S/MIME Working Group S. Santesson (Microsoft)
INTERNET-DRAFT
Expires April 2005
October 2004
Certificate extension for S/MIME Capabilities
<draft-ietf-smime-certcapa-00.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 public key certificates defined by RFC 3280.
S/MIME Capabilities provides an optional method to communicate
cryptographic capabilities of the certified subject as a complement
to use of the S/MIME Capabilities signed attribute in S/MIME messages
according to RFC 3851.
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Table of Contents
1 Introduction ................................................ 2
2 S/MIME Capabilities Extension ............................... 3
3 Use in applications ......................................... 3
4 Security Considerations ..................................... 3
5 References .................................................. 4
A ASN.1 definitions ........................................... 4
Authors' Addresses ............................................. 4
Disclaimer ..................................................... 5
Copyright Statement ............................................ 5
1. Introduction
The purpose of this specification is to specify means to minimize the
need to revert to default cryptographic settings in encrypted S/MIME
exchanges due to lack of knowledge about the opponents cryptographic
capabilities.
The S/MIME Capabilities attribute is defined in RFC 3851 to identify
the 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.
The use of S/MIME does however introduce the scenario where 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 its default
configuration for encrypted messages to recipients with unknown
capabilities. The problem is however that this default configuration
may not be compatible with the recipient's capabilities and/or
security policy.
The solution defined in this specification leverage on the fact that
S/MIME encryption requires possession of the recipient's public key
certificate. This certificate 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 will also have the capacity to identify the subject's
cryptographic capabilities for use with S/MIME, to be used as an
optional additional information resource when knowledge about the
subject capabilities is considered insufficient.
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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 S/MIME
capabilities attribute defined in RFC 3633.
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 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
encrypting 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
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certificate may require revocation of the certificate. Revocation
should however only be motivated if a listed algorithm is considered
broken and/or considered too week to use for the adopted encryption
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.
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
A. ASN.1 definitions
TBD
Authors' Addresses
Stefan Santesson
Microsoft
Tuborg Boulevard 12
2900 Hellerup
Denmark
EMail: stefans@microsoft.com
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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.
Expires April 2005
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