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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

TLS                                                         S. Santesson
Internet-Draft                                           3xA Security AB
Intended status: Standards Track                           H. Tschofenig
Expires: June 16, 2012                            Nokia Siemens Networks
                                                       December 14, 2011


      Transport Layer Security (TLS) Cached Information Extension
                   draft-ietf-tls-cached-info-10.txt

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 handshakes, allowing
   the server to omit sending cached static information to the client
   during the TLS handshake protocol exchange.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 16, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as



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   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Cached Information Extension . . . . . . . . . . . . . . . . .  5
   4.  Extension Exchange . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Cached Information . . . . . . . . . . . . . . . . . . . .  7
     4.2.  Reconnaissance . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Cached Information Substitution  . . . . . . . . . . . . . . .  8
     5.1.  Substitution Syntax for certificate_chain  . . . . . . . .  8
     5.2.  Substitution Syntax for trusted_cas  . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14






























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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
   handshake.

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

































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2.  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 [RFC2119].














































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3.  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 {
              CachedInformationType type;
              HashAlgorithm hash;
              opaque hash_value<1..255>;
         } CachedObject;

         struct {
              CachedObject cached_info<1..2^16-1>;
         } CachedInformation;

   When CachedInformationType identifies certificate_chain, then
   hash_value 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.

   When CachedInformationType identifies trusted_cas, then 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.

   The hash algorithm used to calculate hash values SHALL be the hash
   algorithm that was used to generate the Finished message in the
   handshake exchange from which the hashed information was cached.
   Hash algorithm identifiers are defined in the RFC 5246 [RFC5246]
   HashAlgorithm registry.



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   Other specifications MAY define more CachedInformationType types.


















































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4.  Extension Exchange

4.1.  Cached Information

   Clients MAY include a "cached_information" extension in the
   (extended) client hello, which MAY contain zero or more cached
   objects (CachedObject).

   Servers that receive an extended client hello containing a
   "cached_information" extension MAY indicate that they support cached
   information objects by including a cached_information extension in
   their (extended) server hello.

   A cached_information extension provided in the server hello has the
   following semantics:

   o  An empty cached_information extension indicates that the server
      supports information caching but provides no information about
      what information types it supports.

   o  A non-empty cached information extension indicates that the server
      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.

   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
   [I-D.ietf-tls-rfc4366-bis] value.

4.2.  Reconnaissance

   A client MAY include an empty cached_information extension (with
   empty extension_data field) in its (extended) client hello to query
   whether the server supports cached information.

   Upon receiving an empty cached_information extension, a server MAY
   indicate that it supports cached information in handshakes by
   including a cached_information extension in its (extended) server
   hello according to any of the available options in Section 4.1.











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5.  Cached Information Substitution

   Following a successful exchange of "cached_information" extensions,
   the server MAY substitute cached information in the handshake
   exchange with a matching CachedObject from the client hello
   "cached_information" extension.

   A substitution syntax that defines how the CachedObject structure is
   carried in the handshake message MUST be defined for each
   CachedInformationType in a way that does not violate the syntax of
   the handshake message.  The substitution syntax for
   certificate_chain(1) and trusted_cas(2) is provided below.

   The handshake protocol SHALL proceed using the cached information as
   if it was provided in the handshake protocol.  The Finished message
   SHALL 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.

   The server MUST NOT include more than one CachedObject as
   substitution for the cached information.

5.1.  Substitution Syntax for certificate_chain

   When an object of type certificate_chain is provided in the client
   hello, the server MAY substitute the cached information with a
   matching hash value received from the client by expanding the
   Certificate handshake message as follows.

   Original handshake message syntax defined in RFC 5246 [RFC5246]:



         opaque ASN.1Cert<1..2^24-1>;

         struct {
             ASN.1Cert certificate_list<0..2^24-1>;
         } Certificate;

   Substitution syntax is defined by expanding the syntax of the opaque
   ASN.1Cert structure:



         CachedObject ASN.1Cert<1..2^24-1>;





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5.2.  Substitution Syntax for trusted_cas

   When a hash for an object of type trusted_cas is provided in the
   client hello, the server MAY substitute the cached information with a
   matching hash value received from the client by expanding the
   CertificateRequest handshake message as follows.

   Original handshake message syntax defined in RFC 5246 [RFC5246]:



         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

   The substitution syntax is defined by expanding the syntax of the
   opaque DistinguishedName structure:



         CachedObject DistinguishedName<1..2^16-1>;

























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6.  Security Considerations

   The hash algorithm used in this specification is required to have
   reasonable random properties in order to provide reasonably unique
   identifiers.  There is no clearly identified 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.

   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.




























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7.  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.






































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8.  Acknowledgments

   The author acknowledges input from many members of the TLS working
   group.















































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9.  References

9.1.  Normative References

   [I-D.ietf-tls-rfc4366-bis]
              3rd, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", draft-ietf-tls-rfc4366-bis-12
              (work in progress), September 2010.

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

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

9.2.  Informative References

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.































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Authors' Addresses

   Stefan Santesson
   3xA Security AB
   Scheelev. 17
   Lund  223 70
   Sweden

   Email: sts@aaa-sec.com


   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600
   Finland

   Phone: +358 (50) 4871445
   Email: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at































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