draft-ietf-smime-rfc2632bis-07.txt   rfc3850.txt 
Internet Draft Editor: Blake Ramsdell, Network Working Group B. Ramsdell, Editor
draft-ietf-smime-rfc2632bis-07.txt Sendmail, Inc. Request for Comments: 3850 Sendmail, Inc.
June 4, 2004 Obsoletes: 2632 July 2004
Expires December 4, 2004 Category: Standards Track
S/MIME Version 3.1 Certificate Handling
Status of this memo
This document is an Internet-Draft and is in full conformance with all Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.1
provisions of Section 10 of RFC2026. Certificate Handling
Internet-Drafts are working documents of the Internet Engineering Task Status of this Memo
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Internet-Drafts are draft documents valid for a maximum of six months This document specifies an Internet standards track protocol for the
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or to cite them other than as "work in progress." Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
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Abstract Abstract
This document specifies conventions for X.509 certificate usage by This document specifies conventions for X.509 certificate usage by
S/MIME (Secure/Multipurpose Internet Mail Extensions) agents. S/MIME Secure/Multipurpose Internet Mail Extensions (S/MIME) agents. S/MIME
provides a method to send and receive secure MIME messages, and provides a method to send and receive secure MIME messages, and
certificates are an integral part of S/MIME agent processing. S/MIME certificates are an integral part of S/MIME agent processing. S/MIME
agents validate certificates as described in RFC 3280, the Internet agents validate certificates as described in RFC 3280, the Internet
X.509 Public Key Infrastructure Certificate and CRL Profile. S/MIME X.509 Public Key Infrastructure Certificate and CRL Profile. S/MIME
agents must meet the certificate processing requirements in this agents must meet the certificate processing requirements in this
document as well as those in RFC 3280. document as well as those in RFC 3280.
Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Definitions. . . . . . . . . . . . . . . . . . . . . . . 2
1.2. Compatibility with Prior Practice of S/MIME. . . . . . . 3
1.3. Terminology. . . . . . . . . . . . . . . . . . . . . . . 3
1.4. Changes Since S/MIME v3 (RFC 2632) . . . . . . . . . . . 3
2. CMS Options. . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 . CertificateRevocationLists . . . . . . . . . . . . . . . 4
2.2. CertificateChoices . . . . . . . . . . . . . . . . . . . 4
2.3. CertificateSet . . . . . . . . . . . . . . . . . . . . . 5
3. Using Distinguished Names for Internet Mail . . . . . . . . . . 6
4. Certificate Processing . . . . . . . . . . . . . . . . . . . . 7
4.1. Certificate Revocation Lists . . . . . . . . . . . . . . 8
4.2. Certification Path Validation. . . . . . . . . . . . . . 8
4.3. Certificate and CRL Signing Algorithms . . . . . . . . . 9
4.4. PKIX Certificate Extensions. . . . . . . . . . . . . . . 9
5. Security Considerations. . . . . . . . . . . . . . . . . . . . 11
A. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
A.1. Normative References . . . . . . . . . . . . . . . . . . 13
A.2. Informative References . . . . . . . . . . . . . . . . . 14
B. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
C. Editor's Address . . . . . . . . . . . . . . . . . . . . . . . 15
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 16
1. Overview 1. Overview
S/MIME (Secure/Multipurpose Internet Mail Extensions), described in S/MIME (Secure/Multipurpose Internet Mail Extensions), described in
[SMIME-MSG], provides a method to send and receive secure MIME [SMIME-MSG], provides a method to send and receive secure MIME
messages. Before using a public key to provide security services, the messages. Before using a public key to provide security services,
S/MIME agent MUST verify that the public key is valid. S/MIME agents the S/MIME agent MUST verify that the public key is valid. S/MIME
MUST use PKIX certificates to validate public keys as described in the agents MUST use PKIX certificates to validate public keys as
Internet X.509 Public Key Infrastructure (PKIX) Certificate and CRL described in the Internet X.509 Public Key Infrastructure (PKIX)
Profile [KEYM]. S/MIME agents MUST meet the certificate processing Certificate and CRL Profile [KEYM]. S/MIME agents MUST meet the
requirements documented in this document in addition to those stated certificate processing requirements documented in this document in
in [KEYM]. addition to those stated in [KEYM].
This specification is compatible with the Cryptographic Message Syntax This specification is compatible with the Cryptographic Message
[CMS] in that it uses the data types defined by CMS. It also inherits Syntax [CMS] in that it uses the data types defined by CMS. It also
all the varieties of architectures for certificate-based key inherits all the varieties of architectures for certificate-based key
management supported by CMS. management supported by CMS.
1.1 Definitions 1.1. Definitions
For the purposes of this document, the following definitions apply. For the purposes of this document, the following definitions apply.
ASN.1: Abstract Syntax Notation One, as defined in ITU-T X.208. ASN.1: Abstract Syntax Notation One, as defined in ITU-T X.208
[X.208-88].
Attribute Certificate (AC): An X.509 AC is a separate structure from a
subject's public key X.509 Certificate. A subject may have multiple
X.509 ACs associated with each of its public key X.509 Certificates.
Each X.509 AC binds one or more Attributes with one of the subject's
public key X.509 Certificates. The X.509 AC syntax is defined in
[ACAUTH].
BER: Basic Encoding Rules for ASN.1, as defined in ITU-T X.209. Attribute Certificate (AC): An X.509 AC is a separate structure from
a subject's public key X.509 Certificate. A subject may have
multiple X.509 ACs associated with each of its public key X.509
Certificates. Each X.509 AC binds one or more Attributes with one of
the subject's public key X.509 Certificates. The X.509 AC syntax is
defined in [ACAUTH].
Certificate: A type that binds an entity's name to a public key with a Certificate: A type that binds an entity's name to a public key with
digital signature. This type is defined in the Internet X.509 Public a digital signature. This type is defined in the Internet X.509
Key Infrastructure (PKIX) Certificate and CRL Profile [KEYM]. This Public Key Infrastructure (PKIX) Certificate and CRL Profile [KEYM].
type also contains the distinguished name of the certificate issuer This type also contains the distinguished name of the certificate
(the signer), an issuer-specific serial number, the issuer's signature issuer (the signer), an issuer-specific serial number, the issuer's
algorithm identifier, a validity period, and extensions also defined signature algorithm identifier, a validity period, and extensions
in that document. also defined in that document.
