S/MIME Working Group S. Santesson (Microsoft) INTERNET-DRAFT Expires February 2005 August 2004 S/MIME Capabilities in X.509 certificates Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract This document defines a certificate extension for inclusion of S/MIME capabilities in public key certificates as defined by RFC 3280. S/MIME Capabilities provides a method of broadcasting the cryptographic S/MIME capabilities of the certified subject as a complement to use of S/MIME Capabilities signed attributes as defined in RFC 3851. Table of Contents 1 Introduction ................................................ 2 2 S/MIME Capabilities Extension ............................... 3 3 Use in applications ......................................... 3 4 Security Considerations ..................................... 3 5 References .................................................. 4 6 Intellectual Property Rights ................................ 4 A ASN.1 definitions ........................................... 6 Santesson [Page 1] INTERNET DRAFT S/MIME Capabilities extension August 2004 Authors' Addresses ............................................. 6 Full Copyright Statement ....................................... 7 1. Introduction S/MIME defines in RFC 3851 the S/MIME Capabilities attribute as a means to communicate the cryptographic capabilities of a sender of a signed S/MIME message. This information can then be used by the recipient in subsequent S/MIME secured exchanges with the sender to select appropriate cryptographic properties for the exchange. However, the use of S/MIME in electronic mail also introduces the scenario where a sender of an encrypted e-mail has no prior established knowledge of the recipient's cryptographic capabilities through recent S/MIME exchanges. In this case the sender has no other option but to fall back to a default assumption of the recipient which may be incompatible with the recipient's capabilities and/or security policy. Sending an encrypted e-mail does however require possession of the recipient's public key certificate which in turn contains information about the recipient's public key cryptographic capabilities but not its symmetric cryptographic capabilities needed for S/MIME. The approach of this document to minimize the need to revert to default cryptographic settings due to lack of knowledge, is enable the capacity to include S/MIME Capabilities also in public key certificates. Santesson [Page 2] INTERNET DRAFT S/MIME Capabilities extension August 2004 2. S/MIME Capabilities Extension Data objects and their syntax and semantics needed to communicate S/MIME capabilities have already been defined in RFC 3633 through the sMIMECapabilities attribute and there is no need to re-invent this structure. The S/MIME Capabilities extension does therefore reuse the data structure of the sMIMECapabilities attribute as well as the defined object identifier for this structure. 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 provided in this certificate extension if more recent S/MIME capabilities information is available from signed messages from the certified subject. In this case the information in the S/MIME capabilities extension SHOULD be ignored. 4 Security Considerations Implementers should be aware that the S/MIME capabilities extension contains a statement about the subject's capabilities at the time of certificate issuance, which may change over the lifetime of the certificate. Santesson [Page 3] INTERNET DRAFT S/MIME Capabilities extension August 2004 Certification Authorities should therefore renew a certificate including S/MIME Capabilities, if the subjects cryptographic capabilities changes in a way that is no longer compatible with the current certificate. It is however, despite any available information about the recipient's cryptographic capabilities, still the responsibility of the sender to always choose sufficiently strong encryption for its information disclosure and the responsibility of the recipient to only accept data that has been protected according to the recipient's security policy. 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 6 Intellectual Property Rights The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. Santesson [Page 4] INTERNET DRAFT S/MIME Capabilities extension August 2004 The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. Santesson [Page 5] INTERNET DRAFT S/MIME Capabilities extension August 2004 A. ASN.1 definitions TBD Authors' Addresses Stefan Santesson Microsoft Tuborg Boulevard 12 2900 Hellerup Denmark EMail: stefans@microsoft.com Santesson [Page 6] INTERNET DRAFT S/MIME Capabilities extension August 2004 Full Copyright Statement Copyright (C) The Internet Society (2004). All Rights Reserved. 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The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS 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. Santesson [Page 7]