest payload provided by the TLS server in an earlier exchange, excluding the
two length bytes of the certificate_authorities vector.
</t>
<t>The hash algorithm used to calculate hash values is conveyed in the 'hash' field of the CachedObject element. The list of registered hash algorithms can be found in the TLS HashAlgorithm Registry, which was created by RFC 5246 <xref target="RFC5246"/>. The value zero (0) for 'none' is not an allowed choice for a hash algorithm and MUST NOT be used.</t>
<t>This document establishes a registry for CachedInformationType types and additional values can be added following the policy described in <xref target="IANA"/>.
</t>
</section>
<!-- ******************************************************************************** -->
<section title="Exchange Specification">
<t>Clients supporting this extension MAY include the "cached_information" extension in the
(extended) client hello, which MAY contain zero or more CachedObject attributes.
</t>
<t>
A server supporting this extension MAY include the "cached_information" extension
in the (extended) server hello, which MAY contain one or more CachedObject attributes it supports.
By returning the "cached_information" extension the server indicates that it
supports caching of each present CachedObject that matches the specified hash value.
The server MAY support other cached objects that are not present in the extension.
</t>
<t>Note: If clients make use of the Server Name Indication <xref target="RFC6066"/> then clients may need to cache
multiple data items for a single server since servers may host multiple 'virtual' servers at a single underlying
network address.
</t>
<t>Following a successful exchange of the "cached_information" extensions in the client and server hello,
the server omits sending the corresponding handshake message. How information is omitted from the handshake message is defined per cached info type. <xref target="certificate_chain"/> and <xref target="trusted_cas"/> defines the syntax of the fingerprinted information.</t>
<!--
<t>The handshake protocol MUST proceed using the information as
if it was provided in the handshake protocol. The Finished message
MUST be calculated over the actual data exchanged in the handshake
protocol. That is, the Finished message will not be calculated over the
information that was omitted from transmission.</t>
-->
<t>The handshake protocol MUST proceed using the information as if it was
provided in the handshake protocol. The Finished message MUST be
calculated over the actual data exchanged in the handshake protocol. That
is, the Finished message will be calculated over the information that was
omitted from transmission by means of its present hash in the client hello
and not through its presence in the handshake exchange.</t>
<t>The server MUST NOT include more than one fingerprint for a single information element, i.e., at maximum only one CachedObject structure per replaced information is provided.</t>
<section anchor="certificate_chain" title="Omitting the Certificate Chain">
<t>When an object of type 'certificate_chain' is provided in the client hello, the server MAY replace the sequence of
certificates with an empty sequence with an actual length field of zero (=empty vector).</t>
<t>The original handshake message syntax is defined in RFC 5246 <xref target="RFC5246"/> and has the following
structure:</t>
<t><figure>
<artwork>
<![CDATA[
opaque ASN.1Cert<1..2^24-1>;
struct {
ASN.1Cert certificate_list<0..2^24-1>;
} Certificate;
]]></artwork>
</figure>
</t>
<t>Note that <xref target="I-D.ietf-tls-oob-pubkey"/> allows the certificate payload to contain only the SubjectPublicKeyInfo instead of the full information typically found in a certificate. Hence, when this specification is used in combination with <xref target="I-D.ietf-tls-oob-pubkey"/> and the negotiated certificate type is a raw public key then the TLS server omits sending a Certificate payload that contains an ASN.1Cert structure of the SubjectPublicKeyInfo.</t>
</section>
<section anchor="trusted_cas" title="Omitting the Trusted CAs">
<!--
<t>When a fingerprint for an object of type 'trusted_cas' is provided in the
client hello, the server MAY send a DistinguishedName in the Certificate Request message
with an actual length field of zero (=empty vector).</t>
-->
<t>When a fingerprint for an object of type 'trusted_cas' is provided in the
client hello, the server MAY send a DistinguishedName in the Certificate
Request message with an actual length field of zero (=empty vector).</t>
<t>The original handshake message syntax is defined in RFC 5246 <xref target="RFC5246"/> and has the following
structure:</t>
<t><figure>
<artwork>
<![CDATA[
opaque DistinguishedName<1..2^16-1>;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
SignatureAndHashAlgorithm
supported_signature_algorithms<2^16-1>;
DistinguishedName certificate_authorities<0..2^16-1>;
} CertificateRequest;
]]></artwork>
</figure>
</t>
</section>
</section>
<!-- ******************************************************************************** -->
<section title="Example">
<t><xref target="example"/> illustrates an example exchange using the TLS cached info extension.
