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rfc:rfc3850

Network Working Group B. Ramsdell, Editor Request for Comments: 3850 Sendmail, Inc. Obsoletes: 2632 July 2004 Category: Standards Track

 Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.1
                        Certificate Handling

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 (2004).

Abstract

 This document specifies conventions for X.509 certificate usage by
 Secure/Multipurpose Internet Mail Extensions (S/MIME) 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.

Table of Contents

 1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1.  Definitions. . . . . . . . . . . . . . . . . . . . . . .  2
     1.2.  Compatibility with Prior Practice of S/MIME. . . . . . .  3
     1.3.  Terminology. . . . . . . . . . . . . . . . . . . . . . .  3
     1.4.  Changes Since S/MIME v3 (RFC 2632) . . . . . . . . . . .  3
 2.  CMS Options. . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1 . CertificateRevocationLists . . . . . . . . . . . . . . .  4
     2.2.  CertificateChoices . . . . . . . . . . . . . . . . . . .  4
     2.3.  CertificateSet . . . . . . . . . . . . . . . . . . . . .  5
 3. Using Distinguished Names for Internet Mail . . . . . . . . . .  6
 4.  Certificate Processing . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Certificate Revocation Lists . . . . . . . . . . . . . .  8
     4.2.  Certification Path Validation. . . . . . . . . . . . . .  8
     4.3.  Certificate and CRL Signing Algorithms . . . . . . . . .  9

Ramsdell Standards Track [Page 1] RFC 3850 S/MIME 3.1 Certificate Handling July 2004

     4.4.  PKIX Certificate Extensions. . . . . . . . . . . . . . .  9
 5.  Security Considerations. . . . . . . . . . . . . . . . . . . . 11
 A.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     A.1.  Normative References . . . . . . . . . . . . . . . . . . 13
     A.2.  Informative References . . . . . . . . . . . . . . . . . 14
 B.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
 C.  Editor's Address . . . . . . . . . . . . . . . . . . . . . . . 15
 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 16

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
 [X.208-88].
 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].
 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.

Ramsdell Standards Track [Page 2] RFC 3850 S/MIME 3.1 Certificate Handling July 2004

 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].
 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.

Ramsdell Standards Track [Page 3] RFC 3850 S/MIME 3.1 Certificate Handling July 2004

 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.

Ramsdell Standards Track [Page 4] RFC 3850 S/MIME 3.1 Certificate Handling July 2004

 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

Ramsdell Standards Track [Page 5] RFC 3850 S/MIME 3.1 Certificate Handling July 2004

 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.  Agents are allowed to provide an alternative
 mechanism for associating an email address with a certificate that
 does not contain an email address, such as through the use of the
 agent's address book, if available.  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.

Ramsdell Standards Track [Page 6] RFC 3850 S/MIME 3.1 Certificate Handling July 2004

 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 an
 "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].

Ramsdell Standards Track [Page 7] RFC 3850 S/MIME 3.1 Certificate Handling July 2004

 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

Ramsdell Standards Track [Page 8] RFC 3850 S/MIME 3.1 Certificate Handling July 2004

 (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

Ramsdell Standards Track [Page 9] RFC 3850 S/MIME 3.1 Certificate Handling July 2004

 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 that 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.

Ramsdell Standards Track [Page 10] RFC 3850 S/MIME 3.1 Certificate Handling July 2004

 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(s) 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 that 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

Ramsdell Standards Track [Page 11] RFC 3850 S/MIME 3.1 Certificate Handling July 2004

 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:
  1. no Internet mail addresses in a certificate matches the sender of

a message, if the certificate contains at least one mail address

  1. no certificate chain leads to a trusted CA
  2. no ability to check the CRL for a certificate
  3. an invalid CRL was received
  4. the CRL being checked is expired
  5. the certificate is expired
  6. 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

Ramsdell Standards Track [Page 12] RFC 3850 S/MIME 3.1 Certificate Handling July 2004

 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. References

A.1. Normative References

 [ACAUTH]     Farrell, S. and R. Housley, "An Internet Attribute
              Certificate Profile for Authorization", RFC 3281, April
              2002.
 [CMS]        Housely, R., "Cryptographic Message Syntax (CMS)", RFC
              3852, July 2004.
 [CMSALG]     Housley, R., "Cryptographic Message Syntax (CMS)
              Algorithms", RFC 3370, August 2002.
 [KEYM]       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.
 [KEYMALG]    Bassham, L., Polk, W., and R. Housley, "Algorithms and
              Identifiers for the Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation
              List (CRL) Profile", RFC 3279, April 2002.
 [MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
 [PKCS9]      Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
              Classes and Attribute Types Version 2.0", RFC 2985,
              November 2000.
 [RFC-2822],  Resnick, P., "Internet Message Format", RFC 2822, April
              2001.
 [SMIME-MSG]  Ramsdell, B., Ed., "S/MIME Version 3.1 Message
              Specification", RFC 3851, July 2004.
 [x.208-88]   ITU-T. Recommendation X.208: Specification of Abstract
              Syntax Notation One (ASN.1). 1988.

Ramsdell Standards Track [Page 13] RFC 3850 S/MIME 3.1 Certificate Handling July 2004

A.2. Informative References

 [CERTV2]     Dusse, S., Hoffman, P., Ramsdell, B., and J. Weinstein,
              "S/MIME Version 2 Certificate Handling", RFC 2312, March
              1998.
 [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]   Nicolls, W., "Implementing Company Classification Policy
              with the S/MIME Security Label", RFC 3114, May 2002.
 [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.

Ramsdell Standards Track [Page 14] RFC 3850 S/MIME 3.1 Certificate Handling July 2004

B. 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

C. Editor's Address

 Blake Ramsdell
 Sendmail, Inc.
 704 228th Ave NE #775
 Sammamish, WA  98074
 EMail: blake@sendmail.com

Ramsdell Standards Track [Page 15] RFC 3850 S/MIME 3.1 Certificate Handling July 2004

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Ramsdell Standards Track [Page 16]

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