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rfc:rfc2312

Network Working Group S. Dusse Request for Comments: 2312 RSA Data Security Category: Informational P. Hoffman

                                              Internet Mail Consortium
                                                           B. Ramsdell
                                                             Worldtalk
                                                          J. Weinstein
                                                              Netscape
                                                            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:
  1. "PKCS #1: RSA Encryption", [PKCS-1].
  2. "PKCS #7: Cryptographic Message Syntax", [PKCS-7]
  3. "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
 X.509.

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
 certificates.

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
 environment.
 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.
 CountryName
 StateOrProvinceName
 Locality
 CommonName
 Title
 Organization
 OrganizationalUnit
 StreetAddress
 PostalCode
 PhoneNumber
 EmailAddress

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
 database.
 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
 [SMIME-MSG].

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
 messages.
 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
 messages.

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
 certificate.
 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
 certified.
 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
 [SMIME-MSG].
 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
 challengePassword
 unstructuredAddress
 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 (e.g., UNIVERSAL STRING), 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:
  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
 fails.

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}

CountryName OBJECT IDENTIFIER ::=

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

StateOrProvinceName OBJECT IDENTIFIER ::=

   {joint-iso-ccitt(2) ds(5) attributeType(4) 8}

locality OBJECT IDENTIFIER ::=

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

CommonName OBJECT IDENTIFIER ::=

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

Title OBJECT IDENTIFIER ::=

   {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}

StreetAddress OBJECT IDENTIFIER ::=

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

Postal Code OBJECT IDENTIFIER ::=

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

Phone Number OBJECT IDENTIFIER ::=

   {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 }

keyUsage OBJECT IDENTIFIER ::=

   {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
 1998.
 [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
 EMail: spock@rsa.com
 Paul Hoffman
 Internet Mail Consortium
 127 Segre Place
 Santa Cruz, CA  95060
 Phone: (408) 426-9827
 EMail: phoffman@imc.org
 Blake Ramsdell
 Worldtalk
 13122 NE 20th St., Suite C
 Bellevue, WA 98005
 Phone: (425) 882-8861
 EMail: blaker@deming.com
 Jeff Weinstein
 Netscape Communications Corporation
 501 East Middlefield Road
 Mountain View, CA  94043
 Phone: (415) 254-1900
 EMail: jsw@netscape.com

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
 English.
 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
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS 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.

Dusse, et. al. Informational [Page 20]

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