Network Working Group                                           S. Dusse
Request for Comments: 2312                             RSA Data Security
Category: Informational                                       P. Hoffman
                                                Internet Mail Consortium
                                                             B. Ramsdell
                                                            J. Weinstein
                                                              March 1998

                 S/MIME Version 2 Certificate Handling

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  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. In order to validate the keys of a message sent to it, an
   S/MIME agent needs to certify that the key is valid. This memo
   describes the mechanisms S/MIME uses to create and validate keys
   using certificates.

   This specification is compatible with PKCS #7 in that it uses the
   data types defined by PKCS #7. It also inherits all the varieties of
   architectures for certificate-based key management supported by PKCS
   #7.  Note that the method S/MIME messages make certificate requests
   is defined in [SMIME-MSG].

   In order to handle S/MIME certificates, an agent has to follow
   specifications in this memo, as well as some of the specifications
   listed in the following documents:

    - "PKCS #1: RSA Encryption", [PKCS-1].
    - "PKCS #7: Cryptographic Message Syntax", [PKCS-7]
    - "PKCS #10: Certification Request Syntax", [PKCS-10].

Dusse, et. al.               Informational                      [Page 1]

RFC 2312        S/MIME Version 2 Certificate Handling         March 1998

   Please note: The information in this document is historical material
   being published for the public record. It is not an IETF standard.
   The use of the word "standard" in this document indicates a standard
   for adopters of S/MIME version 2, not an IETF standard.

1.1 Definitions

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

   ASN.1: Abstract Syntax Notation One, as defined in CCITT X.208.

   BER: Basic Encoding Rules for ASN.1, as defined in CCITT X.209.

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

   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, and a list of certificate serial
   numbers and their associated revocation times. 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 CCITT

1.2 Compatibility with Prior Practice of S/MIME

   Appendix C contains important information about how S/MIME agents
   following this specification should act in order to have the greatest
   interoperability with earlier implementations of S/MIME.

1.3 Terminology

   Throughout this memo, the terms MUST, MUST NOT, SHOULD, and SHOULD
   NOT are used in capital letters. This conforms to the definitions in
   [MUSTSHOULD].  [MUSTSHOULD] defines the use of these key words to
   help make the intent of standards track documents as clear as
   possible. The same key words are used in this document to help
   implementors achieve interoperability.

Dusse, et. al.               Informational                      [Page 2]

RFC 2312        S/MIME Version 2 Certificate Handling         March 1998

2. PKCS #7 Options

   The PKCS #7 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 PKCS #7 format for S/MIME messages is defined in [SMIME-MSG].

2.1 CertificateRevocationLists

   Receiving agents MUST support for 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 validate CRLs and check certificates against CRLs, if
   available, in accordance with [KEYM]. All agents SHOULD check the
   nextUpdate field in the CRL against the current time. If the current
   time is later than the nextUpdate time, the action that the agent
   takes is a local decision. For instance, it could warn a human user,
   it could retrieve a new CRL if able, and so on.

   Receiving agents MUST recognize CRLs in received S/MIME messages.

   Clients MUST use revocation information included as a CRL in an
   S/MIME message when verifying the signature and certificate path
   validity in that message.  Clients SHOULD store CRLs received in
   messages for use in processing later messages.

   Clients 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 ExtendedCertificateOrCertificate

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

2.2.1 Historical Note About PKCS #7 Certificates

   The PKCS #7 message format supports a choice of certificate two
   formats for public key content types: X.509 and PKCS #6 Extended
   Certificates. The PKCS #6 format is not in widespread use. In
   addition, proposed revisions of X.509 certificates 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

Dusse, et. al.               Informational                      [Page 3]

RFC 2312        S/MIME Version 2 Certificate Handling         March 1998

2.3 ExtendedCertificateAndCertificates

   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.

   Clients 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.

   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.

Dusse, et. al.               Informational                      [Page 4]

RFC 2312        S/MIME Version 2 Certificate Handling         March 1998

3. Distinguished Names in Certificates

3.1 Using Distinguished Names for Internet Mail

   The format of an X.509 certificate includes fields for the subject
   name and issuer name. The subject name identifies the owner of a
   particular public key/private key pair while the issuer name is meant
   to identify the entity that "certified" the subject (that is, who
   signed the subject's certificate). The subject name and issuer name
   are defined by X.509 as Distinguished Names.