Certificate Revocation List (CRL): A type that contains information Certificate Revocation List (CRL): A type that contains information
about certificates whose validity an issuer has prematurely revoked. about certificates whose validity an issuer has prematurely revoked.
The information consists of an issuer name, the time of issue, the The information consists of an issuer name, the time of issue, the
next scheduled time of issue, a list of certificate serial numbers and next scheduled time of issue, a list of certificate serial numbers
their associated revocation times, and extensions as defined in and their associated revocation times, and extensions as defined in
[KEYM]. The CRL is signed by the issuer. The type intended by this [KEYM]. The CRL is signed by the issuer. The type intended by this
specification is the one defined in [KEYM]. specification is the one defined in [KEYM].
DER: Distinguished Encoding Rules for ASN.1, as defined in ITU-T
X.690.
Receiving agent: software that interprets and processes S/MIME CMS Receiving agent: software that interprets and processes S/MIME CMS
objects, MIME body parts that contain CMS objects, or both. objects, MIME body parts that contain CMS objects, or both.
Sending agent: software that creates S/MIME CMS objects, MIME body Sending agent: software that creates S/MIME CMS objects, MIME body
parts that contain CMS objects, or both. parts that contain CMS objects, or both.
S/MIME agent: user software that is a receiving agent, a sending S/MIME agent: user software that is a receiving agent, a sending
agent, or both. agent, or both.
1.2 Compatibility with Prior Practice of S/MIME 1.2. Compatibility with Prior Practice of S/MIME
S/MIME version 3.1 agents should attempt to have the greatest S/MIME version 3.1 agents should attempt to have the greatest
interoperability possible with agents for prior versions of S/MIME. interoperability possible with agents for prior versions of S/MIME.
S/MIME version 2 is described in RFC 2311 through RFC 2315, inclusive S/MIME version 2 is described in RFC 2311 through RFC 2315, inclusive
and S/MIME version 3 is described in RFC 2630 through RFC 2634 and S/MIME version 3 is described in RFC 2630 through RFC 2634
inclusive. RFC 2311 also has historical information about the inclusive. RFC 2311 also has historical information about the
development of S/MIME. development of S/MIME.
1.3 Terminology 1.3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [MUSTSHOULD]. document are to be interpreted as described in [MUSTSHOULD].
1.4 Changes Since S/MIME v3 (RFC 2632) 1.4. Changes Since S/MIME v3 (RFC 2632)
Version 1 and Version 2 CRLs MUST be supported. Version 1 and Version 2 CRLs MUST be supported.
Multiple CA certificates with the same subject and public key, but Multiple CA certificates with the same subject and public key, but
with overlapping validity periods, MUST be supported. with overlapping validity periods, MUST be supported.
Version 2 attribute certificates SHOULD be supported, and version 1 Version 2 attribute certificates SHOULD be supported, and version 1
attributes certificates MUST NOT be used. attributes certificates MUST NOT be used.
The use of the MD2 digest algorithm for certificate signatures is The use of the MD2 digest algorithm for certificate signatures is
discouraged, and security language added. discouraged and security language added.
Clarified use of email address use in certificates. Certificates that Clarified use of email address use in certificates. Certificates
do not contain an email address have no requirements for verifying the that do not contain an email address have no requirements for
email address associated with the certificate. verifying the email address associated with the certificate.
Receiving agents SHOULD display certificate information when Receiving agents SHOULD display certificate information when
displaying the results of signature verification. displaying the results of signature verification.
Receiving agents MUST NOT accept a signature made with a certificate Receiving agents MUST NOT accept a signature made with a certificate
that does not have the digitalSignature or nonRepudiation bit set. that does not have the digitalSignature or nonRepudiation bit set.
Clarifications for the interpretation of the key usage and extended Clarifications for the interpretation of the key usage and extended
key usage extensions. key usage extensions.
2. CMS Options 2. CMS Options
The CMS message format allows for a wide variety of options in content The CMS message format allows for a wide variety of options in
and algorithm support. This section puts forth a number of support content and algorithm support. This section puts forth a number of
requirements and recommendations in order to achieve a base level of support requirements and recommendations in order to achieve a base
interoperability among all S/MIME implementations. Most of the CMS level of interoperability among all S/MIME implementations. Most of
format for S/MIME messages is defined in [SMIME-MSG]. the CMS format for S/MIME messages is defined in [SMIME-MSG].
2.1 CertificateRevocationLists 2.1. CertificateRevocationLists
Receiving agents MUST support the Certificate Revocation List (CRL) Receiving agents MUST support the Certificate Revocation List (CRL)
format defined in [KEYM]. If sending agents include CRLs in outgoing format defined in [KEYM]. If sending agents include CRLs in outgoing
messages, the CRL format defined in [KEYM] MUST be used. In all cases, messages, the CRL format defined in [KEYM] MUST be used. In all
both v1 and v2 CRLs MUST be supported. cases, both v1 and v2 CRLs MUST be supported.
All agents MUST be capable of performing revocation checks using CRLs All agents MUST be capable of performing revocation checks using CRLs
as specified in [KEYM]. All agents MUST perform revocation status as specified in [KEYM]. All agents MUST perform revocation status
checking in accordance with [KEYM]. Receiving agents MUST recognize checking in accordance with [KEYM]. Receiving agents MUST recognize
CRLs in received S/MIME messages. CRLs in received S/MIME messages.
Agents SHOULD store CRLs received in messages for use in processing Agents SHOULD store CRLs received in messages for use in processing
later messages. later messages.
