path: root/doc/rfc/rfc3850.txt

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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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]
^L
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:

   -  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




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


Full 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.

   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.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.









Ramsdell                    Standards Track                    [Page 16]
^L