   Distinguished Names are defined by a CCITT standard X.501 [X.501]. A
   Distinguished Name is broken into one or more Relative Distinguished
   Names.  Each Relative Distinguished Name is comprised of one or more
   Attribute-Value Assertions. Each Attribute-Value Assertion consists
   of a Attribute Identifier and its corresponding value information,
   such as CountryName=US. Distinguished Names were intended to identify
   entities in the X.500 directory tree [X.500]. Each Relative
   Distinguished Name can be thought of as a node in the tree which is
   described by some collection of Attribute-Value Assertions. The
   entire Distinguished Name is some collection of nodes in the tree
   that traverse a path from the root of the tree to some end node which
   represents a particular entity.

   The goal of the directory was to provide an infrastructure to
   uniquely name every communications entity everywhere. However,
   adoption of a global X.500 directory infrastructure has been slower
   than expected. Consequently, there is no requirement for X.500
   directory service provision in the S/MIME environment, although such
   provision would almost undoubtedly be of great value in facilitating
   key management for S/MIME.

   The use of Distinguished Names in accordance with the X.500 directory
   is not very widespread. By contrast, Internet mail addresses, as
   described in RFC 822 [RFC-822], are used almost exclusively in the
   Internet environment to identify originators and recipients of
   messages. However, Internet mail addresses bear no resemblance to
   X.500 Distinguished Names (except, perhaps, that they are both
   hierarchical in nature). Some method is needed to map Internet mail
   addresses to entities that hold public keys. Some people have

Dusse, et. al.               Informational                      [Page 5]

RFC 2312        S/MIME Version 2 Certificate Handling         March 1998

   suggested that the X.509 certificate format should be abandoned in
   favor of other binding mechanisms. Instead, S/MIME keeps the X.509
   certificate and Distinguished Name mechanisms while tailoring the
   content of the naming information to suit the Internet mail

   End-entity certificates MUST 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.

   Receiving agents MUST recognize email addresses in the subjectAltName
   field. Receiving agents MUST recognize email addresses in the
   Distinguished Name field.

   Sending agents SHOULD make the address in the From 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 header of a
   mail message matches an Internet mail address in the signer's
   certificate. A receiving agent MUST provide some explicit alternate
   processing of the message if this comparison fails, which may be to
   reject the message.

3.2 Required Name Attributes

   Receiving agents MUST support parsing of zero, one, or more instances
   of each of the following set of name attributes within the
   Distinguished Names in certificates.

   Sending agents MUST include the Internet mail address during
   Distinguished Name creation. Guidelines for the inclusion, omission,
   and ordering of the remaining name attributes during the creation of
   a distinguished name will most likely be dictated by the policies
   associated with the certification service which will certify the
   corresponding name and public key.


Dusse, et. al.               Informational                      [Page 6]

RFC 2312        S/MIME Version 2 Certificate Handling         March 1998

   All attributes other than EmailAddress are described in X.520
   [X.520].  EmailAddress is an IA5String that can have multiple
   attribute values.

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. Another method under consideration by
   the IETF is to provide certificate retrieval services as part of the
   existing Domain Name System (DNS). Until such mechanisms are widely
   used, their utility may be limited by the small number of
   correspondent's certificates that can be retrieved. At a minimum, for
   initial S/MIME deployment, a user agent could automatically generate
   a message to an intended recipient requesting that recipient's
   certificate in a signed return message.

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

   Receiving and sending agents SHOULD provide a mechanism for the
   import and export of certificates, using a PKCS #7 certs-only
   message. This allows for import and export of full certificate chains
   as opposed to just a single certificate. This is described in

Dusse, et. al.               Informational                      [Page 7]

RFC 2312        S/MIME Version 2 Certificate Handling         March 1998

4.1 Certificate Revocation Lists

   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
   retrieval. However, it is always better to get the latest information
   from the CA than to get information stored away from incoming

   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
   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.

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 when validating a correspondent's public
   key. This is necessary when a) verifying a signature from a
   correspondent and, b) creating a digital envelope with the
   correspondent as the intended recipient.

   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

Dusse, et. al.               Informational                      [Page 8]

RFC 2312        S/MIME Version 2 Certificate Handling         March 1998

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

4.4 X.509 Version 3 Certificate Extensions

   The X.509 v3 standard 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. As such, there is still a fair amount of profiling work
   to be done before there is widespread agreement on which v3
   extensions will be used. Further, there are active efforts underway
   to issue X.509 v3 certificates for business purposes. This memo
   identifies the minumum required set of certificate extensions which
   have the greatest value in the S/MIME environment. The
   basicConstraints, and keyUsage extensions are defined in [X.509].

