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
Santesson Standards Track [Page 4]RFC 4262 S/MIME Capabilities Extensions December 2005
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Acknowledgement
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Internet Society.
Santesson Standards Track [Page 5]
