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+Network Working Group                                       S. Santesson
+Request for Comments: 4262                                     Microsoft
+Category: Standards Track                                  December 2005
+
+
+                    X.509 Certificate Extension for
+   Secure/Multipurpose Internet Mail Extensions (S/MIME) Capabilities
+
+Status of This Memo
+
+   This document specifies an Internet standards track protocol for the
+   Internet community, and requests discussion and suggestions for
+   improvements.  Please refer to the current edition of the "Internet
+   Official Protocol Standards" (STD 1) for the standardization state
+   and status of this protocol.  Distribution of this memo is unlimited.
+
+Copyright Notice
+
+   Copyright (C) The Internet Society (2005).
+
+Abstract
+
+   This document defines a certificate extension for inclusion of
+   Secure/Multipurpose Internet Mail Extensions (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.
+
+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
+   [RFC3280].
+
+   The S/MIME Capabilities attribute, defined in RFC 3851 [RFC3851], 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.
+
+   However, S/MIME does involve also the scenario where, for example, a
+   sender of an encrypted message has no prior established knowledge of
+   the recipient's cryptographic capabilities through recent S/MIME
+   exchanges.
+
+
+
+
+
+Santesson                   Standards Track                     [Page 1]
+
+RFC 4262             S/MIME Capabilities Extensions        December 2005
+
+
+   In such a case, the sender is forced to rely on out-of-band means or
+   its default configuration to select a 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
+   retrieval of certified dynamically updateable 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 [RFC3851].  (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 }
+
+
+
+
+Santesson                   Standards Track                     [Page 2]
+
+RFC 4262             S/MIME Capabilities Extensions        December 2005
+
+
+   All content requirements defined for the SMIMECapabilities attribute
+   in RFC 3851 apply 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.
+
+   Certification Authorities (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 extension MAY at their own
+   discretion ignore any present S/MIME Capabilities and SHOULD always
+   gracefully ignore any present S/MIME Capabilities that are not
+   considered relevant to the particular use of the certificate.
+
+   This extension MUST NOT be marked critical.
+
+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 is available to the
+   application.
+
+   It is outside the scope of this specification to define what is, or
+   is not, regarded as a more reliable source of information by the
+   application that is using the certificate.
+
+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.
+
+
+
+
+
+
+
+Santesson                   Standards Track                     [Page 3]
+
+RFC 4262             S/MIME Capabilities Extensions        December 2005
+
+
+   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.  Normative References
+
+   [STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate
+              Requirement Levels", BCP 14, RFC 2119, March 1997.
+
+   [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
+              X.509 Public Key Infrastructure Certificate and
+              Certificate Revocation List (CRL) Profile", RFC 3280,
+              April 2002.
+
+   [RFC3851]  Ramsdell, B., "Secure/Multipurpose Internet Mail
+              Extensions (S/MIME) Version 3.1 Message Specification",
+              RFC 3851, July 2004.
+
+Author's Address
+
+   Stefan Santesson
+   Microsoft
+   Tuborg Boulevard 12
+   2900 Hellerup
+   Denmark
+
+   EMail: stefans@microsoft.com
+
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+Santesson                   Standards Track                     [Page 4]
+
+RFC 4262             S/MIME Capabilities Extensions        December 2005
+
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+Full 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.
+
+   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.
+
+Intellectual Property
+
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+
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+
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+
+Acknowledgement
+
+   Funding for the RFC Editor function is currently provided by the
+   Internet Society.
+
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+Santesson                   Standards Track                     [Page 5]
+