2.2 CertificateChoices 2.2. CertificateChoices
Receiving agents MUST support v1 X.509 and v3 X.509 identity Receiving agents MUST support v1 X.509 and v3 X.509 identity
certificates as profiled in [KEYM]. End entity certificates MAY certificates as profiled in [KEYM]. End entity certificates MAY
include an Internet mail address, as described in section 3. include an Internet mail address, as described in section 3.
Receiving agents SHOULD support X.509 version 2 attribute Receiving agents SHOULD support X.509 version 2 attribute
certificates. See [ACAUTH] for details about the profile for attribute certificates. See [ACAUTH] for details about the profile for
certificates. attribute certificates.
2.2.1 Historical Note About CMS Certificates 2.2.1. Historical Note About CMS Certificates
The CMS message format supports a choice of certificate formats for The CMS message format supports a choice of certificate formats for
public key content types: PKIX, PKCS #6 Extended Certificates [PKCS6] public key content types: PKIX, PKCS #6 Extended Certificates [PKCS6]
and PKIX Attribute Certificates. and PKIX Attribute Certificates.
The PKCS #6 format is not in widespread use. In addition, PKIX The PKCS #6 format is not in widespread use. In addition, PKIX
certificate extensions address much of the same functionality and certificate extensions address much of the same functionality and
flexibility as was intended in the PKCS #6. Thus, sending and flexibility as was intended in the PKCS #6. Thus, sending and
receiving agents MUST NOT use PKCS #6 extended certificates. receiving agents MUST NOT use PKCS #6 extended certificates.
X.509 version 1 attribute certificates are also not widely X.509 version 1 attribute certificates are also not widely
implemented, and have been superseded with version 2 attribute implemented, and have been superseded with version 2 attribute
certificates. Sending agents MUST NOT send version 1 attribute certificates. Sending agents MUST NOT send version 1 attribute
certificates. certificates.
2.3 CertificateSet 2.3. CertificateSet
Receiving agents MUST be able to handle an arbitrary number of Receiving agents MUST be able to handle an arbitrary number of
certificates of arbitrary relationship to the message sender and to certificates of arbitrary relationship to the message sender and to
each other in arbitrary order. In many cases, the certificates each other in arbitrary order. In many cases, the certificates
included in a signed message may represent a chain of certification included in a signed message may represent a chain of certification
from the sender to a particular root. There may be, however, from the sender to a particular root. There may be, however,
situations where the certificates in a signed message may be unrelated situations where the certificates in a signed message may be
and included for convenience. unrelated and included for convenience.
Sending agents SHOULD include any certificates for the user's public Sending agents SHOULD include any certificates for the user's public
key(s) and associated issuer certificates. This increases the key(s) and associated issuer certificates. This increases the
likelihood that the intended recipient can establish trust in the likelihood that the intended recipient can establish trust in the
originator's public key(s). This is especially important when sending originator's public key(s). This is especially important when
a message to recipients that may not have access to the sender's sending a message to recipients that may not have access to the
public key through any other means or when sending a signed message to sender's public key through any other means or when sending a signed
a new recipient. The inclusion of certificates in outgoing messages message to a new recipient. The inclusion of certificates in
can be omitted if S/MIME objects are sent within a group of outgoing messages can be omitted if S/MIME objects are sent within a
correspondents that has established access to each other's group of correspondents that has established access to each other's
certificates by some other means such as a shared directory or manual certificates by some other means such as a shared directory or manual
certificate distribution. Receiving S/MIME agents SHOULD be able to certificate distribution. Receiving S/MIME agents SHOULD be able to
handle messages without certificates using a database or directory handle messages without certificates using a database or directory
lookup scheme. lookup scheme.
A sending agent SHOULD include at least one chain of certificates up A sending agent SHOULD include at least one chain of certificates up
to, but not including, a Certificate Authority (CA) that it believes to, but not including, a Certificate Authority (CA) that it believes
that the recipient may trust as authoritative. A receiving agent that the recipient may trust as authoritative. A receiving agent
MUST be able to handle an arbitrarily large number of certificates MUST be able to handle an arbitrarily large number of certificates
and chains. and chains.
Agents MAY send CA certificates, that is, certificates which can be Agents MAY send CA certificates, that is, certificates which can be
considered the "root" of other chains, and which MAY be self-signed. considered the "root" of other chains, and which MAY be self-signed.
Note that receiving agents SHOULD NOT simply trust any self-signed Note that receiving agents SHOULD NOT simply trust any self-signed
certificates as valid CAs, but SHOULD use some other mechanism to certificates as valid CAs, but SHOULD use some other mechanism to
determine if this is a CA that should be trusted. Also note that when determine if this is a CA that should be trusted. Also note that
certificates contain DSA public keys the parameters may be located in when certificates contain DSA public keys the parameters may be
the root certificate. This would require that the recipient possess located in the root certificate. This would require that the
both the end-entity certificate as well as the root certificate to recipient possess both the end-entity certificate as well as the root
perform a signature verification, and is a valid example of a case certificate to perform a signature verification, and is a valid
where transmitting the root certificate may be required. example of a case where transmitting the root certificate may be
required.
Receiving agents MUST support chaining based on the distinguished name Receiving agents MUST support chaining based on the distinguished
fields. Other methods of building certificate chains MAY be supported. name fields. Other methods of building certificate chains MAY be
supported.
Receiving agents SHOULD support the decoding of X.509 attribute Receiving agents SHOULD support the decoding of X.509 attribute
certificates included in CMS objects. All other issues regarding the certificates included in CMS objects. All other issues regarding the
generation and use of X.509 attribute certificates are outside of the generation and use of X.509 attribute certificates are outside of the
scope of this specification. One specification that addresses scope of this specification. One specification that addresses
attribute certificate use is defined in [SECLABEL]. attribute certificate use is defined in [SECLABEL].
3. Using Distinguished Names for Internet Mail 3. Using Distinguished Names for Internet Mail
End-entity certificates MAY contain an Internet mail address as End-entity certificates MAY contain an Internet mail address as
described in [RFC-2822]. The address must be an "addr-spec" as defined described in [RFC-2822]. The address must be an "addr-spec" as
in Section 3.4.1 of that specification. The email address SHOULD be in defined in Section 3.4.1 of that specification. The email address
the subjectAltName extension, and SHOULD NOT be in the subject SHOULD be in the subjectAltName extension, and SHOULD NOT be in the
distinguished name. subject distinguished name.
