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Network Working Group B. Ramsdell, Editor Request for Comments: 2632 Worldtalk Category: Standards Track June 1999

               S/MIME Version 3 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 (1999).  All Rights Reserved.

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 certify 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 memo, the following definitions apply.
 ASN.1: Abstract Syntax Notation One, as defined in ITU-T X.680-689.
 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 [X.509]

Ramsdell Standards Track [Page 1] RFC 2632 S/MIME Version 3 Certificate Handling June 1999

 BER: Basic Encoding Rules for ASN.1, as defined in ITU-T X.690.
 Certificate: A type that binds an entity's distinguished 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
 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 agents should attempt to have the greatest
 interoperability possible with S/MIME version 2 agents. S/MIME
 version 2 is described in RFC 2311 through RFC 2315, 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",
 document are to be interpreted as described in [MUSTSHOULD].

Ramsdell Standards Track [Page 2] RFC 2632 S/MIME Version 3 Certificate Handling June 1999

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

2.2 CertificateChoices

 Receiving agents MUST support PKIX v1 and PKIX v3 certificates. See
 [KEYM] for details about the profile for certificate formats. End
 entity certificates MAY include an Internet mail address, as
 described in section 3.1.
 Receiving agents SHOULD support X.509 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 and
 X.509 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

Ramsdell Standards Track [Page 3] RFC 2632 S/MIME Version 3 Certificate Handling June 1999

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
 SHOULD be able to handle an arbitrarily large number of certificates
 and chains.
 Agents MAY send CA certificates, that is, certificates that are
 self-signed and can be considered the "root" of other chains. 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 in
 the case of DSA certificates the parameters may be located in the
 root certificate.  This would require that the recipient possess the
 root certificate in order 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 but are not currently recommended.

Ramsdell Standards Track [Page 4] RFC 2632 S/MIME Version 3 Certificate Handling June 1999

 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.

3. Using Distinguished Names for Internet Mail

 End-entity certificates MAY contain an Internet mail address as
 described in [RFC-822]. The address must be an "addr-spec" as defined
 in Section 6.1 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 email addresses in the subjectAltName
 field. Receiving agents MUST recognize email addresses in the
 Distinguished Name field in the PKCS #9 emailAddress attribute.
 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
 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.
 All subject and issuer names MUST be populated (i.e. not an empty
 SEQUENCE) in S/MIME-compliant PKIX 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. The PKIX Working Group is
 investigating other mechanisms such as directory servers. 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

Ramsdell Standards Track [Page 5] RFC 2632 S/MIME Version 3 Certificate Handling June 1999

 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
 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-

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 certificate
 chains.  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
 Receiving and sending agents SHOULD retrieve and utilize CRL
 information every time a certificate is verified as part of a
 certificate chain 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

Ramsdell Standards Track [Page 6] RFC 2632 S/MIME Version 3 Certificate Handling June 1999

 CRL information in a particular context is beyond the scope of this
 memo but may be governed by the policies associated with particular
 certificate hierarchies.
 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 Certificate Chain Validation

 In creating a user agent for secure messaging, certificate, CRL, and
 certificate chain validation SHOULD be highly automated while still
 acting in the best interests of the user. Certificate, CRL, and chain
 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 chain 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 chain 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 chain validation on incoming signed

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 [DSS].
 A receiving agent SHOULD be capable of verifying the signatures on
 certificates and CRLs made with md2WithRSAEncryption,
 md5WithRSAEncryption and sha-1WithRSAEncryption signature algorithms
 with key sizes from 512 bits to 2048 bits described in [PKCS#1V2].

Ramsdell Standards Track [Page 7] RFC 2632 S/MIME Version 3 Certificate Handling June 1999

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 minumum 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 Certificate Extension, the Key Usage Certificate
 Extension, authorityKeyID, subjectKeyID, and the subjectAltNames when
 they appear in end-user certificates. Some mechanism SHOULD exist to
 handle the defined certificate extensions when they appear in
 intermediate 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
 chain of certificates.
 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
 Certificates SHOULD contain a basicConstraints extension in CA
 certificates, and SHOULD NOT contain that extension in end entity

Ramsdell Standards Track [Page 8] RFC 2632 S/MIME Version 3 Certificate Handling June 1999

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 keyUsage extension which specifies that
 the corresponding public key can be used to sign end user certs and
 sign CRLs.
 If a key usage extension is included in a PKIX certificate, then it
 MUST be marked as critical. Key Usage in Diffie-Hellman Key Exchange Certificates

 For Diffie-Hellman key exchange certificates (certificates in which
 the subject public key algorithm is dhpublicnumber), if the keyUsage
 keyAgreement bit is set to 1 AND if the public key is to be used to
 form a pairwise key to decrypt data, then the S/MIME agent MUST only
 use the public key if the keyUsage encipherOnly bit is set to 0. If
 the keyUsage keyAgreement bit is set to 1 AND if the key is to be
 used to form a pairwise key to encrypt data, then the S/MIME agent
 MUST only use the public key if the keyUsage decipherOnly bit is set
 to 0.

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-822 email address(es) that
 correspond to the entity for this certificate. Any RFC-822 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-822 email addresses MAY be present.

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.

Ramsdell Standards Track [Page 9] RFC 2632 S/MIME Version 3 Certificate Handling June 1999

 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 noticable 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 match the sender

of a message

  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

Ramsdell Standards Track [Page 10] RFC 2632 S/MIME Version 3 Certificate Handling June 1999

A. References

 [CERTV2]     Dusse, S., Hoffman, P. and B. Ramsdell,"S/MIME Version 2
              Certificate Handling", RFC 2312, March 1998.
 [CMS]        Housley, R., "Cryptographic Message Syntax", RFC 2630,
              June 1999.
 [DSS]        NIST FIPS PUB 186, "Digital Signature Standard", 18 May
 [KEYM]       Housley, R., Ford, W., Polk, W. and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and CRL
              Profile", RFC 2459, January 1999.
 [MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
 [PKCS#1V2]   Kaliski, B., "PKCS #1: RSA Cryptography Specifications
              Version 2.0", RFC 2437, October 1998.
 [RFC-822]    Crocker, D., "Standard For The Format Of ARPA Internet
              Text Messages", STD 11, RFC 822, August 1982.
 [SMIME-MSG]  Ramsdell, B., Editor, "S/MIME Version 3 Message
              Specification", RFC 2633, June 1999.
 [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
 [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 11] RFC 2632 S/MIME Version 3 Certificate Handling June 1999

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 Elliott Ginsburg Paul Hoffman Russ Housley Michael
 Myers John Pawling Denis Pinkas Jim Schaad

Editor's Address

 Blake Ramsdell
 17720 NE 65th St Ste 201
 Redmond, WA 98052
 Phone: +1 425 376 0225

Ramsdell Standards Track [Page 12] RFC 2632 S/MIME Version 3 Certificate Handling June 1999

Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
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 or assist in its implementation may be prepared, copied, published
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 followed, or as required to translate it into languages other than
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an


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

Ramsdell Standards Track [Page 13]

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