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Versions: (RFC 2632) 00 01 02 03 04 05 06 07 RFC 3850

Internet Draft                                Editor:  Blake Ramsdell,
draft-ietf-smime-rfc2632bis-07.txt            Sendmail, Inc.
June 4, 2004
Expires December 4, 2004


                S/MIME Version 3.1 Certificate Handling

Status of this memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

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 as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.


Abstract

This document specifies conventions for X.509 certificate usage by
S/MIME (Secure/Multipurpose Internet Mail Extensions) agents. S/MIME
provides a method to send and receive secure MIME messages, and
certificates are an integral part of S/MIME agent processing. S/MIME
agents validate certificates as described in RFC 3280, the Internet
X.509 Public Key Infrastructure Certificate and CRL Profile. S/MIME
agents must meet the certificate processing requirements in this
document as well as those in RFC 3280.


1. Overview

S/MIME (Secure/Multipurpose Internet Mail Extensions), described in
[SMIME-MSG], provides a method to send and receive secure MIME
messages. Before using a public key to provide security services, the
S/MIME agent MUST verify that the public key is valid. S/MIME agents
MUST use PKIX certificates to validate public keys as described in the
Internet X.509 Public Key Infrastructure (PKIX) Certificate and CRL
Profile [KEYM]. S/MIME agents MUST meet the certificate processing
requirements documented in this document in addition to those stated
in [KEYM].

This specification is compatible with the Cryptographic Message Syntax
[CMS] in that it uses the data types defined by CMS. It also inherits
all the varieties of architectures for certificate-based key
management supported by CMS.



1.1 Definitions

For the purposes of this document, the following definitions apply.

ASN.1: Abstract Syntax Notation One, as defined in ITU-T X.208.

Attribute Certificate (AC): An X.509 AC is a separate structure from a
subject's public key X.509 Certificate. A subject may have multiple
X.509 ACs associated with each of its public key X.509 Certificates.
Each X.509 AC binds one or more Attributes with one of the subject's
public key X.509 Certificates. The X.509 AC syntax is defined in
[ACAUTH].

BER: Basic Encoding Rules for ASN.1, as defined in ITU-T X.209.

Certificate: A type that binds an entity's name to a public key with a
digital signature. This type is defined in the Internet X.509 Public
Key Infrastructure (PKIX) Certificate and CRL Profile [KEYM]. This
type also contains the distinguished name of the certificate issuer
(the signer), an issuer-specific serial number, the issuer's signature
algorithm identifier, a validity period, and extensions also defined
in that document.

Certificate Revocation List (CRL): A type that contains information
about certificates whose validity an issuer has prematurely revoked.
The information consists of an issuer name, the time of issue, the
next scheduled time of issue, a list of certificate serial numbers and
their associated revocation times, and extensions as defined in
[KEYM]. The CRL is signed by the issuer. The type intended by this
specification is the one defined in [KEYM].

DER: Distinguished Encoding Rules for ASN.1, as defined in ITU-T
X.690.

Receiving agent: software that interprets and processes S/MIME CMS
objects, MIME body parts that contain CMS objects, or both.

Sending agent: software that creates S/MIME CMS objects, MIME body
parts that contain CMS objects, or both.

S/MIME agent: user software that is a receiving agent, a sending
agent, or both.


1.2 Compatibility with Prior Practice of S/MIME

S/MIME version 3.1 agents should attempt to have the greatest
interoperability possible with agents for prior versions of S/MIME.
S/MIME version 2 is described in RFC 2311 through RFC 2315, inclusive
and S/MIME version 3 is described in RFC 2630 through RFC 2634
inclusive. RFC 2311 also has historical information about the
development of S/MIME.


1.3 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 [MUSTSHOULD].


1.4 Changes Since S/MIME v3 (RFC 2632)

Version 1 and Version 2 CRLs MUST be supported.

Multiple CA certificates with the same subject and public key, but
with overlapping validity periods, MUST be supported.

Version 2 attribute certificates SHOULD be supported, and version 1
attributes certificates MUST NOT be used.