Receiving agents MUST recognize and accept certificates that contain Receiving agents MUST recognize and accept certificates that contain
no email address. Receiving agents MUST recognize email addresses in no email address. Agents are allowed to provide an alternative
the subjectAltName field. Receiving agents MUST recognize email mechanism for associating an email address with a certificate that
addresses in the Distinguished Name field in the PKCS #9 [PKCS9] does not contain an email address, such as through the use of the
emailAddress attribute: agent's address book, if available. Receiving agents MUST recognize
email addresses in the subjectAltName field. Receiving agents MUST
recognize email addresses in the Distinguished Name field in the PKCS
#9 [PKCS9] emailAddress attribute:
pkcs-9-at-emailAddress OBJECT IDENTIFIER ::= pkcs-9-at-emailAddress OBJECT IDENTIFIER ::=
{iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1 } {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1 }
Note that this attribute MUST be encoded as IA5String. Note that this attribute MUST be encoded as IA5String.
Sending agents SHOULD make the address in the From or Sender header in Sending agents SHOULD make the address in the From or Sender header
a mail message match an Internet mail address in the signer's in a mail message match an Internet mail address in the signer's
certificate. Receiving agents MUST check that the address in the From certificate. Receiving agents MUST check that the address in the
or Sender header of a mail message matches an Internet mail address, From or Sender header of a mail message matches an Internet mail
if present, in the signer's certificate, if mail addresses are present address, if present, in the signer's certificate, if mail addresses
in the certificate. A receiving agent SHOULD provide some explicit are present in the certificate. A receiving agent SHOULD provide
alternate processing of the message if this comparison fails, which some explicit alternate processing of the message if this comparison
may be to display a message that shows the recipient the addresses in fails, which may be to display a message that shows the recipient the
the certificate or other certificate details. addresses in the certificate or other certificate details.
A receiving agent SHOULD display a subject name or other certificate A receiving agent SHOULD display a subject name or other certificate
details when displaying an indication of successful or unsuccessful details when displaying an indication of successful or unsuccessful
signature verification. signature verification.
All subject and issuer names MUST be populated (i.e. not an empty All subject and issuer names MUST be populated (i.e., not an empty
SEQUENCE) in S/MIME-compliant X.509 identity certificates, except that SEQUENCE) in S/MIME-compliant X.509 identity certificates, except
the subject DN in a user's (i.e. end-entity) certificate MAY be an that the subject DN in a user's (i.e., end-entity) certificate MAY be
empty SEQUENCE in which case the subjectAltName extension will include an empty SEQUENCE in which case the subjectAltName extension will
the subject's identifier and MUST be marked as critical. include the subject's identifier and MUST be marked as critical.
4. Certificate Processing 4. Certificate Processing
A receiving agent needs to provide some certificate retrieval A receiving agent needs to provide some certificate retrieval
mechanism in order to gain access to certificates for recipients of mechanism in order to gain access to certificates for recipients of
digital envelopes. There are many ways to implement certificate digital envelopes. There are many ways to implement certificate
retrieval mechanisms. X.500 directory service is an excellent example retrieval mechanisms. X.500 directory service is an excellent
of a certificate retrieval-only mechanism that is compatible with example of a certificate retrieval-only mechanism that is compatible
classic X.500 Distinguished Names. Another method under consideration with classic X.500 Distinguished Names. Another method under
by the IETF is to provide certificate retrieval services as part of consideration by the IETF is to provide certificate retrieval
the existing Domain Name System (DNS). Until such mechanisms are services as part of the existing Domain Name System (DNS). Until
widely used, their utility may be limited by the small number of such mechanisms are widely used, their utility may be limited by the
correspondent's certificates that can be retrieved. At a minimum, for small number of correspondent's certificates that can be retrieved.
initial S/MIME deployment, a user agent could automatically generate a At a minimum, for initial S/MIME deployment, a user agent could
message to an intended recipient requesting that recipient's automatically generate a message to an intended recipient requesting
certificate in a signed return message. that recipient's certificate in a signed return message.
Receiving and sending agents SHOULD also provide a mechanism to allow Receiving and sending agents SHOULD also provide a mechanism to allow
a user to "store and protect" certificates for correspondents in such a user to "store and protect" certificates for correspondents in such
a way so as to guarantee their later retrieval. In many environments, a way so as to guarantee their later retrieval. In many
it may be desirable to link the certificate retrieval/storage environments, it may be desirable to link the certificate
mechanisms together in some sort of certificate database. In its retrieval/storage mechanisms together in some sort of certificate
simplest form, a certificate database would be local to a particular database. In its simplest form, a certificate database would be
user and would function in a similar way as a "address book" that local to a particular user and would function in a similar way as an
stores a user's frequent correspondents. In this way, the certificate "address book" that stores a user's frequent correspondents. In this
retrieval mechanism would be limited to the certificates that a user way, the certificate retrieval mechanism would be limited to the
has stored (presumably from incoming messages). A comprehensive certificates that a user has stored (presumably from incoming
certificate retrieval/storage solution may combine two or more messages). A comprehensive certificate retrieval/storage solution
mechanisms to allow the greatest flexibility and utility to the user. may combine two or more mechanisms to allow the greatest flexibility
For instance, a secure Internet mail agent may resort to checking a and utility to the user. For instance, a secure Internet mail agent
centralized certificate retrieval mechanism for a certificate if it may resort to checking a centralized certificate retrieval mechanism
can not be found in a user's local certificate storage/retrieval for a certificate if it can not be found in a user's local
database. certificate storage/retrieval database.
Receiving and sending agents SHOULD provide a mechanism for the import Receiving and sending agents SHOULD provide a mechanism for the
and export of certificates, using a CMS certs-only message. This import and export of certificates, using a CMS certs-only message.
allows for import and export of full certificate chains as opposed to This allows for import and export of full certificate chains as
just a single certificate. This is described in [SMIME-MSG]. opposed to just a single certificate. This is described in [SMIME-
MSG].