   Sending and receiving agents MUST correctly handle the v3 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 v3 certificate extensions when they appear in
   intermediate or CA certificates.

   Certificates issued for the S/MIME environment SHOULD NOT contain any
   critical extensions 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.

4.4.1 Basic Constraints Certificate Extension

   The basic constraints extension serves to delimit the role and
   position of an issuing authority or end-user certificate plays in a
   chain of certificates.

Dusse, et. al.               Informational                      [Page 9]

RFC 2312        S/MIME Version 2 Certificate Handling         March 1998

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

   Certificates SHOULD contain a basicContstraints extension.

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.

5. Generating Keys and Certification Requests

5.1 Binding Names and Keys

   An S/MIME agent or some related administrative utility or function
   MUST be capable of generating a certification request given a user's
   public key and associated name information. In most cases, the user's
   public key/private key pair will be generated simultaneously.
   However, there are cases where the keying information may be
   generated by an external process (such as when a key pair is
   generated on a cryptographic token or by a "key recovery" service).

   There SHOULD NOT be multiple valid (that is, non-expired and non-
   revoked) certificates for the same key pair bound to different
   Distinguished Names.  Otherwise, a security flaw exists where an
   attacker can substitute one valid certificate for another in such a
   way that can not be detected by a message recipient. If a users
   wishes to change their name (or create an alternate name), the user
   agent SHOULD generate a new key pair. If the user wishes to reuse an
   existing key pair with a new or alternate name, the user SHOULD first
   have any valid certificates for the existing public key revoked.

   In general, it is possible for a user to request certification for
   the same name and different public key from the same or different
   certification authorities.  This is acceptable both for end-entity
   and issuer certificates and can be useful in supporting a change of
   issuer keys in a smooth fashion.

Dusse, et. al.               Informational                     [Page 10]

RFC 2312        S/MIME Version 2 Certificate Handling         March 1998

   CAs that re-use their own name with distinct keys MUST include the
   AuthorityKeyIdentifier extension in certificates that they issue, and
   MUST have the SubjectKeyIdentifier extension in their own
   certificate. CAs SHOULD use these extensions uniformly.

   Clients SHOULD handle multiple valid CA certificates that certify
   different public keys but contain the same subject name (in this
   case, that CA's name).

   When selecting an appropriate issuer's certificate to use to verify a
   given certificate, clients SHOULD process the AuthorityKeyIdentifier
   and SubjectKeyIdentifier extensions.

   5.2 Using PKCS #10 for Certification Requests

   PKCS #10 is a flexible and extensible message format for representing
   the results of cryptographic operations on some data. The choice of
   naming information is largely dictated by the policies and procedures
   associated with the intended certification service.

   In addition to key and naming information, the PKCS #10 format
   supports the inclusion of optional attributes, signed by the entity
   requesting certification. This allows for information to be conveyed
   in a certification request which may be useful to the request
   process, but not necessarily part of the Distinguished Name being

   Receiving agents MUST support the identification of an RSA key with
   the rsa defined in X.509 and the rsaEncryption OID. Certification
   authorities MUST support sha-1WithRSAEncryption and
   md5WithRSAEncryption and SHOULD support MD2WithRSAEncryption for
   verification of signatures on certificate requests as described in

   For the creation and submission of certification-requests, RSA keys
   SHOULD be identified with the rsaEncryption OID and signed with the
   sha-1WithRSAEncryption signature algorithm.  Certification-requests
   MUST NOT be signed with the md2WithRSAEncryption signature algorithm.

   Certification requests MUST include a valid Internet mail address,
   either as part of the certificate (as described in 3.2) or as part of
   the PKCS #10 attribute list. Certification authorities MUST check
   that the address in the "From:" header matches either of these
   addresses. CAs SHOULD allow the CA operator to configure processing
   of messages whose addresses do not match.

Dusse, et. al.               Informational                     [Page 11]

RFC 2312        S/MIME Version 2 Certificate Handling         March 1998

   Certification authorities SHOULD support parsing of zero or one
   instance of each of the following set of certification-request
   attributes on incoming messages. Attributes that a particular
   implementation does not support may generate a warning message to the
   requestor, or may be silently ignored.  Inclusion of the following
   attributes during the creation and submission of a certification-
   request will most likely be dictated by the policies associated with
   the certification service which will certify the corresponding name
   and public key.


   postalAddress is described in [X.520].

