INTERNET-DRAFT                               S. Santesson (3xA Security)
Intended Status: Proposed Standard
Expires: March 19, 2010                               September 15, 2009

      Transport Layer Security (TLS) Cached Information Extension

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   This document defines a Transport Layer Security (TLS) extension for
   cached information. This extension allows the TLS client to inform a
   server of cached information from previous TLS sessions, allowing the
   server to omit sending cached static information to the client during
   the TLS handshake protocol exchange.

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Table of Contents

   1  Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2  Cached Information Extension . . . . . . . . . . . . . . . . . . 3
   4  Message flow . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   5  Security Considerations  . . . . . . . . . . . . . . . . . . . . 4
   6  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . . 5
   7  Normative References . . . . . . . . . . . . . . . . . . . . . . 5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 6

1  Introduction

   TLS handshakes often include fairly static information such as server
   certificate and a list of trusted Certification Authorities (CAs).
   Static information such as a server certificate can be of
   considerable size. This is the case in particular if the server
   certificate is bundled with a complete certificate path, including
   all intermediary certificates up to the trust anchor public key.

   Significant benefits can be achieved in low bandwidth and high
   latency networks, in particular if the communication channel also has
   a relatively high rate of transmission errors, if a known and
   previously cached server certificate path can be omitted from the TLS

   This specification defines the Cached Information TLS extension,
   which may be used by a client and a server to exclude transmission of
   known cached parameters from the TLS handshake.

1.1  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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2  Cached Information Extension

   A new extension type (cached_information(TBD)) is defined and used in
   both the client hello and server hello messages. The extension type
   is specified as follows.

      enum {
           cached_information(TBD), (65535)
      } ExtensionType;

   The "extension_data" field of this extension, when included in the
   client hello, SHALL contain "CachedInformation" according to the
   following structure:

      enum {
           certificate_chain(1), trusted_cas(2), (255)
      } CachedInformationType;

      struct {
           HashAlgorithm hash;
           opaque hash_value<1..255>;
      } CachedInformationHash;

      struct {
           CachedInformationType type;
           CachedInformationHash hashes<1..2^16-1>;
      } CachedObject;

      struct {
           CachedObject cached_info<1..2^24-1>;;
      } CachedInformation;

   Hash algorithm identifiers are provided by the RFC 5246 [RFC5246]
   HashAlgorithm registry. Compliant implementations MUST support
   sha1(2) as HashAlgorithm.

   When CachedInformationType identifies certificate_chain, then
   hash_value MUST include at least one hash value calculated over the
   certificate_list element of a server side Certificate message.

   When CachedInformationType identifies trusted_cas, then hash_value
   MUST include at least one hash value calculated over the
   certificate_authorities element of a server side CertificateRequest

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   The client MUST NOT include hashes for multiple objects in the same
   CachedObject structure. If more than one hash is present in the
   CachedObject structure, they MUST be hashes over the same information
   object using different hash algorithms.

   Other specifications MAY define more CachedInformationType types.

4  Message flow

   Clients MAY include an extension of type "cached_information" in the
   (extended) client hello, which SHALL contain at least one
   CachedObject as specified in section 2. 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 [RFC4366] value.

   Servers that receive an extended client hello containing a
   "cached_information" extension, MAY indicate that they support
   caching of information objects by including an extension of type
   "cached_information" with an empty extension_data field in their
   (extended) server hello.

   Following a successful exchange of "cached_information" extensions,
   the server may replace data objects identified through the client
   extension with any of the CachedInformationHash values received from
   the client, which matches the replaced object.

   The handshake protocol will proceed using the cached data as if it
   was provided in the handshake protocol. The Finished message will
   however 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
   objects that were omitted from transmission.

5  Security Considerations

   Hash algorithms used in this specification are required to have
   reasonable random properties in order to provide reasonably unique
   identifiers. Failure of a provided hash to correctly and uniquely
   identify the correct set of hashed parameters may at most lead to a
   failed TLS handshake followed by a new attempt without the cached
   information extension. No serious security threat requires selected
   hash algorithms to have strong collision resistance.

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6  IANA Considerations

   1) Create an entry, cached_information(TBD), in the existing registry
      for ExtensionType (defined in RFC 5246 [RFC5246]).

   2) Establish a registry for TLS CachedInformationType values.  The
      first entries in the registry are certificate_chain(1) and
      trusted_cas(2). TLS CachedInformationType values in the inclusive
      range 0-63 (decimal) are assigned via RFC 5226 [RFC5226] Standards
      Action.  Values from the inclusive range 64-223 (decimal) are
      assigned via RFC 5226 Specification Required.  Values from the
      inclusive range 224-255 (decimal) are reserved for RFC 5226
      Private Use.

7  Normative References

   [RFC2119]   S. Bradner, "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997

   [RFC5226]   T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
               Considerations Section in RFCs", RFC 5226, May 2008

   [RFC5246]   T. Dierks, E. Rescorla, "The Transport Layer Security
               (TLS) Protocol Version 1.2", RFC 5246, August 2008

   [RFC4366]   S. Blake-Wilson, M. Nystrom, D. Hopwood, J. Mikkelsen, T.
               Wright, "Transport Layer Security (TLS) Extensions", RFC
               4366, April 2006

   NOTE: RFC 4366 will be updated by RFC4366bis, currently in IESG

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Authors' Addresses

   Stefan Santesson

   3xA Security AB
   Bjornstorp 744
   247 98 Genarp


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