Agents MUST handle multiple valid Certification Authority (CA) Agents MUST handle multiple valid Certification Authority (CA)
certificates containing the same subject name and the same public keys certificates containing the same subject name and the same public
but with overlapping validity intervals. keys but with overlapping validity intervals.
4.1 Certificate Revocation Lists 4.1. Certificate Revocation Lists
In general, it is always better to get the latest CRL information from In general, it is always better to get the latest CRL information
a CA than to get information stored away from incoming messages. A from a CA than to get information stored away from incoming messages.
receiving agent SHOULD have access to some certificate revocation list A receiving agent SHOULD have access to some certificate revocation
(CRL) retrieval mechanism in order to gain access to certificate list (CRL) retrieval mechanism in order to gain access to certificate
revocation information when validating certification paths. A revocation information when validating certification paths. A
receiving or sending agent SHOULD also provide a mechanism to allow a receiving or sending agent SHOULD also provide a mechanism to allow a
user to store incoming certificate revocation information for user to store incoming certificate revocation information for
correspondents in such a way so as to guarantee its later retrieval. correspondents in such a way so as to guarantee its later retrieval.
Receiving and sending agents SHOULD retrieve and utilize CRL Receiving and sending agents SHOULD retrieve and utilize CRL
information every time a certificate is verified as part of a information every time a certificate is verified as part of a
certification path validation even if the certificate was already certification path validation even if the certificate was already
verified in the past. However, in many instances (such as off-line verified in the past. However, in many instances (such as off-line
verification) access to the latest CRL information may be difficult or verification) access to the latest CRL information may be difficult
impossible. The use of CRL information, therefore, may be dictated by or impossible. The use of CRL information, therefore, may be
the value of the information that is protected. The value of the CRL dictated by the value of the information that is protected. The
information in a particular context is beyond the scope of this value of the CRL information in a particular context is beyond the
specification but may be governed by the policies associated with scope of this specification but may be governed by the policies
particular certification paths. associated with particular certification paths.
All agents MUST be capable of performing revocation checks using CRLs All agents MUST be capable of performing revocation checks using CRLs
as specified in [KEYM]. All agents MUST perform revocation status as specified in [KEYM]. All agents MUST perform revocation status
checking in accordance with [KEYM]. Receiving agents MUST recognize checking in accordance with [KEYM]. Receiving agents MUST recognize
CRLs in received S/MIME messages. CRLs in received S/MIME messages.
4.2 Certification Path Validation 4.2. Certification Path Validation
In creating a user agent for secure messaging, certificate, CRL, and In creating a user agent for secure messaging, certificate, CRL, and
certification path validation SHOULD be highly automated while still certification path validation SHOULD be highly automated while still
acting in the best interests of the user. Certificate, CRL, and path acting in the best interests of the user. Certificate, CRL, and path
validation MUST be performed as per [KEYM] when validating a validation MUST be performed as per [KEYM] when validating a
correspondent's public key. This is necessary before using a public correspondent's public key. This is necessary before using a public
key to provide security services such as: verifying a signature; key to provide security services such as: verifying a signature;
encrypting a content-encryption key (ex: RSA); or forming a pairwise encrypting a content-encryption key (ex: RSA); or forming a pairwise
symmetric key (ex: Diffie-Hellman) to be used to encrypt or decrypt a symmetric key (ex: Diffie-Hellman) to be used to encrypt or decrypt a
content-encryption key. content-encryption key.
Certificates and CRLs are made available to the path validation Certificates and CRLs are made available to the path validation
procedure in two ways: a) incoming messages, and b) certificate and procedure in two ways: a) incoming messages, and b) certificate and
CRL retrieval mechanisms. Certificates and CRLs in incoming messages CRL retrieval mechanisms. Certificates and CRLs in incoming messages
are not required to be in any particular order nor are they required 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 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 (although in most cases they will be related to the sender).
certificates and CRLs SHOULD be cached for use in path validation and Incoming certificates and CRLs SHOULD be cached for use in path
optionally stored for later use. This temporary certificate and CRL validation and optionally stored for later use. This temporary
cache SHOULD be used to augment any other certificate and CRL certificate and CRL cache SHOULD be used to augment any other
retrieval mechanisms for path validation on incoming signed messages. certificate and CRL retrieval mechanisms for path validation on
incoming signed messages.
4.3 Certificate and CRL Signing Algorithms 4.3. Certificate and CRL Signing Algorithms
Certificates and Certificate Revocation Lists (CRLs) are signed by the Certificates and Certificate Revocation Lists (CRLs) are signed by
certificate issuer. A receiving agent MUST be capable of verifying the the certificate issuer. A receiving agent MUST be capable of
signatures on certificates and CRLs made with id-dsa-with-sha1 verifying the signatures on certificates and CRLs made with
[CMSALG]. id-dsa-with-sha1 [CMSALG].
A receiving agent MUST be capable of verifying the signatures on A receiving agent MUST be capable of verifying the signatures on
certificates and CRLs made with md5WithRSAEncryption and certificates and CRLs made with md5WithRSAEncryption and
sha1WithRSAEncryption signature algorithms with key sizes from 512 sha1WithRSAEncryption signature algorithms with key sizes from 512
bits to 2048 bits described in [CMSALG]. bits to 2048 bits described in [CMSALG].
Because of the security issues surrounding MD2 [RC95], and in light of Because of the security issues surrounding MD2 [RC95], and in light
current use, md2WithRSAEncryption MAY be supported. of current use, md2WithRSAEncryption MAY be supported.
