draft-ietf-smime-certcapa-03.txt   draft-ietf-smime-certcapa-04.txt 
Stefan Santesson
Microsoft Security Center of Excellence (SCOE)
S/MIME Working Group S. Santesson (Microsoft) S/MIME Working Group S. Santesson (Microsoft)
INTERNET-DRAFT INTERNET-DRAFT
Expires August 2005 Expires September 2005
February 2005 March 2005
Certificate extension for S/MIME Capabilities X.509 Certificate Extension for S/MIME Capabilities
<draft-ietf-smime-certcapa-03.txt> <draft-ietf-smime-certcapa-04.txt>
By submitting this Internet-Draft, I certify that any applicable By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed, patent or other IPR claims of which I am aware have been disclosed,
or will be disclosed, and any of which I become aware will be or will be disclosed, and any of which I become aware will be
disclosed, in accordance with RFC 3668 disclosed, in accordance with RFC 3668
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Abstract Abstract
This document defines a certificate extension for inclusion of S/MIME This document defines a certificate extension for inclusion of S/MIME
capabilities in public key certificates defined by RFC 3280. capabilities in X.509 public key certificates, as defined by RFC
3280.
S/MIME Capabilities provides an optional method to communicate This certificate extension provides an optional method to indicate
cryptographic capabilities of the certified subject as a complement the cryptographic capabilities of an entity as a complement to the
to use of the S/MIME Capabilities signed attribute in S/MIME messages S/MIME Capabilities signed attribute in S/MIME messages according to
according to RFC 3851. RFC 3851.
Table of Contents Table of Contents
1 Introduction ................................................ 2 1 Introduction ................................................ 2
1.1 Terminology ............................................... 3 1.1 Terminology ............................................... 3
2 S/MIME Capabilities Extension ............................... 3 2 S/MIME Capabilities Extension ............................... 3
3 Use in applications ......................................... 3 3 Use in applications ......................................... 4
4 Security Considerations ..................................... 4 4 Security Considerations ..................................... 4
5 References .................................................. 4 5 References .................................................. 5
Authors' Addresses ............................................. 5 Authors' Addresses ............................................. 5
Disclaimer ..................................................... 5 Disclaimer ..................................................... 5
Copyright Statement ............................................ 5 Copyright Statement ............................................ 5
1 Introduction 1 Introduction
This document defines a certificate extension for inclusion of S/MIME This document defines a certificate extension for inclusion of S/MIME
capabilities in public key certificates defined by RFC 3280 [RFC capabilities in X.509 public key certificates, as defined by RFC 3280
3280]. [RFC 3280].
The S/MIME Capabilities attribute is defined in RFC 3851 for The S/MIME Capabilities attribute, defined in RFC 3851, is defined to
specifying cryptographic capabilities of the sender of a signed indicate cryptographic capabilities of the sender of a signed S/MIME
S/MIME message. This information can be used by the recipient in message. This information can be used by the recipient in subsequent
subsequent S/MIME secured exchanges to select appropriate S/MIME secured exchanges to select appropriate cryptographic
cryptographic properties for future exchange with the opponent. properties for future exchange with the opponent.
The use of S/MIME does however introduce the scenario where e.g. a S/MIME does however involve also the scenario where e.g. a sender of
sender of an encrypted message has no prior established knowledge of an encrypted message has no prior established knowledge of the
the recipient's cryptographic capabilities through recent S/MIME recipient's cryptographic capabilities through recent S/MIME
exchanges. exchanges.
In such case the sender is forced to rely on its default In such case the sender is forced to rely on out-of-band means or its
configuration for encrypted messages to recipients with unknown default configuration to select content encryption algorithm for
capabilities. The problem is however that this default configuration encrypted messages to recipients with unknown capabilities. Such
may not be compatible with the recipient's capabilities and/or default configuration may however be incompatible with the
security policy. recipient's capabilities and/or security policy.
The solution defined in this specification leverages on the fact that The solution defined in this specification leverages the fact that
S/MIME encryption requires possession of the recipient's public key S/MIME encryption requires possession of the recipient's public key
certificate. This certificate contains information about the certificate. This certificate already contains information about the
recipient's public key and the cryptographic capabilities of this recipient's public key and the cryptographic capabilities of this
key. Through the extension mechanism defined in this specification key. Through the extension mechanism defined in this specification
the certificate will also have the capacity to identify the subject's the certificate may also identify the subject's cryptographic S/MIME
cryptographic capabilities for use with S/MIME, to be used as an capabilities. This may then be used as an optional information
optional additional information resource when knowledge about the resource to select appropriate encryption settings for the
subject capabilities is considered insufficient. communication.