5.2.1 Challenge Password

   The challenge-password attribute type specifies a password by which
   an entity may request certificate revocation. The interpretation of
   the password is intended to be specified by the issuer of the
   certificate; no particular interpretation is required. The
   challenge-password attribute type is intended for PKCS #10
   certification requests.

Challenge-password attribute values have ASN.1 type ChallengePassword:

ChallengePassword ::= CHOICE {
  PrintableString, T61String }

A challenge-password attribute must have a single attribute value.

It is expected that if UCS becomes an ASN.1 type
ChallengePassword will become a CHOICE type:

ChallengePassword ::= CHOICE {
    PrintableString, T61String, UNIVERSAL STRING }

5.2.2 Unstructured Address

   The unstructured-address attribute type specifies the address or
   addresses of the subject of a certificate as an unstructured ASCII or
   T.61 string.  The interpretation of the addresses is intended to be
   specified by the issuer of the certificate; no particular
   interpretation is required. A likely interpretation is as an
   alternative to the X.520 postalAddress attribute type. The
   unstructured-address attribute type is intended for PKCS #10

Dusse, et. al.               Informational                     [Page 12]

RFC 2312        S/MIME Version 2 Certificate Handling         March 1998

   certification requests.

   Unstructured-address attribute values have
   ASN.1 type UnstructuredAddress:

   UnstructuredAddress ::= CHOICE {
     PrintableString, T61String }

   An unstructured-address attribute can have multiple attribute values.

   Note: T.61's newline character (hexadecimal code 0d) is recommended
   as a line separator in multi-line addresses.

   It is expected that if UCS becomes an ASN.1 type (e.g., UNIVERSAL
   STRING), UnstructuredAddress will become a CHOICE type:

   UnstructuredAddress ::= CHOICE {
       PrintableString, T61String, UNIVERSAL STRING }

5.3 Fulfilling a Certification Request

   Certification authorities SHOULD use the sha-1WithRSAEncryption
   signature algorithms when signing certificates.

5.4 Using PKCS #7 for Fulfilled Certificate Response

   [PKCS-7] supports a degenerate case of the SignedData content type
   where there are no signers on the content (and hence, the content
   value is "irrelevant"). This degenerate case is used to convey
   certificate and CRL information. Certification authorities MUST use
   this format for returning certificate information resulting from the
   successful fulfillment of a certification request. At a minimum, the
   fulfilled certificate response MUST include the actual subject
   certificate (corresponding to the information in the certification
   request). The response SHOULD include other certificates which link
   the issuer to higher level certification authorities and
   corresponding certificate-revocation lists. Unrelated certificates
   and revocation information is also acceptable.

   Receiving agents MUST parse this degenerate PKCS #7 message type and
   handle the certificates and CRLs according to the requirements and
   recommendations in Section 4.

Dusse, et. al.               Informational                     [Page 13]

RFC 2312        S/MIME Version 2 Certificate Handling         March 1998

6. Security Considerations

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

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

Dusse, et. al.               Informational                     [Page 14]

RFC 2312        S/MIME Version 2 Certificate Handling         March 1998

A. Object Identifiers and Syntax

   Sections A.1 through A.4 are adopted from [SMIME-MSG].

A.5 Name Attributes

emailAddress OBJECT IDENTIFIER ::=

     {iso(1) member-body(2) US(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1}

     {joint-iso-ccitt(2) ds(5) attributeType(4) 6}

StateOrProvinceName OBJECT IDENTIFIER ::=
     {joint-iso-ccitt(2) ds(5) attributeType(4) 8}

     {joint-iso-ccitt(2) ds(5) attributeType(4) 7}

     {joint-iso-ccitt(2) ds(5) attributeType(4) 3}

     {joint-iso-ccitt(2) ds(5) attributeType(4) 12}

Organization OBJECT IDENTIFIER ::=
     {joint-iso-ccitt(2) ds(5) attributeType(4) 10}

OrganizationalUnit OBJECT IDENTIFIER ::=
     {joint-iso-ccitt(2) ds(5) attributeType(4) 11}

     {joint-iso-ccitt(2) ds(5) attributeType(4) 9}

     {joint-iso-ccitt(2) ds(5) attributeType(4) 17}

     {joint-iso-ccitt(2) ds(5) attributeType(4) 20}

A.6 Certification Request Attributes

postalAddress OBJECT IDENTIFIER ::=
     {joint-iso-ccitt(2) ds(5) attributeType(4) 16}

challengePassword OBJECT IDENTIFIER ::=
     {iso(1) member-body(2) US(840) rsadsi(113549) pkcs(1) pkcs-9(9) 7}