4.4 PKIX Certificate Extensions 4.4. PKIX Certificate Extensions
PKIX describes an extensible framework in which the basic certificate PKIX describes an extensible framework in which the basic certificate
information can be extended and how such extensions can be used to information can be extended and how such extensions can be used to
control the process of issuing and validating certificates. The PKIX control the process of issuing and validating certificates. The PKIX
Working Group has ongoing efforts to identify and create extensions Working Group has ongoing efforts to identify and create extensions
which have value in particular certification environments. Further, which have value in particular certification environments. Further,
there are active efforts underway to issue PKIX certificates for there are active efforts underway to issue PKIX certificates for
business purposes. This document identifies the minimum required set business purposes. This document identifies the minimum required set
of certificate extensions which have the greatest value in the S/MIME of certificate extensions which have the greatest value in the S/MIME
environment. The syntax and semantics of all the identified extensions environment. The syntax and semantics of all the identified
are defined in [KEYM]. extensions are defined in [KEYM].
Sending and receiving agents MUST correctly handle the basic Sending and receiving agents MUST correctly handle the basic
constraints, key usage, authority key identifier, subject key constraints, key usage, authority key identifier, subject key
identifier, and subject alternative names certificate extensions when identifier, and subject alternative names certificate extensions when
they appear in end-entity and CA certificates. Some mechanism SHOULD they appear in end-entity and CA certificates. Some mechanism SHOULD
exist to gracefully handle other certificate extensions when they exist to gracefully handle other certificate extensions when they
appear in end-entity or CA certificates. appear in end-entity or CA certificates.
Certificates issued for the S/MIME environment SHOULD NOT contain any Certificates issued for the S/MIME environment SHOULD NOT contain any
critical extensions (extensions that have the critical field set to critical extensions (extensions that have the critical field set to
TRUE) other than those listed here. These extensions SHOULD be marked TRUE) other than those listed here. These extensions SHOULD be
as non-critical unless the proper handling of the extension is deemed marked as non-critical unless the proper handling of the extension is
critical to the correct interpretation of the associated certificate. deemed critical to the correct interpretation of the associated
Other extensions may be included, but those extensions SHOULD NOT be certificate. Other extensions may be included, but those extensions
marked as critical. SHOULD NOT be marked as critical.
Interpretation and syntax for all extensions MUST follow [KEYM], Interpretation and syntax for all extensions MUST follow [KEYM],
unless otherwise specified here. unless otherwise specified here.
4.4.1 Basic Constraints Certificate Extension 4.4.1. Basic Constraints Certificate Extension
The basic constraints extension serves to delimit the role and The basic constraints extension serves to delimit the role and
position of an issuing authority or end-entity certificate plays in a position that an issuing authority or end-entity certificate plays in
certification path. a certification path.
For example, certificates issued to CAs and subordinate CAs contain a For example, certificates issued to CAs and subordinate CAs contain a
basic constraint extension that identifies them as issuing authority basic constraint extension that identifies them as issuing authority
certificates. End-entity certificates contain an extension that certificates. End-entity certificates contain an extension that
constrains the certificate from being an issuing authority constrains the certificate from being an issuing authority
certificate. certificate.
Certificates SHOULD contain a basicConstraints extension in CA Certificates SHOULD contain a basicConstraints extension in CA
certificates, and SHOULD NOT contain that extension in end entity certificates, and SHOULD NOT contain that extension in end entity
certificates. certificates.
4.4.2 Key Usage Certificate Extension 4.4.2. Key Usage Certificate Extension
The key usage extension serves to limit the technical purposes for The key usage extension serves to limit the technical purposes for
which a public key listed in a valid certificate may be used. Issuing which a public key listed in a valid certificate may be used.
authority certificates may contain a key usage extension that Issuing authority certificates may contain a key usage extension that
restricts the key to signing certificates, certificate revocation restricts the key to signing certificates, certificate revocation
lists and other data. lists and other data.
For example, a certification authority may create subordinate issuer For example, a certification authority may create subordinate issuer
certificates which contain a key usage extension which specifies that certificates which contain a key usage extension which specifies that
the corresponding public key can be used to sign end user certificates the corresponding public key can be used to sign end user
and sign CRLs. certificates and sign CRLs.
If a key usage extension is included in a PKIX certificate, then it If a key usage extension is included in a PKIX certificate, then it
MUST be marked as critical. MUST be marked as critical.
S/MIME receiving agents MUST NOT accept the signature of a message if S/MIME receiving agents MUST NOT accept the signature of a message if
it was verified using a certificate which contains the key usage it was verified using a certificate which contains the key usage
extension without either the digitalSignature or nonRepudiation bit extension without either the digitalSignature or nonRepudiation bit
set. Sometimes S/MIME is used as a secure message transport for set. Sometimes S/MIME is used as a secure message transport for
applications beyond interpersonal messaging. In such cases, the applications beyond interpersonal messaging. In such cases, the
S/MIME-enabled application can specify additional requirements S/MIME-enabled application can specify additional requirements
concerning the digitalSignature or nonRepudiation bits within this concerning the digitalSignature or nonRepudiation bits within this
extension. extension.
If the key usage extension is not specified, receiving clients MUST If the key usage extension is not specified, receiving clients MUST
presume that the digitalSignature and nonRepudiation bits are set. presume that the digitalSignature and nonRepudiation bits are set.
4.4.3 Subject Alternative Name Extension 4.4.3. Subject Alternative Name Extension
The subject alternative name extension is used in S/MIME as the The subject alternative name extension is used in S/MIME as the
preferred means to convey the RFC-2822 email address(es) that preferred means to convey the RFC-2822 email address(es) that
correspond to the entity for this certificate. Any RFC-2822 email correspond(s) to the entity for this certificate. Any RFC-2822 email
addresses present MUST be encoded using the rfc822Name CHOICE of the addresses present MUST be encoded using the rfc822Name CHOICE of the
GeneralName type. Since the SubjectAltName type is a SEQUENCE OF GeneralName type. Since the SubjectAltName type is a SEQUENCE OF
GeneralName, multiple RFC-2822 email addresses MAY be present. GeneralName, multiple RFC-2822 email addresses MAY be present.