The use of the MD2 digest algorithm for certificate signatures is
discouraged, and security language added.

Clarified use of email address use in certificates.  Certificates that
do not contain an email address have no requirements for verifying the
email address associated with the certificate.

Receiving agents SHOULD display certificate information when
displaying the results of signature verification.

Receiving agents MUST NOT accept a signature made with a certificate
that does not have the digitalSignature or nonRepudiation bit set.

Clarifications for the interpretation of the key usage and extended
key usage extensions.


2. CMS Options

The CMS message format allows for a wide variety of options in content
and algorithm support. This section puts forth a number of support
requirements and recommendations in order to achieve a base level of
interoperability among all S/MIME implementations. Most of the CMS
format for S/MIME messages is defined in [SMIME-MSG].


2.1 CertificateRevocationLists

Receiving agents MUST support the Certificate Revocation List (CRL)
format defined in [KEYM]. If sending agents include CRLs in outgoing
messages, the CRL format defined in [KEYM] MUST be used. In all cases,
both v1 and v2 CRLs MUST be supported.

All agents MUST be capable of performing revocation checks using CRLs
as specified in [KEYM]. All agents MUST perform revocation status
checking in accordance with [KEYM]. Receiving agents MUST recognize
CRLs in received S/MIME messages.

Agents SHOULD store CRLs received in messages for use in processing
later messages.


2.2 CertificateChoices

Receiving agents MUST support v1 X.509 and v3 X.509 identity
certificates as profiled in [KEYM]. End entity certificates MAY
include an Internet mail address, as described in section 3.

Receiving agents SHOULD support X.509 version 2 attribute
certificates. See [ACAUTH] for details about the profile for attribute
certificates.


2.2.1 Historical Note About CMS Certificates

The CMS message format supports a choice of certificate formats for
public key content types: PKIX, PKCS #6 Extended Certificates [PKCS6]
and PKIX Attribute Certificates.

The PKCS #6 format is not in widespread use. In addition, PKIX
certificate extensions address much of the same functionality and
flexibility as was intended in the PKCS #6. Thus, sending and
receiving agents MUST NOT use PKCS #6 extended certificates.

X.509 version 1 attribute certificates are also not widely
implemented, and have been superseded with version 2 attribute
certificates. Sending agents MUST NOT send version 1 attribute
certificates.


2.3 CertificateSet

Receiving agents MUST be able to handle an arbitrary number of
certificates of arbitrary relationship to the message sender and to
each other in arbitrary order. In many cases, the certificates
included in a signed message may represent a chain of certification
from the sender to a particular root. There may be, however,
situations where the certificates in a signed message may be unrelated
and included for convenience.

Sending agents SHOULD include any certificates for the user's public
key(s) and associated issuer certificates. This increases the
likelihood that the intended recipient can establish trust in the
originator's public key(s). This is especially important when sending
a message to recipients that may not have access to the sender's
public key through any other means or when sending a signed message to
a new recipient. The inclusion of certificates in outgoing messages
can be omitted if S/MIME objects are sent within a group of
correspondents that has established access to each other's
certificates by some other means such as a shared directory or manual
certificate distribution. Receiving S/MIME agents SHOULD be able to
handle messages without certificates using a database or directory
lookup scheme.

A sending agent SHOULD include at least one chain of certificates up
to, but not including, a Certificate Authority (CA) that it believes
that the recipient may trust as authoritative. A receiving agent
MUST be able to handle an arbitrarily large number of certificates
and chains.

Agents MAY send CA certificates, that is, certificates which can be
considered the "root" of other chains, and which MAY be self-signed.
Note that receiving agents SHOULD NOT simply trust any self-signed
certificates as valid CAs, but SHOULD use some other mechanism to
determine if this is a CA that should be trusted. Also note that when
certificates contain DSA public keys the parameters may be located in
the root certificate. This would require that the recipient possess
both the end-entity certificate as well as the root certificate to
perform a signature verification, and is a valid example of a case
where transmitting the root certificate may be required.

