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TLS                                                         S. Santesson
Internet-Draft                                           3xA Security AB
Intended status: Standards Track                           H. Tschofenig
Expires: June 28, 2012                            Nokia Siemens Networks
                                                       December 26, 2011


      Transport Layer Security (TLS) Cached Information Extension
                   draft-ietf-tls-cached-info-11.txt

Abstract

   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).

   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.

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 28, 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



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   (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
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Cached Information Extension . . . . . . . . . . . . . . . . .  5
   4.  Exchange Specification . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Fingerprint of the Certificate Chain . . . . . . . . . . .  7
     4.2.  Fingerprint for Trusted CAs  . . . . . . . . . . . . . . .  8
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
     6.1.  New Entry to the TLS ExtensionType Registry  . . . . . . . 11
     6.2.  New Registry for CachedInformationType . . . . . . . . . . 11
     6.3.  New Registry for HashAlgorithm . . . . . . . . . . . . . . 11
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
























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1.  Introduction

   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).

   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 [I-D.iab-smart-object-workshop].

   This specification defines a TLS extension that allows 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", "MUST", "MUST 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

   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.



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

   The extension_data field of this extension, when included in the
   client hello, MUST contain the CachedInformation 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 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.

   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.

   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:

   o  SHA-1: NIST FIPS PUB 180-3 [SHA]




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   o  SHA-224: RFC 3874 [RFC3874]

   o  SHA-256: NIST FIPS PUB 180-3 [SHA]

   o  SHA-384: NIST FIPS PUB 180-3 [SHA]

   o  SHA-512: NIST FIPS PUB 180-3 [SHA]

   This document establishes a registry for CachedInformationType types
   and additional values can be added following the policy described in
   Section 6.








































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4.  Exchange Specification

   Clients supporting this extension MAY include the
   "cached_information" extension in the (extended) client hello, which
   MAY contain zero or more CachedObject attributes.

   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.

   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.

   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.  Section 4.1 and Section 4.2 defines the syntax of the
   fingerprinted information.

   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.

   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.

4.1.  Fingerprint of the Certificate Chain

   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.

   The original handshake message syntax is defined in RFC 5246
   [RFC5246] and has the following structure:







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

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

   By using the extension defined in this document the following
   information is sent:



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

   The opaque ASN.1Cert structure is replaced with the CachedObject
   structure defined in this document.

   Note: [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
   [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.

4.2.  Fingerprint for Trusted CAs

   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.

   The original handshake message syntax is defined in RFC 5246
   [RFC5246] and has the following structure:



         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;

   By using the extension defined in this document the following
   information is sent:



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          struct {
             ClientCertificateType certificate_types<1..2^8-1>;
             SignatureAndHashAlgorithm
               supported_signature_algorithms<2^16-1>;
             CachedObject DistinguishedName<1..2^16-1>;
          } CertificateRequest;

   The opaque DistinguishedName structure is replaced with the
   CachedObject structure defined in this document.










































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5.  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 requirement that this hash algorithm must
   have strong collision resistance.

   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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6.  IANA Considerations

6.1.  New Entry to the TLS ExtensionType Registry

   IANA is requested to add an entry to the existing TLS ExtensionType
   registry, defined in RFC 5246 [RFC5246], for cached_information(TBD)
   defined in this document.

6.2.  New Registry for CachedInformationType

   IANA is requested to establish a registry for TLS
   CachedInformationType values.  The first entries in the registry are

   o  certificate_chain(1)

   o  trusted_cas(2)

   The policy for adding new values to this registry, following the
   terminology defined in RFC 5226 [RFC5226], is as follows:

   o  0-63 (decimal): Standards Action

   o  64-223 (decimal): Specification Required

   o  224-255 (decimal): reserved for Private Use

6.3.  New Registry for HashAlgorithm

   IANA is requested to establish a registry for HashAlgorithm values
   and to populate the registry with an initial set of values listed in
   Section 3.

   The policy for adding new values to this registry, following the
   terminology defined in RFC 5226 [RFC5226], is as follows:

   o  0-63 (decimal): Standards Action

   o  64-223 (decimal): Specification Required

   o  224-255 (decimal): reserved for Private Use











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7.  Acknowledgments

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

   We would like to thank Paul Wouters for his feedback and Nikos
   Mavrogiannopoulos for his document review in December 2011.












































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8.  References

8.1.  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.

   [RFC3874]  Housley, R., "A 224-bit One-way Hash Function: SHA-224",
              RFC 3874, September 2004.

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

   [SHA]      "Federal Information Processing Standards Publication
              (FIPS PUB) 180-3, Secure Hash Standard (SHS)",
              October 2008.

8.2.  Informative References

   [I-D.iab-smart-object-workshop]
              Tschofenig, H. and J. Arkko, "Report from the
              'Interconnecting Smart Objects with the Internet'
              Workshop, 25th March 2011, Prague",
              draft-iab-smart-object-workshop-06 (work in progress),
              October 2011.

   [I-D.wouters-tls-oob-pubkey]
              Wouters, P., Gilmore, J., Weiler, S., Kivinen, T., and H.
              Tschofenig, "TLS out-of-band public key validation",
              draft-wouters-tls-oob-pubkey-02 (work in progress),
              November 2011.

   [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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