4.4.4 Extended Key Usage Extension 4.4.4. Extended Key Usage Extension
The extended key usage extension also serves to limit the technical The extended key usage extension also serves to limit the technical
purposes for which a public key listed in a valid certificate may be purposes for which a public key listed in a valid certificate may be
used. The set of technical purposes for the certificate therefore are used. The set of technical purposes for the certificate therefore
the intersection of the uses indicated in the key usage and extended are the intersection of the uses indicated in the key usage and
key usage extensions. extended key usage extensions.
For example, if the certificate contains a key usage extension For example, if the certificate contains a key usage extension
indicating digital signature and an extended key usage extension which indicating digital signature and an extended key usage extension
includes the email protection OID, then the certificate may be used which includes the email protection OID, then the certificate may be
for signing but not encrypting S/MIME messages. If the certificate used for signing but not encrypting S/MIME messages. If the
contains a key usage extension indicating digital signature, but no certificate contains a key usage extension indicating digital
extended key usage extension then the certificate may also be used to signature, but no extended key usage extension then the certificate
sign but not encrypt S/MIME messages. may also be used to sign but not encrypt S/MIME messages.
If the extended key usage extension is present in the certificate then If the extended key usage extension is present in the certificate
interpersonal message S/MIME receiving agents MUST check it contains then interpersonal message S/MIME receiving agents MUST check that it
either the emailProtection or the anyExtendedKeyUsage OID as defined contains either the emailProtection or the anyExtendedKeyUsage OID as
in [KEYM]. S/MIME uses other than interpersonal messaging MAY require defined in [KEYM]. S/MIME uses other than interpersonal messaging
the explicit presence of the extended key usage extension or other MAY require the explicit presence of the extended key usage extension
OIDs to be present in the extension or both. or other OIDs to be present in the extension or both.
5. Security Considerations 5. Security Considerations
All of the security issues faced by any cryptographic application must All of the security issues faced by any cryptographic application
be faced by a S/MIME agent. Among these issues are protecting the must be faced by a S/MIME agent. Among these issues are protecting
user's private key, preventing various attacks, and helping the user the user's private key, preventing various attacks, and helping the
avoid mistakes such as inadvertently encrypting a message for the user avoid mistakes such as inadvertently encrypting a message for
wrong recipient. The entire list of security considerations is beyond the wrong recipient. The entire list of security considerations is
the scope of this document, but some significant concerns are listed beyond the scope of this document, but some significant concerns are
here. listed here.
When processing certificates, there are many situations where the When processing certificates, there are many situations where the
processing might fail. Because the processing may be done by a user 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 agent, a security gateway, or other program, there is no single way
handle such failures. Just because the methods to handle the failures to handle such failures. Just because the methods to handle the
has not been listed, however, the reader should not assume that they failures has not been listed, however, the reader should not assume
are not important. The opposite is true: if a certificate is not that they are not important. The opposite is true: if a certificate
provably valid and associated with the message, the processing is not provably valid and associated with the message, the processing
software should take immediate and noticeable steps to inform the end software should take immediate and noticeable steps to inform the end
user about it. user about it.
Some of the many places where signature and certificate checking might Some of the many places where signature and certificate checking
fail include: might fail include:
- no Internet mail addresses in a certificate matches the sender of a - no Internet mail addresses in a certificate matches the sender of
message, if the certificate contains at least one mail address a message, if the certificate contains at least one mail address
- no certificate chain leads to a trusted CA - no certificate chain leads to a trusted CA
- no ability to check the CRL for a certificate - no ability to check the CRL for a certificate
- an invalid CRL was received - an invalid CRL was received
- the CRL being checked is expired - the CRL being checked is expired
- the certificate is expired - the certificate is expired
- the certificate has been revoked - the certificate has been revoked
There are certainly other instances where a certificate may be There are certainly other instances where a certificate may be
invalid, and it is the responsibility of the processing software to invalid, and it is the responsibility of the processing software to
check them all thoroughly, and to decide what to do if the check check them all thoroughly, and to decide what to do if the check
skipping to change at line 540 skipping to change at page 12, line 38
At the Selected Areas in Cryptography '95 conference in May 1995, At the Selected Areas in Cryptography '95 conference in May 1995,
Rogier and Chauvaud presented an attack on MD2 that can nearly find Rogier and Chauvaud presented an attack on MD2 that can nearly find
collisions [RC95]. Collisions occur when one can find two different collisions [RC95]. Collisions occur when one can find two different
messages that generate the same message digest. A checksum operation messages that generate the same message digest. A checksum operation
in MD2 is the only remaining obstacle to the success of the attack. in MD2 is the only remaining obstacle to the success of the attack.
For this reason, the use of MD2 for new applications is discouraged. For this reason, the use of MD2 for new applications is discouraged.
It is still reasonable to use MD2 to verify existing signatures, as It is still reasonable to use MD2 to verify existing signatures, as
the ability to find collisions in MD2 does not enable an attacker to the ability to find collisions in MD2 does not enable an attacker to
find new messages having a previously computed hash value. find new messages having a previously computed hash value.
It is possible for there to be multiple unexpired CRLs for a CA. If an It is possible for there to be multiple unexpired CRLs for a CA. If
agent is consulting CRLs for certificate validation, it SHOULD make an agent is consulting CRLs for certificate validation, it SHOULD
sure that the most recently issued CRL for that CA is consulted, since make sure that the most recently issued CRL for that CA is consulted,
an S/MIME message sender could deliberately include an older unexpired since an S/MIME message sender could deliberately include an older
CRL in an S/MIME message. This older CRL might not include recent unexpired CRL in an S/MIME message. This older CRL might not include
revoked certificates, which might lead an agent to accept a recent revoked certificates, which might lead an agent to accept a
certificate that has been revoked in a subsequent CRL. certificate that has been revoked in a subsequent CRL.
When determining the time for a certificate validity check, agents When determining the time for a certificate validity check, agents
have to be careful to use a reliable time. Unless it is from a trusted have to be careful to use a reliable time. Unless it is from a
agent, this time MUST NOT be the SigningTime attribute found in an trusted agent, this time MUST NOT be the SigningTime attribute found
S/MIME message. For most sending agents, the SigningTime attribute in an S/MIME message. For most sending agents, the SigningTime
could be deliberately set to direct the receiving agent to check a CRL attribute could be deliberately set to direct the receiving agent to
that could have out-of-date revocation status for a certificate, or check a CRL that could have out-of-date revocation status for a
cause an improper result when checking the Validity field of a certificate, or cause an improper result when checking the Validity
certificate. field of a certificate.
