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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: September 24, 2015                                     ARM Ltd.
                                                          March 23, 2015


      Transport Layer Security (TLS) Cached Information Extension
                   draft-ietf-tls-cached-info-19.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 chain (i.e., the certificates of
   intermediate CAs up to the root CA).

   This document defines an extension that allows a TLS client to inform
   a server of cached information, allowing the server to omit 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 September 24, 2015.

Copyright Notice

   Copyright (c) 2015 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



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Cached Information Extension  . . . . . . . . . . . . . . . .   3
   4.  Exchange Specification  . . . . . . . . . . . . . . . . . . .   4
     4.1.  Server Certificate Message  . . . . . . . . . . . . . . .   5
     4.2.  CertificateRequest Message  . . . . . . . . . . . . . . .   6
   5.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  New Entry to the TLS ExtensionType Registry . . . . . . .   9
     7.2.  New Registry for CachedInformationType  . . . . . . . . .   9
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Reducing the amount of information exchanged during a Transport Layer
   Security handshake to a minimum helps to improve performance in
   environments where devices are connected to a network with a low
   bandwidth, and lossy radio technology.  With Internet of Things such
   environments exist, for example, when devices use IEEE 802.15.4 or
   Bluetooth Smart.  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 information cached in an earlier TLS
   handshake.

   A typical example exchange may therefore look as follows.  First, the
   client and the server executes the full TLS handshake.  The client
   then caches the certificate provided by the server.  When the TLS
   client connects to the TLS server some time in the future, without
   using session resumption, it then attaches the cached_info 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 has not changed then the TLS server does



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   not need to send its' certificate and the corresponding certificate
   chain again.  In case information has changed, which can be seen from
   the fingerprint provided by the client, the certificate payload is
   transmitted to the client to allow the client to update the cache.

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].

   This document refers to the TLS protocol but the description is
   equally applicable to DTLS as well.

3.  Cached Information Extension

   This document defines a new extension type (cached_info(TBD)), which
   is used in client hello and server hello messages.  The extension
   type is specified as follows.


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

   The extension_data field of this extension, when included in the
   client hello, MUST contain the CachedInformation structure.  The
   client MAY send multiple CachedObjects of the same
   CachedInformationType.  This may, for example, be the case when the
   client has cached multiple certificates from a server.





















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         enum {
              cert(1), cert_req(2) (255)
         } CachedInformationType;

         struct {
              select (type) {
                case client:
                  CachedInformationType type;
                  opaque hash_value<1..255>;
                case server:
                  CachedInformationType type;
              } body;
         } CachedObject;

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

   This document defines the following types:

   Omitting the Server Certificate Message:

      With the type field set to 'cert', the client MUST include the
      message digest of the Certificate message in the hash_value field.
      For this type the message digest MUST be calculated using SHA-256
      [RFC4634].

   Omitting the CertificateRequest Message

      With the type set to 'cert_req', the client MUST include the
      message digest of the CertificateRequest message in the hash_value
      field.  For this type the message digest MUST be calculated using
      SHA-256 [RFC4634].

   New types can be added following the policy described in the IANA
   considerations section, see Section 7.  Different message digest
   algorithms for use with these types can also be added by registering
   a new type that makes use of this updated message digest algorithm.

4.  Exchange Specification

   Clients supporting this extension MAY include the "cached_info"
   extension in the (extended) client hello.  If the client includes the
   extension then it MUST contain one or more CachedObject attributes.

   A server supporting this extension MAY include the "cached_info"
   extension in the (extended) server hello.  By returning the
   "cached_info" extension the server indicates that it supports the



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   cached info types.  For each indicated cached info type the server
   MUST alter the transmission of respective payloads, according to the
   rules outlined with each type.  If the server includes the extension
   it MUST only include CachedObjects of a type also supported by the
   client (as expressed in the client hello).  For example, if a client
   indicates support for 'cert' and 'cert_req' then the server cannot
   respond with a "cached_info" attribute containing support for
   'cert_status'.

   Since the client includes a fingerprint of information it cached (for
   each indicated type) the server is able to determine whether cached
   information is stale.  If the server supports this specification and
   notices a mismatch between the data cached by the client and its own
   information then the server MUST include the information in full and
   MUST NOT list the respective type in the "cached_info" extension.

   Note: If a server is part of a hosting environment then the client
   may have cached multiple data items for a single server.  To allow
   the client to select the appropriate information from the cache it is
   RECOMMENDED that the client utilizes the Server Name Indication
   extension [RFC6066].

   Following a successful exchange of the "cached_info" extension in the
   client and server hello, the server alters sending the corresponding
   handshake message.  How information is altered from the handshake
   messages is defined in Section 4.1, and in Section 4.2 for the types
   defined in this specification.

4.1.  Server Certificate Message

   When a ClientHello message contains the "cached_info" extension with
   a type set to 'cert' then the server MAY send the Certificate message
   shown in Figure 2 under the following conditions:

      The server software implements the "cached_info" extension defined
      in this specification.

      The 'cert' cached info extension is enabled (for example, a policy
      allows the use of this extension).

      The server compared the value in the hash_value field of the
      client-provided "cached_info" extension with the fingerprint of
      the Certificate message it normally sends to clients.  This check
      ensures that the information cached by the client is current.

