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S/MIME Working Group                            S. Santesson (Microsoft)
INTERNET-DRAFT
Expires November 2005                                           May 2005

          X.509 Certificate Extension for S/MIME Capabilities
                   <draft-ietf-smime-certcapa-05.txt>


Status of this Memo

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Abstract

   This document defines a certificate extension for inclusion of S/MIME
   capabilities in X.509 public key certificates, as defined by RFC
   3280.

   This certificate extension provides an optional method to indicate
   the cryptographic capabilities of an entity as a complement to 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
   1.1  Terminology ...............................................    3
   2  S/MIME Capabilities Extension ...............................    3
   3  Use in applications .........................................    4
   4  Security Considerations .....................................    4
   5  References ..................................................    5
   Authors' Addresses .............................................    5
   Disclaimer .....................................................    5
   Copyright Statement ............................................    5

1  Introduction

   This document defines a certificate extension for inclusion of S/MIME
   capabilities in X.509 public key certificates, as defined by RFC 3280
   [RFC 3280].

   The S/MIME Capabilities attribute, defined in RFC 3851, is defined to
   indicate 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.

   S/MIME does however involve also the scenario where e.g. 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 out-of-band means or its
   default configuration to select content encryption algorithm for
   encrypted messages to recipients with unknown capabilities. Such
   default configuration may however be incompatible with the
   recipient's capabilities and/or security policy.

   The solution defined in this specification leverages the fact that
   S/MIME encryption requires possession of the recipient's public key
   certificate. This certificate already 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 may also identify the subject's cryptographic S/MIME
   capabilities. This may then be used as an optional information
   resource to select appropriate encryption settings for the
   communication.

   This document is limited to the "static" approach where asserted
   cryptographic capabilities remain unchanged until the certificate
   expires or is revoked. Other "dynamic" approaches which allow



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   retrieval of certified dynamically updatable capabilities during the
   lifetime of a certificate are out of scope of this document.

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 [STDWORDS].

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
   SMIMECapabilities attribute defined in RFC 3851 [RFC 3851] (The ASN.1
   structure of smimeCapabilities is included below for illustrative
   purposes only).

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

      SMIMECapabilities ::= SEQUENCE OF SMIMECapability

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

   All content requirements defined for the SMIMECapabilities attribute
   in RFC 3851 applies also to this extension.

   There are numerous different types of S/MIME capabilities that have
   been defined in various documents. While all of the different
   capabilities can be placed in this extension, the intended purpose of
   this specification is mainly to support inclusion of S/MIME
   capabilities specifying content encryption algorithms.

   CAs SHOULD limit the type of included S/MIME capabilities in this
   extension to types that are considered relevant to the intended use
   of the certificate.

   Client applications processing this extensions MAY at its own
   discretion ignore any present S/MIME capabilities and SHOULD always
   gracefully ignore any present S/MIME capabilities that is not
   consider relevant to its particular use of the certificate.

   This extension MUST NOT be marked critical.



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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
   certificate using 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
   certificate may require revocation of the certificate. Revocation
   should however only be motivated if a listed algorithm is considered
   broken or considered too weak for the governing security 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.






















Santesson                                                       [Page 4]

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5 References

   Normative references:

   [STDWORDS] 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


Authors' Addresses


   Stefan Santesson
   Microsoft
   Tuborg Boulevard 12
   2900 Hellerup
   Denmark

   EMail: stefans@microsoft.com


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 (2005).

   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 November 2005



Santesson                                                       [Page 5]


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