INTERNET-DRAFT                               S. Santesson (3xA Security)
Intended Status: Proposed Standard
Expires: October 22, 2010                                 April 20, January 13, 2011                                  July 12, 2010

      Transport Layer Security (TLS) Cached Information Extension
                  <draft-ietf-tls-cached-info-08.txt>
                  <draft-ietf-tls-cached-info-09.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 sessions, 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 to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/1id-abstracts.html

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html

Copyright and License Notice

   Copyright (c) 2010 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
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
      1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Cached Information Extension  . . . . . . . . . . . . . . . . . 4
   3.  Extension Exchange  . . . . . . . . . . . . . . . . . . . . . . 5
      3.1. Reconnaissance  . . Cached Information  . . . . . . . . . . . . . . . . . . . . 5
      3.2. Cached Information Reconnaissance  . . . . . . . . . . . . . . . . . . . . . . 5
   4.  Data  Cached Information Substitution . . . . . . . . . . . . . . . . . . . . . . . 6
      4.1.  Data  Substitution Syntax for certificate_chain  . . . . . . . . 6
      4.2.  Data  Substitution Syntax for trusted_cas  . . . . . . . . . 7 . . 6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 8
   8.  Normative References  . . . . . . . . . . . . . . . . . . . . . 9
   Annex A - 64 bit FNV-1a digest  . . . . . . . . . . . . . . . . .  10
      A.1. Definition (Normative)  . . . . . . . . . . . . . . . . .  10
      A.2  Java code sample (Informative)  . . . . . . . . . . . . .  11
      A.3. C code sample (Informative) . . . . . . . . . . . . . . .  12
      A.4. Digest samples (Informative)  . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14  10

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

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

      struct {
           CachedInformationType type;
           HashAlgorithm hash;
           opaque digest_value<0..8>; hash_value<1..255>;
      } CachedObject;

      struct {
           CachedObject cached_info<1..2048>; cached_info<1..2^16-1>;
      } CachedInformation;

   The digest_value of a CachedObject MUST either be empty (0 bytes) or
   contain a 64 bit FNV digest (8 bytes) as specified in Annex A. The 64
   bit integer is represented as an 8 byte digest_value in big-endian
   order (with most significant bits in the first byte and least
   significant bits in the last byte).

   When CachedInformationType identifies certificate_chain, then
   digest_value
   hash_value MUST include a digest 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 digest_value hash_value
   MUST include a digest 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.

   Other specifications MAY define more CachedInformationType types.

3.  Extension Exchange

3.1. Reconnaissance

   A client Cached Information

   Clients MAY include an empty cached_information a "cached_information" extension (with
   empty extension_data field) in its the
   (extended) client hello to query
   whether the server supports hello, which MAY contain zero or more cached information.

   A server indicates
   objects (CachedObject).

   Servers that it supports receive an extended client hello containing a
   "cached_information" extension MAY indicate that they support cached
   information in handshakes
   according to section 3.2. objects by including a cached_information extension in its
   their (extended) server hello.

3.2. Cached Information

   Clients MAY specify cached information from previous handshakes by
   including a "cached_information"

   A cached_information extension provided in the (extended) client
   hello, which contains at least one 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 object (CachedObject) for information extension indicates that the
       server supports caching of each present object type (CachedInformationType), as CachedObject that matches
       the specified hash value. The server MAY support other cached
       objects that are not present in
   section 2. the extension.

   Note: Clients MAY 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. Clients sending a non-empty cached_information
   extension MUST provide a 64 bit (8 byte) digest_value for each cached
   object.

   Servers that receive an extended

3.2. Reconnaissance

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

   A cached_information extension provided in the server client hello has to query
   whether the
   following semantics:

     o An server supports cached information.

   Upon receiving an empty cached_information extension indicates that the extension, a server
       supports information caching but provides no information about
       what information types MAY
   indicate that it supports.

     o A non-empty supports cached information extension indicates that the
       server supports only those CachedInformationType types that are
       identified in handshakes by each present CachedObject.

     o A CachedObject with an empty digest_value indicates that the
   including a cached_information extension in its (extended) server supports caching of the specified object type
       (CachedInformationType), but does not specify
   hello according to any digest values
       it will accept.

     o A present non-empty digest_value indicates that the server will
       honor caching of objects of the specified type that matches the
       present digest value. available options in section 3.1.

4.  Data  Cached Information Substitution

   Following a successful exchange of "cached_information" extensions,
   the server may MAY substitute data objects cached information in the handshake
   exchange with a matching digest_value representing a matching object type. received CachedObject from the client hello
   "cached_information" extension.

   A substitution syntax that defines how the CachedObject structure is
   carried in its client hello. 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 will SHALL proceed using the cached data information as
   if it was provided in the handshake protocol. The Finished message will
   however
   SHALL be calculated over the actual data exchanged in the handshake
   protocol. That is, the Finished message will be calculated over the
   digest
   hash values of cached information objects and not over the cached
   objects
   information that were omitted from transmission.

   Each CachedInformationType MUST specify how actual data is replaced
   by a digest in a way that does not violate the defined syntax of
   existing handshake messages. the data exchange syntax for
   certificate_chain(1) and trusted_cas(2) are provided below.

