TLS                                                         S. Santesson
Internet-Draft                                           3xA Security AB
Intended status: Standards Track                           H. Tschofenig
Expires: March 16, September 29, 2013                       Nokia Siemens Networks
                                                      September 12, 2012
                                                          March 28, 2013

      Transport Layer Security (TLS) Cached Information Extension
                   draft-ietf-tls-cached-info-13.txt
                   draft-ietf-tls-cached-info-14.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
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   This Internet-Draft will expire on March 16, September 29, 2013.

Copyright Notice

   Copyright (c) 2012 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Cached Information Extension . . . . . . . . . . . . . . . . .  5
   4.  Exchange Specification . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Fingerprint of  Omitting the Certificate Chain . . . . . . . . . . . . . .  7
     4.2.  Fingerprint for  Omitting the Trusted CAs . . . . . . . . . . . . . . . . .  8
   5.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 10  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12 11
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13 12
     7.1.  New Entry to the TLS ExtensionType Registry  . . . . . . . 13 12
     7.2.  New Registry for CachedInformationType . . . . . . . . . . 13 12
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 14 13
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15 14
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 15 14
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 15 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16 15

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 [RFC6574].

   This specification defines a TLS extension that allows a client and a
   server to exclude transmission of cached information from the TLS
   handshake.

   A typical example exchange may therefore look as follows.  First, the
   TLS exchange
   client and the server executes the usual TLS handshake.  It may  The client
   may, for example, decide to
   store cache the certificate provided by the server for a future exchange.
   server.  When the TLS client then connects to the TLS server some time in
   the future, without using session resumption, it then attaches the
   cached_information extension defined in this document to the client
   hello message to indicate that it had cached the certificate, and it
   provides the fingerprint of it.  If the server's certificate had not
   changed then the TLS server does not need to send the full
   certificate to the client again.  In case the information had
   changed, the certificate payload is transmitted to the client to
   allow the client to update it's state information.

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].

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 the hash calculated over the
   certificate_list element of the Certificate payload provided by the
   TLS server in an earlier exchange, 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 the CertificateRequest payload
   provided by the TLS server in an earlier exchange, 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.  The list of registered
   hash algorithms can be found in the TLS HashAlgorithm Registry, which
   was created by RFC 5246 [RFC5246].  The value zero (0) for 'none' is
   not an allowed choice for a hash algorithm and MUST NOT be used.

   This document establishes a registry for CachedInformationType types
   and additional values can be added following the policy described in
   Section 7.

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

   A server supporting this extension MAY include the
   "cached_information" extension in the (extended) server hello, which
   MAY contain one or more CachedObject attributes. attributes it supports.  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 If clients make use of the Server Name Indication [RFC6066] value.
   then clients may need to cache multiple data items for a single
   server since servers may host multiple 'virtual' servers at a single
   underlying network address.

   Following a successful exchange of the "cached_information" extensions,
   extensions in the client and server MAY send fingerprints of hello, the cached information in server omits sending
   the corresponding handshake exchange as a replacement for the exchange of message.  How information is omitted from
   the full
   data. handshake message is defined per cached info type.  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 was omitted from transmission. transmission by means of its present hash in
   the client hello and not through its presence in the handshake
   exchange.

   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  Omitting the Certificate Chain

   When an object of type 'certificate_chain' is provided in the client
   hello, the server MAY send a fingerprint instead of replace the complete
   certificate chain as shown below. sequence of certificates with an
   empty sequence with an actual length field of zero (=empty vector).

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

         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 cached_objects<1..2^24-1>;
   } Certificate;

   The certificate_list vector of opaque ASN.1Cert elements in the
   original syntax is replaced with a vector holding CachedObject
   structures as defined in this document.

   Note:

   Note that [I-D.ietf-tls-oob-pubkey] allows a PKIX the certificate containing payload to
   contain only the SubjectPublicKeyInfo instead of the full information
   typically found in a certificate.  Hence, when this specification is
   used in combination with [I-D.ietf-tls-oob-pubkey] and the negotiated
   certificate type is a raw public key then the TLS server sends the
   hashed omits
   sending a Certificate payload that contains a an ASN.1Cert structure of
   the SubjectPublicKeyInfo.