Receiving agents MUST support chaining based on the distinguished name
fields. Other methods of building certificate chains MAY be supported.

Receiving agents SHOULD support the decoding of X.509 attribute
certificates included in CMS objects. All other issues regarding the
generation and use of X.509 attribute certificates are outside of the
scope of this specification. One specification that addresses
attribute certificate use is defined in [SECLABEL].


3. Using Distinguished Names for Internet Mail

End-entity certificates MAY contain an Internet mail address as
described in [RFC-2822]. The address must be an "addr-spec" as defined
in Section 3.4.1 of that specification. The email address SHOULD be in
the subjectAltName extension, and SHOULD NOT be in the subject
distinguished name.

Receiving agents MUST recognize and accept certificates that contain
no email address. Receiving agents MUST recognize email addresses in
the subjectAltName field. Receiving agents MUST recognize email
addresses in the Distinguished Name field in the PKCS #9 [PKCS9]
emailAddress attribute:

pkcs-9-at-emailAddress OBJECT IDENTIFIER ::=
  {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1 }

Note that this attribute MUST be encoded as IA5String.

Sending agents SHOULD make the address in the From or Sender header in
a mail message match an Internet mail address in the signer's
certificate. Receiving agents MUST check that the address in the From
or Sender header of a mail message matches an Internet mail address,
if present, in the signer's certificate, if mail addresses are present
in the certificate. A receiving agent SHOULD provide some explicit
alternate processing of the message if this comparison fails, which
may be to display a message that shows the recipient the addresses in
the certificate or other certificate details.

A receiving agent SHOULD display a subject name or other certificate
details when displaying an indication of successful or unsuccessful
signature verification.

All subject and issuer names MUST be populated (i.e. not an empty
SEQUENCE) in S/MIME-compliant X.509 identity certificates, except that
the subject DN in a user's (i.e. end-entity) certificate MAY be an
empty SEQUENCE in which case the subjectAltName extension will include
the subject's identifier and MUST be marked as critical.


4. Certificate Processing

A receiving agent needs to provide some certificate retrieval
mechanism in order to gain access to certificates for recipients of
digital envelopes. There are many ways to implement certificate
retrieval mechanisms. X.500 directory service is an excellent example
of a certificate retrieval-only mechanism that is compatible with
classic X.500 Distinguished Names. Another method under consideration
by the IETF is to provide certificate retrieval services as part of
the existing Domain Name System (DNS). Until such mechanisms are
widely used, their utility may be limited by the small number of
correspondent's certificates that can be retrieved. At a minimum, for
initial S/MIME deployment, a user agent could automatically generate a
message to an intended recipient requesting that recipient's
certificate in a signed return message.

Receiving and sending agents SHOULD also provide a mechanism to allow
a user to "store and protect" certificates for correspondents in such
a way so as to guarantee their later retrieval. In many environments,
it may be desirable to link the certificate retrieval/storage
mechanisms together in some sort of certificate database. In its
simplest form, a certificate database would be local to a particular
user and would function in a similar way as a "address book" that
stores a user's frequent correspondents. In this way, the certificate
retrieval mechanism would be limited to the certificates that a user
has stored (presumably from incoming messages). A comprehensive
certificate retrieval/storage solution may combine two or more
mechanisms to allow the greatest flexibility and utility to the user.
For instance, a secure Internet mail agent may resort to checking a
centralized certificate retrieval mechanism for a certificate if it
can not be found in a user's local certificate storage/retrieval
database.

Receiving and sending agents SHOULD provide a mechanism for the import
and export of certificates, using a CMS certs-only message. This
allows for import and export of full certificate chains as opposed to
just a single certificate. This is described in [SMIME-MSG].

Agents MUST handle multiple valid Certification Authority (CA)
certificates containing the same subject name and the same public keys
but with overlapping validity intervals.


4.1 Certificate Revocation Lists

In general, it is always better to get the latest CRL information from
a CA than to get information stored away from incoming messages. A
receiving agent SHOULD have access to some certificate revocation list
(CRL) retrieval mechanism in order to gain access to certificate
revocation information when validating certification paths. A
receiving or sending agent SHOULD also provide a mechanism to allow a
user to store incoming certificate revocation information for
correspondents in such a way so as to guarantee its later retrieval.

