S/MIME Working Group S. Santesson (Microsoft) INTERNET-DRAFT Expires November 2005 May 2005 X.509 Certificate Extension for S/MIME Capabilities Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 a "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html 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] INTERNET DRAFT S/MIME Capabilities Extension May 2005 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 Santesson [Page 2] INTERNET DRAFT S/MIME Capabilities Extension May 2005 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. Santesson [Page 3] INTERNET DRAFT S/MIME Capabilities Extension May 2005 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] INTERNET DRAFT S/MIME Capabilities Extension May 2005 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]