4.2.  Fingerprint for  Omitting the Trusted CAs

   When a hash fingerprint for an object of type 'trusted_cas' is provided in
   the client hello, the server MAY send a fingerprint instead of DistinguishedName in the
   complete certificate authorities information as shown below.
   Certificate Request message with an actual length field of zero
   (=empty vector).

   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:

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

   The certificate_authorities vector of opaque DistinguishedName
   elements in the original syntax is replaced with a vector holding
   CachedObject structures as defined in this document.

5.  Example

   Figure 1 illustrates an example exchange using the TLS cached info
   extension.  In the normal TLS handshake exchange shown in flow (A)
   the TLS server provides its certificate in the Certificate payload to
   the client, see step [1].  This allows the client to store the
   certificate for future use.  After some time the TLS client again
   interacts with the same TLS server and makes use of the TLS cached
   info extension, as shown in flow (B).  The TLS client indicates
   support for this specification via the cached_information extension,
   see [2], and indicates that it has stored the certificate_chain from
   the earlier exchange.  With [3] the TLS server indicates that it also
   supports this specification and informs the client that it also
   supports caching of other objects beyond the 'certificate_chain',
   namely 'trusted_cas' (also defined in this document), and the 'foo-
   bar' extension (i.e., an imaginary extension that yet needs to be
   defined).  With [4] the TLS server provides the fingerprint of omits sending the certificate chain
   chain, as described in Section 4.1.

   (A) Initial (full) Exchange

   client_hello  ->
                          <-  server_hello,
                              certificate, // [1]
                              server_key_exchange,
                              server_hello_done

   client_key_exchange,
   change_cipher_spec,
   finished                  ->

                          <- change_cipher_spec,
                             finished

   Application Data        <------->     Application Data

   (B) TLS Cached Extension Usage

   client_hello,
   cached_information=(certificate_chain)   -> // [2]
                          <-  server_hello,
                              cached_information= // [3]
                              (certificate_chain, trusted_cas, foo-bar)
                              certificate, // [4]
                              server_key_exchange,
                              server_hello_done

   client_key_exchange,
   change_cipher_spec,
   finished                  ->

                          <- change_cipher_spec,
                             finished

   Application Data        <------->     Application Data

                    Figure 1: Example Message Exchange

6.  Security Considerations

   This specification defines a mechanism to reference stored state
   using a fingerprint.  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  Sending a hash fingerprint of that cached information in
   an unencrypted handshake might introduce integrity or data disclosure
   issues handshake, as it enables the client and server hello is, may
   allow an attacker or observer to identify if a known object (such
   as a known server certificate) has been correlate independent TLS exchanges.
   While some information elements 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 others may be.  Those who implement and deploy this
   specification should be aware if any therefore make an informed decision whether the
   cached information are subject to such is inline with their security concerns and in such privacy goals.
   In case SHOULD NOT send a hash over encrypted of concerns, it is advised to avoid sending the fingerprint
   of the data objects in unencrypted
   handshake. clear.

   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.

7.  IANA Considerations

7.1.  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.

7.2.  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

8.  Acknowledgments

   We would like to thank the following persons for your detailed
   document reviews:

   o  Paul Wouters and Nikos Mavrogiannopoulos (December 2011)

   o  Rob Stradling (February 2012)

   o  Ondrej Mikle in (in March 2012)

   Additionally, we would like to thank the TLS working group chairs,
   Eric Rescorla and Joe Salowey, as well as the security area
   directors, Sean Turner and Stephen Farrell, for their feedback and
   support.

9.  References

9.1.  Normative References

   [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.

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

9.2.  Informative References

   [I-D.ietf-tls-oob-pubkey]
              Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., Kivinen, T., and H.
              Tschofenig,
              T. Kivinen, "Out-of-Band Public Key Validation for
              Transport Layer Security", draft-ietf-tls-oob-pubkey-04 Security (TLS)",
              draft-ietf-tls-oob-pubkey-07 (work in progress), July 2012.
              February 2013.

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

   [RFC6574]  Tschofenig, H. and J. Arkko, "Report from the Smart Object
              Workshop", RFC 6574, April 2012.

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