Receiving and sending agents SHOULD retrieve and utilize CRL
information every time a certificate is verified as part of a
certification path validation even if the certificate was already
verified in the past. However, in many instances (such as off-line
verification) access to the latest CRL information may be difficult or
impossible. The use of CRL information, therefore, may be dictated by
the value of the information that is protected. The value of the CRL
information in a particular context is beyond the scope of this
specification but may be governed by the policies associated with
particular certification paths.

All agents MUST be capable of performing revocation checks using CRLs
as specified in [KEYM]. All agents MUST perform revocation status
checking in accordance with [KEYM]. Receiving agents MUST recognize
CRLs in received S/MIME messages.


4.2 Certification Path Validation

In creating a user agent for secure messaging, certificate, CRL, and
certification path validation SHOULD be highly automated while still
acting in the best interests of the user. Certificate, CRL, and path
validation MUST be performed as per [KEYM] when validating a
correspondent's public key. This is necessary before using a public
key to provide security services such as: verifying a signature;
encrypting a content-encryption key (ex: RSA); or forming a pairwise
symmetric key (ex: Diffie-Hellman) to be used to encrypt or decrypt a
content-encryption key.

Certificates and CRLs are made available to the path validation
procedure in two ways: a) incoming messages, and b) certificate and
CRL retrieval mechanisms. Certificates and CRLs in incoming messages
are not required to be in any particular order nor are they required
to be in any way related to the sender or recipient of the message
(although in most cases they will be related to the sender). Incoming
certificates and CRLs SHOULD be cached for use in path validation and
optionally stored for later use. This temporary certificate and CRL
cache SHOULD be used to augment any other certificate and CRL
retrieval mechanisms for path validation on incoming signed messages.


4.3 Certificate and CRL Signing Algorithms

Certificates and Certificate Revocation Lists (CRLs) are signed by the
certificate issuer. A receiving agent MUST be capable of verifying the
signatures on certificates and CRLs made with id-dsa-with-sha1
[CMSALG].

A receiving agent MUST be capable of verifying the signatures on
certificates and CRLs made with md5WithRSAEncryption and
sha1WithRSAEncryption signature algorithms with key sizes from 512
bits to 2048 bits described in [CMSALG].

Because of the security issues surrounding MD2 [RC95], and in light of
current use, md2WithRSAEncryption MAY be supported.


4.4 PKIX Certificate Extensions

PKIX describes an extensible framework in which the basic certificate
information can be extended and how such extensions can be used to
control the process of issuing and validating certificates. The PKIX
Working Group has ongoing efforts to identify and create extensions
which have value in particular certification environments. Further,
there are active efforts underway to issue PKIX certificates for
business purposes. This document identifies the minimum required set
of certificate extensions which have the greatest value in the S/MIME
environment. The syntax and semantics of all the identified extensions
are defined in [KEYM].

Sending and receiving agents MUST correctly handle the basic
constraints, key usage, authority key identifier, subject key
identifier, and subject alternative names certificate extensions when
they appear in end-entity and CA certificates. Some mechanism SHOULD
exist to gracefully handle other certificate extensions when they
appear in end-entity or CA certificates.

Certificates issued for the S/MIME environment SHOULD NOT contain any
critical extensions (extensions that have the critical field set to
TRUE) other than those listed here. These extensions SHOULD be marked
as non-critical unless the proper handling of the extension is deemed
critical to the correct interpretation of the associated certificate.
Other extensions may be included, but those extensions SHOULD NOT be
marked as critical.

Interpretation and syntax for all extensions MUST follow [KEYM],
unless otherwise specified here.


4.4.1 Basic Constraints Certificate Extension

The basic constraints extension serves to delimit the role and
position of an issuing authority or end-entity certificate plays in a
certification path.

