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S/MIME Working Group                            S. Santesson (Microsoft)
INTERNET-DRAFT
Expires April 2005
                                                            October 2004


             Certificate extension for S/MIME Capabilities
                   <draft-ietf-smime-certcapa-00.txt>



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   or will be disclosed, and any of which I become aware will be
   disclosed, in accordance with RFC 3668


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Abstract


   This document defines a certificate extension for inclusion of S/MIME
   capabilities in public key certificates defined by RFC 3280.


   S/MIME Capabilities provides an optional method to communicate
   cryptographic capabilities of the certified subject as a complement
   to use of the S/MIME Capabilities signed attribute in S/MIME messages
   according to RFC 3851.










Santesson                                                       [Page 1]

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Table of Contents


   1  Introduction ................................................    2
   2  S/MIME Capabilities Extension ...............................    3
   3  Use in applications .........................................    3
   4  Security Considerations .....................................    3
   5  References ..................................................    4
   A  ASN.1 definitions ...........................................    4
   Authors' Addresses .............................................    4
   Disclaimer .....................................................    5
   Copyright Statement ............................................    5


1. Introduction


   The purpose of this specification is to specify means to minimize the
   need to revert to default cryptographic settings in encrypted S/MIME
   exchanges due to lack of knowledge about the opponents cryptographic
   capabilities.


   The S/MIME Capabilities attribute is defined in RFC 3851 to identify
   the cryptographic capabilities of the sender of a signed S/MIME
   message. This information can be used by the recipient in subsequent
   S/MIME secured exchanges to select appropriate cryptographic
   properties for future exchange with the opponent.


   The use of S/MIME does however introduce the scenario where a sender
   of an encrypted message has no prior established knowledge of the
   recipient's cryptographic capabilities through recent S/MIME
   exchanges.


   In such case the sender is forced to rely on its default
   configuration for encrypted messages to recipients with unknown
   capabilities. The problem is however that this default configuration
   may not be compatible with the recipient's capabilities and/or
   security policy.


   The solution defined in this specification leverage on the fact that
   S/MIME encryption requires possession of the recipient's public key
   certificate. This certificate contains information about the
   recipient's public key and the cryptographic capabilities of this
   key. Through the extension mechanism defined in this specification
   the certificate will also have the capacity to identify the subject's
   cryptographic capabilities for use with S/MIME, to be used as an
   optional additional information resource when knowledge about the
   subject capabilities is considered insufficient.







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2. S/MIME Capabilities Extension


   This section defines the S/MIME Capabilities extension.


   The S/MIME capabilities extension data structure used in this
   specification is identical to the data structure of the S/MIME
   capabilities attribute defined in RFC 3633.


   The S/MIME Capabilities extension MUST have the following definition:


      smimeCapabilities OBJECT IDENTIFIER ::=
         {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
         pkcs-9(9) 15}



      sMIMECapabilitiesExt EXTENSION ::= {
            SYNTAX             SMIMECapabilities
            IDENTIFIED BY      smimeCapabilities }


      SMIMECapabilities ::= SEQUENCE OF SMIMECapability


      SMIMECapability ::= SEQUENCE {
         capabilityID OBJECT IDENTIFIER,
         parameters ANY DEFINED BY capabilityID OPTIONAL }


   Algorithms should be ordered by preference


3. Use in applications


   Applications using the S/MIME Capabilities extension SHOULD NOT use
   information in the extension if more reliable and relevant
   authenticated capabilities information are available to the
   application.


   It is outside the scope of this specification to define what is, or
   is not, regarded as more reliable source of information by the
   encrypting application.



4  Security Considerations


   The S/MIME capabilities extension contains a statement about the
   subject's capabilities made at the time of certificate issuance.
   Implementers should therefore take into account any effect caused by
   the change of these capabilities during the lifetime of the
   certificate.


   Change in the subject's capabilities during the lifetime of a




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   certificate may require revocation of the certificate. Revocation
   should however only be motivated if a listed algorithm is considered
   broken and/or considered too week to use for the adopted encryption
   policy.


   Implementers should take into account that the use of this extension
   does not change the fact that it is always the responsibility of the
   sender to choose sufficiently strong encryption for its information
   disclosure.


5 References



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


   [RFC 3280] R. Housley, W. Polk, W. Ford, and D. Solo, "Internet
              X.509 Public Key Infrastructure: Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.


   [RFC 3851] B. Ramsdell, "Secure/Multipurpose Internet Mail
              Extensions (S/MIME) Version 3.1 Message Specification",
              RFC 3851, July 2004




A. ASN.1 definitions


   TBD


Authors' Addresses



   Stefan Santesson
   Microsoft
   Tuborg Boulevard 12
   2900 Hellerup
   Denmark


   EMail: stefans@microsoft.com











Santesson                                                       [Page 4]

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Disclaimer


   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.



Copyright Statement


   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.





Expires April 2005






























Santesson                                                       [Page 5]


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