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Versions: (draft-santesson-tls-certcache) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 RFC 7924

INTERNET-DRAFT                              S. Santesson (3xA Security)
Intended Status: Proposed Standard               Q. Dang         (NIST)
Expires January 2010                                          June 2009


      Transport Layer Security (TLS) Cached Information Extension
                  <draft-ietf-tls-cached-info-00.txt>


Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Drafts.

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   The list of current Internet-Drafts can be accessed at
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   The list of Internet-Draft Shadow Directories can be accessed at
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Abstract

   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.


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



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   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
   handshake.

   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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].


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 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;




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      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
   message.

   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.

   Servers that receive an extended client hello containing a
   "cached_information" extension, MAY indicate that they support one or
   more of the cached information objects by including an extension of
   type "cached_information" in the (extended) server hello, which SHALL
   contain at least one CachedObject received from the client. The
   CachedObject's returned by the server MUST include the types the
   server supports and has accepted to replace with a hash of the cached
   data.

   After negotiation of the use of cached certificates has been
   successfully completed (by exchanging hello messages including
   "cached_certs" extensions), the server MUST replace agreed cached
   information objects in its handshake messages with a corresponding
   hash_value from CachedInformationHash that was included in the
   cached_information extension of the server hello message.

   The handshake protocol will proceed using the cached data as if it
   they were 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.




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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.


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










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Authors' Addresses


   Stefan Santesson

   3xA Security AB
   Bjornstorp 744
   247 98 Genarp
   Sweden

   EMail: sts@aaa-sec.com


   Quynh Dang

   NIST
   100 Bureau Drive, Stop 8930
   Gaithersburg, MD 20899-8930
   USA

   Email: quynh.dang@nist.gov


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Expires January 2009




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