Dusse, et. al.               Informational                     [Page 15]

RFC 2312        S/MIME Version 2 Certificate Handling         March 1998

unstructuredAddress OBJECT IDENTIFIER ::=
     {iso(1) member-body(2) US(840) rsadsi(113549) pkcs(1) pkcs-9(9) 8}

A.7 X.509 V3 Certificate Extensions

basicConstraints OBJECT IDENTIFIER ::=

     {joint-iso-ccitt(2) ds(5) 29 19 }

The ASN.1 definition of basicConstraints certificate extension is:

basicConstraints basicConstraints EXTENSION ::= {
     SYNTAX  BasicConstraintsSyntax
     IDENTIFIED BY { id-ce 19 } }

BasicConstraintsSyntax ::= SEQUENCE {
     cA                 BOOLEAN DEFAULT FALSE,
     pathLenConstraint  INTEGER (0..MAX) OPTIONAL }

     {joint-iso-ccitt(2) ds(5) 29 15 }

The ASN.1 definition of keyUsage certificate extension is:

keyUsage EXTENSION ::= {
     SYNTAX  KeyUsage
     IDENTIFIED BY { id-ce 15 }}

KeyUsage ::= BIT STRING {
     digitalSignature      (0),
     nonRepudiation        (1),
     keyEncipherment       (2),
     dataEncipherment      (3),
     keyAgreement          (4),
     keyCertSign           (5),
     cRLSign               (6)}

Dusse, et. al.               Informational                     [Page 16]

RFC 2312        S/MIME Version 2 Certificate Handling         March 1998

B. References

   [KEYM] PKIX Part 1. At the time of this writing, PKIX is a Work in
   Progress, but it is expected that there will be standards-track RFCs
   at some point in the future.

   [MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Levels", BCP 1l4, RFC 2119, March 1997.

   [PKCS-1] Kaliski, B., "PKCS #1: RSA Encryption Version 1.5", RFC
   2313, March 1998.

   [PKCS-7] Kaliski, B., "PKCS #7: Cryptographic Message Syntax Version
   1.5", RFC 2315, March 1998.

   [PKCS-10] Kaliski, B., "PKCS #10: Certification Request Syntax
   Version 1.5", RFC 2314, March 1998.

   [RFC-822] Crocker, D., "Standard For The Format Of ARPA Internet Text
   Messages", STD 11, RFC 822, August 1982.

   [SMIME-MSG] Dusse, S., Hoffman, P., Ramsdell, R., Lundblade, L., and
   L. Repka, "S/MIME Version 2 Message Specification", RFC 2311, March

   [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.

Dusse, et. al.               Informational                     [Page 17]

RFC 2312        S/MIME Version 2 Certificate Handling         March 1998

C. Compatibility with Prior Practice in S/MIME

   S/MIME was originally developed by RSA Data Security, Inc. Many
   developers implemented S/MIME agents before this document was
   published. All S/MIME receiving agents SHOULD make every attempt to
   interoperate with these earlier implementations of S/MIME.

D. Acknowledgements

   Significant contributions to the content of this memo were made by
   many people, including David Solo, Anil Gangolli, Jeff Thompson, and
   Lisa Repka.

Dusse, et. al.               Informational                     [Page 18]

RFC 2312        S/MIME Version 2 Certificate Handling         March 1998

E. Authors' Addresses

   Steve Dusse
   RSA Data Security, Inc.
   100 Marine Parkway, #500
   Redwood City, CA  94065  USA

   Phone: (415) 595-8782

   Paul Hoffman
   Internet Mail Consortium
   127 Segre Place
   Santa Cruz, CA  95060

   Phone: (408) 426-9827

   Blake Ramsdell
   13122 NE 20th St., Suite C
   Bellevue, WA 98005

   Phone: (425) 882-8861

   Jeff Weinstein
   Netscape Communications Corporation
   501 East Middlefield Road
   Mountain View, CA  94043

   Phone: (415) 254-1900

Dusse, et. al.               Informational                     [Page 19]

RFC 2312        S/MIME Version 2 Certificate Handling         March 1998

F.  Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   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

Dusse, et. al.               Informational                     [Page 20]