S/MIME Working Group S. Santesson (Microsoft)
INTERNET-DRAFT
Expires September 2005
March 2005
X.509 Certificate Extension for S/MIME Capabilities
<draft-ietf-smime-certcapa-04.txt>
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disclosed, in accordance with RFC 3668
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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.
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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 for future exchange with the opponent.
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}
sMIMECapabilitiesExt EXTENSION ::= {
SYNTAX SMIMECapabilities
IDENTIFIED BY smimeCapabilities }
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
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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.
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 and/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.
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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 (2004). 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.
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Expires September 2005
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