This document is limited to the "static" approach where asserted This document is limited to the "static" approach where asserted
cryptographic capabilities remain unchanged until the certificate cryptographic capabilities remain unchanged until the certificate
expires or is revoked. Other "dynamic" approaches which allow expires or is revoked. Other "dynamic" approaches which allow
retrieval of certified dynamically updatable capabilities during the retrieval of certified dynamically updatable capabilities during the
lifetime of a certificate are out of scope of this document. lifetime of a certificate are out of scope of this document.
1.1 Terminology 1.1 Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [STDWORDS]. document are to be interpreted as described in RFC 2119 [STDWORDS].
2 S/MIME Capabilities Extension 2 S/MIME Capabilities Extension
This section defines the S/MIME Capabilities extension. This section defines the S/MIME Capabilities extension.
The S/MIME capabilities extension data structure used in this The S/MIME capabilities extension data structure used in this
specification is identical to the data structure of the specification is identical to the data structure of the
SMIMECapabilities attribute defined in RFC 3851 [RFC 3851]. SMIMECapabilities attribute defined in RFC 3851 [RFC 3851] (The ASN.1
structure of smimeCapabilities is included below for illustrative
The S/MIME Capabilities extension MUST follow the requirements and purposes only).
definitions defined in RFC 3851. (The ASN.1 structure of
smimeCapabilities is included below for illustrative purposes only)
smimeCapabilities OBJECT IDENTIFIER ::= smimeCapabilities OBJECT IDENTIFIER ::=
{iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs-9(9) 15} pkcs-9(9) 15}
sMIMECapabilitiesExt EXTENSION ::= { sMIMECapabilitiesExt EXTENSION ::= {
SYNTAX SMIMECapabilities SYNTAX SMIMECapabilities
IDENTIFIED BY smimeCapabilities } IDENTIFIED BY smimeCapabilities }
SMIMECapabilities ::= SEQUENCE OF SMIMECapability SMIMECapabilities ::= SEQUENCE OF SMIMECapability
SMIMECapability ::= SEQUENCE { SMIMECapability ::= SEQUENCE {
capabilityID OBJECT IDENTIFIER, capabilityID OBJECT IDENTIFIER,
parameters ANY DEFINED BY capabilityID OPTIONAL } parameters ANY DEFINED BY capabilityID OPTIONAL }
This extension is expected to hold the same type of capability data All content requirements defined for the SMIMECapabilities attribute
as the SMIMECapabilities attribute defined in RFC 3851. 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. This extension MUST NOT be marked critical.
3 Use in applications 3 Use in applications
Applications using the S/MIME Capabilities extension SHOULD NOT use Applications using the S/MIME Capabilities extension SHOULD NOT use
information in the extension if more reliable and relevant information in the extension if more reliable and relevant
authenticated capabilities information are available to the authenticated capabilities information are available to the
application. application.
It is outside the scope of this specification to define what is, or It is outside the scope of this specification to define what is, or
is not, regarded as more reliable source of information by the is not, regarded as more reliable source of information by the
encrypting application. certificate using application.
4 Security Considerations 4 Security Considerations
The S/MIME capabilities extension contains a statement about the The S/MIME capabilities extension contains a statement about the
subject's capabilities made at the time of certificate issuance. subject's capabilities made at the time of certificate issuance.
Implementers should therefore take into account any effect caused by Implementers should therefore take into account any effect caused by
the change of these capabilities during the lifetime of the the change of these capabilities during the lifetime of the
certificate. certificate.
Change in the subject's capabilities during the lifetime of a Change in the subject's capabilities during the lifetime of a
certificate may require revocation of the certificate. Revocation certificate may require revocation of the certificate. Revocation
should however only be motivated if a listed algorithm is considered should however only be motivated if a listed algorithm is considered
broken and/or considered too weak to use for the adopted encryption broken and/or considered too weak for the governing security policy.
policy.
Implementers should take into account that the use of this extension Implementers should take into account that the use of this extension
does not change the fact that it is always the responsibility of the does not change the fact that it is always the responsibility of the
sender to choose sufficiently strong encryption for its information sender to choose sufficiently strong encryption for its information
disclosure. disclosure.
5 References 5 References
Normative references: Normative references:
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INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights. except as set forth therein, the authors retain all their rights.
Expires August 2005 Expires September 2005
 End of changes. 

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