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

4.1.  Data  Substitution Syntax for certificate_chain

   When a digest for an object of type certificate_chain is provided in the client
   hello, the server MAY substitute the cached data information with a
   matching digest 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 definition syntax of the opaque
   ASN.1Cert structure:

      DigestInfo

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

      struct {
          opaque digest_value<0..8>;
      } DigestInfo;

4.2.  Data  Substitution Syntax for trusted_cas

   When a digest hash for an object of type trusted_cas is provided in the
   client hello, the server MAY substitute the cached data information with a
   matching digest 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 definition syntax of the
   opaque DistinguishedName structure:

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

      struct {
          opaque digest_value<0..8>;
      } DigestInfo;

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

5.  Security Considerations

   The digest 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 digest
   hash algorithm must have strong collision resistance. A non unique digest may at most
   lead 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 failed TLS 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 followed by (such as a new attempt without the renegotiated
   handshake) and sending a hash of that cached information extension. There 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 no identified security
   threats that require the selected digest algorithm public objects and usually not
   sensitive in this regard, but implementers should be aware if any
   cached information are subject to have strong
   collision resistance. such security concerns and in such
   case SHOULD NOT send a hash over encrypted data in en unencrypted
   handshake.

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.  Acknowledgements

   The author acknowledge acknowledges input from many members of the TLS working
   group, Martin Rex for extensive review and input, Marsh Ray and Simon
   Josefsson for coding and test vectors.
   group.

8.  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
         process.

Annex A - 64 bit FNV-1a digest

A.1. Definition (Normative)

   FNV-1 digest algorithm is a non-cryptographic hash function created
   by Glenn Fowler, Landon Curt Noll, and Phong Vo. The FNV digest
   algorithms and sample FNV source code have been released into the
   public domain. FNV-1 has two defined variants, FNV-1 and FNV-1a. The
   algorithm specified in this annex specifies the FNV-1a variant.

   The FNV-1a digest is generated as follows:

      digest = FNV_offset_basis
      for each octet_of_data to be digested {
             digest = digest XOR octet_of_data
             digest = digest * FNV_prime }
      return digest

   In the above pseudocode, all variables are unsigned integers. All
   variables, except for octet_of_data, have the same number of bits as
   the FNV digest (64 Bits). The variable, octet_of_data, is an 8 bit
   unsigned integer. Specifically for a 64 bit FNV-1a digest the
   following applies:

     o All variables, except for octet_of_data, are 64-bit unsigned
       integers.

     o The variable, octet_of_data, is an 8 bit unsigned integer.

     o The FNV_offset_basis is the 64-bit FNV offset basis value:
       14695981039346656037.

     o The FNV_prime is the 64-bit FNV prime value: 1099511628211.

     o The multiply function (indicated by the '*' symbol) returns the
       lower 64-bits of the product.

     o The XOR is an 8-bit operation that modifies only the lower 8-bits
       of the digest value.

     o The digest value returned is an 64-bit unsigned integer.

A.2  Java code sample (Informative)

   /**
    * Java code sample, implementing 64 bit FNV-1a
    * By Stefan Santesson
    */

   import java.math.BigInteger;

   public class FNV {

       static public BigInteger getFNV1a64Digest (String inpString) {

           BigInteger m = new BigInteger("2").pow(64);
           BigInteger fnvPrime = new BigInteger("1099511628211");
           BigInteger fnvOffsetBasis = new BigInteger
                                       ("14695981039346656037");

           BigInteger digest = fnvOffsetBasis;

           for (int i = 0; i < inpString.length(); i++) {
               digest = digest.xor(BigInteger.valueOf(
                        (int) inpString.charAt(i)));
               digest = digest.multiply(fnvPrime).mod(m);
           }

           return (digest);

       }
   }

A.3. C code sample (Informative)

   fnv1a64.h:

   #ifndef FNV1A64_H
   #define FNV1A64_H

   #include <string.h> /* For size_t */
   #include <stdint.h> /* For uint64_t */

   extern uint64_t fnv1a64 (const uint8_t *buffer, size_t len);

   #endif

   fnv1a64.c:

   /* fnv1a.c -- Implementation of the FNV-1A non-cryptographic
    * hash function.
    * By Simon Josefsson <simon@josefsson.org> on 2010-03-30.
    */

   #include "fnv1a64.h"

   #define FNV1A64_OFFSET_BASIS 14695981039346656037ULL
   #define FNV1A64_PRIME 1099511628211ULL

   uint64_t
   fnv1a64 (const uint8_t *buffer, size_t len)
   {
     uint64_t hash;
     size_t i;

     hash = FNV1A64_OFFSET_BASIS;
     for (i = 0; i < len; i++)
       {
         hash = hash ^ buffer[i];
         hash = hash * FNV1A64_PRIME;
       }

     return hash;
   }

A.4. Digest samples (Informative)

   Digest samples for 64 bit FNV-1a

   For input data:
      null ("")
      0 bytes

   Digest is: CB F2 9C E4 84 22 23 25

   For input data:
      hex: 61 ("a")
      1 byte

   Digest is: AF 63 DC 4C 86 01 EC 8C

   For input data:
      hex: FF 00 00 01
      4 bytes

   Digest is: 69 61 19 64 91 CC 68 2D

   For input data:
      hex: 68 74 74 70 3A 2F 2F 65 6E 2E 77 69 6B 69 70 65
           64 69 61 2E 6F 72 67 2F 77 69 6B 69 2F 46 6F 77
           6C 65 72 5F 4E 6F 6C 6C 5F 56 6F 5F 68 61 73 68
           ("http://en.wikipedia.org/wiki/Fowler_Noll_Vo_hash")
      48 bytes

   Digest is: D9 B9 57 FB 7F E7 94 C5

Authors' Addresses

   Stefan Santesson

   3xA Security AB
   Bjornstorp 744
   247 98 Genarp
   Scheelev. 17
   223 70 Sweden

   EMail: sts@aaa-sec.com