For example, certificates issued to CAs and subordinate CAs contain a
basic constraint extension that identifies them as issuing authority
certificates. End-entity certificates contain an extension that
constrains the certificate from being an issuing authority
certificate.

Certificates SHOULD contain a basicConstraints extension in CA
certificates, and SHOULD NOT contain that extension in end entity
certificates.


4.4.2 Key Usage Certificate Extension

The key usage extension serves to limit the technical purposes for
which a public key listed in a valid certificate may be used. Issuing
authority certificates may contain a key usage extension that
restricts the key to signing certificates, certificate revocation
lists and other data.

For example, a certification authority may create subordinate issuer
certificates which contain a key usage extension which specifies that
the corresponding public key can be used to sign end user certificates
and sign CRLs.

If a key usage extension is included in a PKIX certificate, then it
MUST be marked as critical.

S/MIME receiving agents MUST NOT accept the signature of a message if
it was verified using a certificate which contains the key usage
extension without either the digitalSignature or nonRepudiation bit
set. Sometimes S/MIME is used as a secure message transport for
applications beyond interpersonal messaging. In such cases, the
S/MIME-enabled application can specify additional requirements
concerning the digitalSignature or nonRepudiation bits within this
extension.

If the key usage extension is not specified, receiving clients MUST
presume that the digitalSignature and nonRepudiation bits are set.


4.4.3 Subject Alternative Name Extension

The subject alternative name extension is used in S/MIME as the
preferred means to convey the RFC-2822 email address(es) that
correspond to the entity for this certificate. Any RFC-2822 email
addresses present MUST be encoded using the rfc822Name CHOICE of the
GeneralName type. Since the SubjectAltName type is a SEQUENCE OF
GeneralName, multiple RFC-2822 email addresses MAY be present.


4.4.4 Extended Key Usage Extension

The extended key usage extension also serves to limit the technical
purposes for which a public key listed in a valid certificate may be
used. The set of technical purposes for the certificate therefore are
the intersection of the uses indicated in the key usage and extended
key usage extensions.

For example, if the certificate contains a key usage extension
indicating digital signature and an extended key usage extension which
includes the email protection OID, then the certificate may be used
for signing but not encrypting S/MIME messages. If the certificate
contains a key usage extension indicating digital signature, but no
extended key usage extension then the certificate may also be used to
sign but not encrypt S/MIME messages.

If the extended key usage extension is present in the certificate then
interpersonal message S/MIME receiving agents MUST check it contains
either the emailProtection or the anyExtendedKeyUsage OID as defined
in [KEYM]. S/MIME uses other than interpersonal messaging MAY require
the explicit presence of the extended key usage extension or other
OIDs to be present in the extension or both.


5. Security Considerations

All of the security issues faced by any cryptographic application must
be faced by a S/MIME agent. Among these issues are protecting the
user's private key, preventing various attacks, and helping the user
avoid mistakes such as inadvertently encrypting a message for the
wrong recipient. The entire list of security considerations is beyond
the scope of this document, but some significant concerns are listed
here.

When processing certificates, there are many situations where the
processing might fail. Because the processing may be done by a user
agent, a security gateway, or other program, there is no single way to
handle such failures. Just because the methods to handle the failures
has not been listed, however, the reader should not assume that they
are not important. The opposite is true: if a certificate is not
provably valid and associated with the message, the processing
software should take immediate and noticeable steps to inform the end
user about it.

Some of the many places where signature and certificate checking might
fail include:

- no Internet mail addresses in a certificate matches the sender of a
  message, if the certificate contains at least one mail address
- no certificate chain leads to a trusted CA
- no ability to check the CRL for a certificate
- an invalid CRL was received
- the CRL being checked is expired
- the certificate is expired
- the certificate has been revoked

There are certainly other instances where a certificate may be
invalid, and it is the responsibility of the processing software to
check them all thoroughly, and to decide what to do if the check
fails.