In the normal TLS handshake exchange shown in flow (A) the TLS server provides its certificate
in the Certificate payload to the client, see step [1]. This allows the client to store the certificate
for future use. After some time the TLS client again interacts with the same TLS server and makes use of the
TLS cached info extension, as shown in flow (B). The TLS client indicates support for this specification
via the cached_information extension, see [2], and indicates that it has stored the certificate_chain from the earlier
exchange. With [3] the TLS server indicates that it also supports this specification and informs the client that
it also supports caching of other objects beyond the 'certificate_chain', namely 'trusted_cas' (also defined in this document), and the 'foo-bar' extension (i.e., an imaginary extension that yet needs to be defined). With [4] the TLS server omits sending the certificate chain, as described in <xref target="certificate_chain"/>.
</t>
<t><figure anchor="example" title="Example Message Exchange">
<artwork>
<![CDATA[
(A) Initial (full) Exchange
client_hello ->
<- server_hello,
certificate, // [1]
server_key_exchange,
server_hello_done
client_key_exchange,
change_cipher_spec,
finished ->
<- change_cipher_spec,
finished
Application Data <-------> Application Data
(B) TLS Cached Extension Usage
client_hello,
cached_information=(certificate_chain) -> // [2]
<- server_hello,
cached_information= // [3]
(certificate_chain, trusted_cas, foo-bar)
certificate, // [4]
server_key_exchange,
server_hello_done
client_key_exchange,
change_cipher_spec,
finished ->
<- change_cipher_spec,
finished
Application Data <-------> Application Data
]]></artwork>
</figure></t>
</section>
<!-- ******************************************************************************** -->
<section title="Security Considerations">
<t>This specification defines a mechanism to reference stored state using a fingerprint.
Sending a fingerprint of cached information in an
unencrypted handshake, as the client and server hello is, may allow an attacker or observer to
correlate independent TLS exchanges. While some information elements used in this specification,
such as server certificates, are public objects and usually not
sensitive in this regard, others may be. Those who implement and deploy this specification should therefore
make an informed decision whether the cached information is inline with their security and privacy goals.
In case of concerns, it is advised to avoid sending the fingerprint of the data objects in clear.</t>
<t>
The hash algorithm used in this specification is required to have
reasonable random properties in order to provide reasonably unique
identifiers. There is no requirement that this
hash algorithm must have strong collision resistance. <!-- However, since
the hash algorithm is used to represent data in the finished
calculation, the security properties of the finished calculation will
change if a weaker hash algorithm is used to represent cached
information compared with the hash algorithm used to calculate the
finished message.--> </t>
</section>
<!-- ******************************************************************************** -->
<section anchor="IANA" title="IANA Considerations">
<section title="New Entry to the TLS ExtensionType Registry">
<t>IANA is requested to add an entry to the existing TLS ExtensionType registry,
defined in RFC 5246 <xref target="RFC5246"/>, for cached_information(TBD) defined in this document.</t>
</section>
<section title="New Registry for CachedInformationType">
<t>IANA is requested to establish a registry for TLS CachedInformationType values. The
first entries in the registry are <list style="symbols">
<t>certificate_chain(1)</t>
<t>trusted_cas(2)</t>
</list>
</t>
<t>The policy for adding new values to this registry, following the terminology defined in RFC 5226 <xref target="RFC5226"/>, is as follows:
<list style="symbols">
<t>0-63 (decimal): Standards Action</t>
<t>64-223 (decimal): Specification Required</t>
<t>224-255 (decimal): reserved for Private Use</t>
</list>
</t>
</section>
</section>
<!-- ******************************************************************************** -->
<section title="Acknowledgments">
<t>We would like to thank the following persons for your detailed document reviews:
<list style="symbols">
<t>Paul Wouters and Nikos Mavrogiannopoulos (December 2011)</t>
<t>Rob Stradling (February 2012)</t>
<t>Ondrej Mikle (in March 2012)</t>
</list>
</t>
<t>Additionally, we would like to thank the TLS working group chairs, Eric Rescorla and Joe Salowey, as well as the security area directors, Sean Turner and Stephen Farrell, for their feedback and support. </t>
</section>
<!-- ******************************************************************************** -->
</middle>
<back>
<references title="Normative References"> &RFC2119; &RFC5246; &RFC6066;
<!--
<reference anchor="SHA">
<front>
<title>Federal Information Processing Standards Publication (FIPS PUB) 180-3, Secure Hash Standard (SHS)</title>
<author initials="" surname="" fullname="NIST">
<organization/>
</author>
<date month="October" year="2008"/>
</front>
</reference>
-->
&RFC3874;
<!-- &RFC1321; -->
</references>
<references title="Informative References"> &RFC5226; &RFC6574; &I-D.ietf-tls-oob-pubkey; </references>
</back>
</rfc>
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