A. Normative References A. References
[ACAUTH] "An Internet Attribute Certificate Profile for A.1. Normative References
Authorization", RFC 3281
[CMS] "Cryptographic Message Syntax (CMS)", [ACAUTH] Farrell, S. and R. Housley, "An Internet Attribute
draft-ietf-smime-3369bis-04 Certificate Profile for Authorization", RFC 3281, April
2002.
[CMSALG] "Cryptographic Message Syntax (CMS) Algorithms", RFC 3370 [CMS] Housely, R., "Cryptographic Message Syntax (CMS)", RFC
3852, July 2004.
[KEYM] "Internet X.509 Public Key Infrastructure Certificate and CRL [CMSALG] Housley, R., "Cryptographic Message Syntax (CMS)
Profile", RFC 3280 Algorithms", RFC 3370, August 2002.
[KEYMALG] "Algorithms and Identifiers for the Internet X.509 Public [KEYM] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
Key Infrastructure Certificate and CRL Profile ", RFC 3279 X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[MUSTSHOULD] "Key words for use in RFCs to Indicate Requirement [KEYMALG] Bassham, L., Polk, W., and R. Housley, "Algorithms and
Levels", RFC 2119 Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 3279, April 2002.
[PKCS9] "PKCS #9: Selected Object Classes and Attribute Types Version [MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate
2.0", RFC 2985 Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC-2822], "Internet Message Format", RFC 2822 [PKCS9] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
Classes and Attribute Types Version 2.0", RFC 2985,
November 2000.
[SMIME-MSG] "S/MIME Version 3 Message Specification ", Internet Draft [RFC-2822], Resnick, P., "Internet Message Format", RFC 2822, April
draft-ietf-smime-msg 2001.
B. Informative References [SMIME-MSG] Ramsdell, B., Ed., "S/MIME Version 3.1 Message
Specification", RFC 3851, July 2004.
[CERTV2] "S/MIME Version 2 Certificate Handling", RFC 2312 [x.208-88] ITU-T. Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1). 1988.
A.2. Informative References
[CERTV2] Dusse, S., Hoffman, P., Ramsdell, B., and J. Weinstein,
"S/MIME Version 2 Certificate Handling", RFC 2312, March
1998.
[PKCS6] RSA Laboratories, "PKCS #6: Extended-Certificate Syntax [PKCS6] RSA Laboratories, "PKCS #6: Extended-Certificate Syntax
Standard", November 1993 Standard", November 1993.
[RC95] Rogier, N. and Chauvaud, P., "The compression function of MD2 [RC95] Rogier, N. and Chauvaud, P., "The compression function
is not collision free," Presented at Selected Areas in Cryptography of MD2 is not collision free," Presented at Selected
'95, May 1995 Areas in Cryptography '95, May 1995.
[SECLABEL] "Implementing Company Classification Policy with the S/MIME [SECLABEL] Nicolls, W., "Implementing Company Classification Policy
Security Label", RFC 3114 with the S/MIME Security Label", RFC 3114, May 2002.
[X.500] ITU-T Recommendation X.500 (1997) | ISO/IEC 9594-1:1997, [X.500] ITU-T Recommendation X.500 (1997) | ISO/IEC 9594-1:1997,
Information technology - Open Systems Interconnection - The Directory: Information technology - Open Systems Interconnection -
Overview of concepts, models and services The Directory: Overview of concepts, models and
services.
[X.501] ITU-T Recommendation X.501 (1997) | ISO/IEC 9594-2:1997, [X.501] ITU-T Recommendation X.501 (1997) | ISO/IEC 9594-2:1997,
Information technology - Open Systems Interconnection - The Directory: Information technology - Open Systems Interconnection -
Models The Directory: Models.
[X.509] ITU-T Recommendation X.509 (1997) | ISO/IEC 9594-8:1997, [X.509] ITU-T Recommendation X.509 (1997) | ISO/IEC 9594-8:1997,
Information technology - Open Systems Interconnection - The Directory: Information technology - Open Systems Interconnection -
Authentication framework The Directory: Authentication framework.
[X.520] ITU-T Recommendation X.520 (1997) | ISO/IEC 9594-6:1997, [X.520] ITU-T Recommendation X.520 (1997) | ISO/IEC 9594-6:1997,
Information technology - Open Systems Interconnection - The Directory: Information technology - Open Systems Interconnection -
Selected attribute types. The Directory: Selected attribute types.
C. Acknowledgements B. Acknowledgements
Many thanks go out to the other authors of the S/MIME v2 RFC: Steve 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 Dusse, Paul Hoffman and Jeff Weinstein. Without v2, there wouldn't
a v3. be a v3.
A number of the members of the S/MIME Working Group have also worked 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 very hard and contributed to this document. Any list of people is
doomed to omission and for that I apologize. In alphabetical order, 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 the following people stand out in my mind due to the fact that they
made direct contributions to this document. made direct contributions to this document.
Bill Flanigan Bill Flanigan
Trevor Freeman Trevor Freeman
Elliott Ginsburg Elliott Ginsburg
Paul Hoffman Paul Hoffman
Russ Housley Russ Housley
David P. Kemp David P. Kemp
Michael Myers Michael Myers
John Pawling John Pawling
Denis Pinkas Denis Pinkas
Jim Schaad Jim Schaad
D. Editor's address C. Editor's Address
Blake Ramsdell Blake Ramsdell
Sendmail, Inc. Sendmail, Inc.
704 228th Ave NE #775 704 228th Ave NE #775
Sammamish, WA 98074 Sammamish, WA 98074
blake@sendmail.com EMail: blake@sendmail.com
Full Copyright Statement
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except as set forth therein, the authors retain all their rights.
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