One document matched: draft-ietf-tls-cached-info-11.xml
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<rfc ipr="trust200902" category="std" docName="draft-ietf-tls-cached-info-11.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 Siemens 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="2011"/>
<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="I-D.iab-smart-object-workshop"/>.</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>
</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 a hash calculated over the certificate_list
element of a server side Certificate message, 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 a server side CertificateRequest message, 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. This document defines the following hash algorithms:
<list style="symbols">
<t>SHA-1: NIST FIPS PUB 180-3 <xref target="SHA"/></t>
<t>SHA-224: RFC 3874 <xref target="RFC3874"/></t>
<t>SHA-256: NIST FIPS PUB 180-3 <xref target="SHA"/></t>
<t>SHA-384: NIST FIPS PUB 180-3 <xref target="SHA"/></t>
<t>SHA-512: NIST FIPS PUB 180-3 <xref target="SHA"/></t>
<!-- <t>MD5: RFC 1321 <xref target="RFC1321"/></t> -->
</list>
</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>
Server supporting this extension MAY include the "cached_information" extension
in the (extended) server hello, which MAY contain one or more CachedObject attributes.
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: Clients may need the ability to cache different values
depending on other information in the Client Hello that modify what
values the server uses, in particular the Server Name Indication
<xref target="I-D.ietf-tls-rfc4366-bis"/> value.
</t>
<t>Following a successful exchange of "cached_information" extensions,
the server MAY send fingerprints of the cached information in the handshake
exchange as a replacement for the exchange of the full data.
<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 be calculated over the
hash values of cached information objects and not over the cached
information that were omitted from transmission.</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="Fingerprint of the Certificate Chain">
<t>When an object of type 'certificate_chain' is provided in the client
hello, the server MAY send a fingerprint instead of the complete certificate chain as shown below.</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>By using the extension defined in this document the following information is sent:</t>
<t><figure>
<artwork>
<![CDATA[
struct {
CachedObject ASN.1Cert<1..2^24-1>;
} Certificate;
]]></artwork>
</figure></t>
<t>The opaque ASN.1Cert structure is replaced with the CachedObject structure defined in this document.</t>
<t>Note: <xref target="I-D.wouters-tls-oob-pubkey"/> allows a PKIX
certificate containing 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.wouters-tls-oob-pubkey"/>
and the negotiated certificate type is RawPublicKey then the TLS server sends the hashed
Certificate element that contains a ASN.1Cert with the mentioned raw public key.</t>
</section>
<section anchor="trusted_cas" title="Fingerprint for Trusted CAs">
<t>When a hash for an object of type 'trusted_cas' is provided in the
client hello, the server MAY send a fingerprint instead of the complete certificate authorities information as shown below.</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>
<t>By using the extension defined in this document the following information is sent:</t>
<t><figure>
<artwork>
<![CDATA[
struct {
ClientCertificateType certificate_types<1..2^8-1>;
SignatureAndHashAlgorithm
supported_signature_algorithms<2^16-1>;
CachedObject DistinguishedName<1..2^16-1>;
} CertificateRequest;
]]></artwork>
</figure></t>
<t>The opaque DistinguishedName structure is replaced with the CachedObject structure defined in this document.</t>
</section>
</section>
<!-- ******************************************************************************** -->
<section title="Security Considerations">
<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>
<t>Caching information in an encrypted handshake (such as a renegotiated
handshake) and sending a hash of that cached information in an
unencrypted handshake might introduce integrity or data disclosure
issues as it enables an attacker to identify if a known object (such
as a known server certificate) has been used in previous encrypted
handshakes. Information object types defined in this specification,
such as server certificates, are public objects and usually not
sensitive in this regard, but implementers should be aware if any
cached information are subject to such security concerns and in such
case SHOULD NOT send a hash over encrypted data in en unencrypted
handshake.</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 title="New Registry for HashAlgorithm">
<t>IANA is requested to establish a registry for HashAlgorithm values and to populate
the registry with an initial set of values listed in <xref target="cached-info"/>.
</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>The author acknowledges input from many members of the TLS working
group.</t>
<t>We would like to thank Paul Wouters for his feedback and Nikos Mavrogiannopoulos for his document review in December 2011.</t>
</section>
<!-- ******************************************************************************** -->
</middle>
<back>
<references title="Normative References"> &RFC2119; &RFC5246; &I-D.ietf-tls-rfc4366-bis;
<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; &I-D.iab-smart-object-workshop; &I-D.wouters-tls-oob-pubkey; </references>
</back>
</rfc>
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