At the Selected Areas in Cryptography '95 conference in May 1995,
Rogier and Chauvaud presented an attack on MD2 that can nearly find
collisions [RC95]. Collisions occur when one can find two different
messages that generate the same message digest. A checksum operation
in MD2 is the only remaining obstacle to the success of the attack.
For this reason, the use of MD2 for new applications is discouraged.
It is still reasonable to use MD2 to verify existing signatures, as
the ability to find collisions in MD2 does not enable an attacker to
find new messages having a previously computed hash value.

It is possible for there to be multiple unexpired CRLs for a CA. If an
agent is consulting CRLs for certificate validation, it SHOULD make
sure that the most recently issued CRL for that CA is consulted, since
an S/MIME message sender could deliberately include an older unexpired
CRL in an S/MIME message. This older CRL might not include recent
revoked certificates, which might lead an agent to accept a
certificate that has been revoked in a subsequent CRL.

When determining the time for a certificate validity check, agents
have to be careful to use a reliable time. Unless it is from a trusted
agent, this time MUST NOT be the SigningTime attribute found in an
S/MIME message. For most sending agents, the SigningTime attribute
could be deliberately set to direct the receiving agent to check a CRL
that could have out-of-date revocation status for a certificate, or
cause an improper result when checking the Validity field of a
certificate.


A. Normative References

[ACAUTH] "An Internet Attribute Certificate Profile for
Authorization", RFC 3281

[CMS] "Cryptographic Message Syntax (CMS)",
draft-ietf-smime-3369bis-04

[CMSALG] "Cryptographic Message Syntax (CMS) Algorithms", RFC 3370

[KEYM] "Internet X.509 Public Key Infrastructure Certificate and CRL
Profile", RFC 3280

[KEYMALG] "Algorithms and Identifiers for the Internet X.509 Public
Key Infrastructure Certificate and CRL Profile ", RFC 3279

[MUSTSHOULD] "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119

[PKCS9] "PKCS #9: Selected Object Classes and Attribute Types Version
2.0", RFC 2985

[RFC-2822], "Internet Message Format", RFC 2822

[SMIME-MSG] "S/MIME Version 3 Message Specification ", Internet Draft
draft-ietf-smime-msg


B. Informative References

[CERTV2] "S/MIME Version 2 Certificate Handling", RFC 2312

[PKCS6] RSA Laboratories, "PKCS #6: Extended-Certificate Syntax
Standard", November 1993

[RC95] Rogier, N. and Chauvaud, P., "The compression function of MD2
is not collision free," Presented at Selected Areas in Cryptography
'95, May 1995

[SECLABEL] "Implementing Company Classification Policy with the S/MIME
Security Label", RFC 3114

[X.500] ITU-T Recommendation X.500 (1997) | ISO/IEC 9594-1:1997,
Information technology - Open Systems Interconnection - The Directory:
Overview of concepts, models and services

[X.501] ITU-T Recommendation X.501 (1997) | ISO/IEC 9594-2:1997,
Information technology - Open Systems Interconnection - The Directory:
Models

[X.509] ITU-T Recommendation X.509 (1997) | ISO/IEC 9594-8:1997,
Information technology - Open Systems Interconnection - The Directory:
Authentication framework

[X.520] ITU-T Recommendation X.520 (1997) | ISO/IEC 9594-6:1997,
Information technology - Open Systems Interconnection - The Directory:
Selected attribute types.


C. Acknowledgements

Many thanks go out to the other authors of the S/MIME v2 RFC: Steve
Dusse, Paul Hoffman and Jeff Weinstein. Without v2, there wouldn't be
a v3.

A number of the members of the S/MIME Working Group have also worked
very hard and contributed to this document. Any list of people is
doomed to omission and for that I apologize. In alphabetical order,
the following people stand out in my mind due to the fact that they
made direct contributions to this document.

Bill Flanigan
Trevor Freeman
Elliott Ginsburg
Paul Hoffman
Russ Housley
David P. Kemp
Michael Myers
John Pawling
Denis Pinkas
Jim Schaad


D. Editor's address

Blake Ramsdell
Sendmail, Inc.
704 228th Ave NE #775
Sammamish, WA  98074

blake@sendmail.com


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