   The original Certificate handshake message syntax is defined in RFC
   5246 [RFC5246] and has the structure shown in Figure 1.




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

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

           Figure 1: Certificate Message as defined in RFC 5246.

   The new structure of the CertificateRequest message is shown in
   Figure 2.


         struct {
             opaque hash_value<1..255>;
         } CertificateRequest;

                Figure 2: Cached Info Certificate Message.

   The fingerprint MUST be computed as follows: hash_value:=SHA-
   256(Certificate)

   Note that RFC 7250 [RFC7250] allows the certificate 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 [RFC7250] and the negotiated certificate
   type is a raw public key then the TLS server omits sending a
   Certificate payload that contains an ASN.1 Certificate structure with
   the included SubjectPublicKeyInfo rather than the full certificate.
   As such, this extension is compatible with the raw public key
   extension defined in RFC 7250.

4.2.  CertificateRequest Message

   When a fingerprint for an object of type 'cert_req' is provided in
   the client hello, the server MAY omit the CertificateRequest message
   under the following conditions:

      The server software implements the "cached_info" extension defined
      in this specification.

      The 'cert_req' cached info extension is enabled (for example, a
      policy allows the use of this extension).

      The server compared the value in the hash_value field of the
      client-provided "cached_info" extension with the fingerprint of
      the CertificateRequest message it normally sends to clients.  This
      check ensures that the information cached by the client is
      current.



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      The server wants to request a certificate from the client.

   The original CertificateRequest 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;

       Figure 3: CertificateRequest Message as defined in RFC 5246.

   The new structure of the CertificateRequest message is shown in
   Figure 4.


         struct {
             opaque hash_value<1..255>;
         } CertificateRequest;

             Figure 4: Cached Info CertificateRequest Message.

   The fingerprint MUST be computed as follows: hash_value:=SHA-
   256(CertificateRequest)

5.  Example

   Figure 5 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_info" extension, see
   [2], and indicates that it has stored the certificate from the
   earlier exchange (by indicating the 'cert' type).  With [3] the TLS
   server acknowledges the supports of the 'cert' type and by including
   the value in the server hello informs the client that the content of
   the certificate payload contains the fingerprint of the certificate
   instead of the RFC 5246-defined payload of the certificate message in
   message [4].




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   (A) Initial (full) Exchange

   ClientHello            ->
                          <-  ServerHello
                              Certificate* // [1]
                              ServerKeyExchange*
                              CertificateRequest*
                              ServerHelloDone

   Certificate*
   ClientKeyExchange
   CertificateVerify*
   [ChangeCipherSpec]
   Finished               ->

                          <- [ChangeCipherSpec]
                             Finished

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


   (B) TLS Cached Extension Usage

   ClientHello
   cached_info=(cert)     -> // [2]
                          <-  ServerHello
                              cached_info=(cert) [3]
                              Certificate [4]
                              ServerKeyExchange*
                              ServerHelloDone

   ClientKeyExchange
   CertificateVerify*
   [ChangeCipherSpec]
   Finished               ->

                          <- [ChangeCipherSpec]
                             Finished

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

                    Figure 5: Example Message Exchange

6.  Security Considerations

   This specification defines a mechanism to reference stored state
   using a fingerprint.  Sending a fingerprint of cached information in
   an unencrypted handshake, as the client and server hello is, may



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   allow an attacker or observer to correlate independent TLS exchanges.
   While some information elements used in this specification, such as
   server certificates, are public objects and usually do not contain
   sensitive information, other (not yet defined cached info types) may.
   Those who implement and deploy this specification should therefore
   make an informed decision whether the cached information is inline
   with their security and privacy goals.  In case of concerns, it is
   advised to avoid sending the fingerprint of the data objects in
   clear.

   The use of the cached info extension allows the server to obmit
   sending certain TLS messages.  Consequently, these omitted messages
   are not included in the transcript of the handshake in the TLS Finish
   message per value.  However, since the client communicates the hash
   values of the cached values in the initial handshake message the
   fingerprints are included in the TLS Finish message.

   Clients MUST ensure that they only cache information from legitimate
   sources.  For example, when the client populates the cache from a TLS
   exchange then it must only cache information after the successful
   completion of a TLS exchange to ensure that an attacker does not
   inject incorrect information into the cache.  Failure to do so allows
   for man-in-the-middle attacks.

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_info(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  cert(1)

   o  cert_req(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




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   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 March 2012)

   o  Ilari Liusvaara, Adam Langley, and Eric Rescorla (in July 2014)

   o  Sean Turner (in August 2014)

   Additionally, we would like to thank the TLS working group chairs,
   Sean Turner and Joe Salowey, as well as the responsible security area
   director, Stephen Farrell, for their 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.

   [RFC4634]  Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and HMAC-SHA)", RFC 4634, July 2006.

   [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

   [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.






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   [RFC7250]  Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and
              T. Kivinen, "Using Raw Public Keys in Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", RFC 7250, June 2014.

Authors' Addresses

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

   Email: sts@aaa-sec.com


   Hannes Tschofenig
   ARM Ltd.
   Hall in Tirol  6060
   Austria

   Email: Hannes.tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at




























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