draft-ietf-smime-cert-07.txt   rfc2632.txt 
Internet Draft Editor: Blake Ramsdell,
draft-ietf-smime-cert-07.txt Worldtalk
March 31, 1999
Expires September 30, 1999
S/MIME Version 3 Certificate Handling Network Working Group B. Ramsdell, Editor
Request for Comments: 2632 Worldtalk
Status of this memo Category: Standards Track June 1999
This document is an Internet-Draft and is in full conformance with all S/MIME Version 3 Certificate Handling
provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Task Status of this Memo
Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months This document specifies an Internet standards track protocol for the
and may be updated, replaced, or obsoleted by other documents at any Internet community, and requests discussion and suggestions for
time. It is inappropriate to use Internet-Drafts as reference improvements. Please refer to the current edition of the "Internet
material 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.
The list of current Internet-Drafts can be accessed at Copyright Notice
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at Copyright (C) The Internet Society (1999). All Rights Reserved.
http://www.ietf.org/shadow.html.
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, the
S/MIME agent MUST certify that the public key is valid. S/MIME agents S/MIME agent MUST certify that the public key is valid. S/MIME agents
MUST use PKIX certificates to validate public keys as described in the MUST use PKIX certificates to validate public keys as described in
Internet X.509 Public Key Infrastructure (PKIX) Certificate and CRL the Internet X.509 Public Key Infrastructure (PKIX) Certificate and
Profile [KEYM]. S/MIME agents MUST meet the certificate processing CRL Profile [KEYM]. S/MIME agents MUST meet the certificate
requirements documented in this document in addition to those stated processing requirements documented in this document in addition to
in [KEYM]. 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 draft, the following definitions apply. For the purposes of this memo, the following definitions apply.
ASN.1: Abstract Syntax Notation One, as defined in ITU-T X.680-689.
Attribute Certificate (AC): An X.509 AC is a separate structure from a ASN.1: Abstract Syntax Notation One, as defined in ITU-T X.680-689.
subject's public key X.509 Certificate. A subject may have multiple
X.509 ACs associated with each of its public key X.509 Certificates.
Each X.509 AC binds one or more Attributes with one of the subject's
public key X.509 Certificates. The X.509 AC syntax is defined in
[X.509]
BER: Basic Encoding Rules for ASN.1, as defined in ITU-T X.690. Attribute Certificate (AC): An X.509 AC is a separate structure from
a subject's public key X.509 Certificate. A subject may have
multiple X.509 ACs associated with each of its public key X.509
Certificates. Each X.509 AC binds one or more Attributes with one of
the subject's public key X.509 Certificates. The X.509 AC syntax is
defined in [X.509]
BER: Basic Encoding Rules for ASN.1, as defined in ITU-T X.690.
Certificate: A type that binds an entity's distinguished name to a Certificate: A type that binds an entity's distinguished name to a
public key with a digital signature. This type is defined in the public key with a digital signature. This type is defined in the
Internet X.509 Public Key Infrastructure (PKIX) Certificate and CRL Internet X.509 Public Key Infrastructure (PKIX) Certificate and CRL
Profile [KEYM]. This type also contains the distinguished name of the Profile [KEYM]. This type also contains the distinguished name of the
certificate issuer (the signer), an issuer-specific serial number, the certificate issuer (the signer), an issuer-specific serial number,
issuer's signature algorithm identifier, a validity period, and the issuer's signature algorithm identifier, a validity period, and
extensions also defined in that document. extensions 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 DER: Distinguished Encoding Rules for ASN.1, as defined in ITU-T
X.690. 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 agents should attempt to have the greatest S/MIME version 3 agents should attempt to have the greatest
interoperability possible with S/MIME version 2 agents. S/MIME version interoperability possible with S/MIME version 2 agents. S/MIME
2 is described in RFC 2311 through RFC 2315, inclusive. RFC 2311 also version 2 is described in RFC 2311 through RFC 2315, inclusive. RFC
has historical information about the development of S/MIME. 2311 also has historical information about the development of S/MIME.
1.3 Terminology 1.3 Terminology
Throughout this draft, the terms MUST, MUST NOT, SHOULD, and SHOULD The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
NOT are used in capital letters. This conforms to the definitions in "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
[MUSTSHOULD]. [MUSTSHOULD] defines the use of these key words to help document are to be interpreted as described in [MUSTSHOULD].
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.
1.4 Discussion of This Draft
This draft is being discussed on the "ietf-smime" mailing list.
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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. messages, the CRL format defined in [KEYM] MUST be used.
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.
Agents MUST handle multiple valid Certificate Authority (CA) Agents MUST handle multiple valid Certificate 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.
2.2 CertificateChoices 2.2 CertificateChoices
Receiving agents MUST support PKIX v1 and PKIX v3 certificates. See Receiving agents MUST support PKIX v1 and PKIX v3 certificates. See
[KEYM] for details about the profile for certificate formats. End [KEYM] for details about the profile for certificate formats. End
entity certificates MAY include an Internet mail address, as described entity certificates MAY include an Internet mail address, as
in section 3.1. described in section 3.1.
Receiving agents SHOULD support X.509 attribute certificates. Receiving agents SHOULD support X.509 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 and public key content types: PKIX, PKCS #6 Extended Certificates and
X.509 Attribute Certificates. The PKCS #6 format is not in widespread X.509 Attribute Certificates. The PKCS #6 format is not in widespread
use. In addition, PKIX certificate extensions address much of the same use. In addition, PKIX certificate extensions address much of the
functionality and flexibility as was intended in the PKCS #6. Thus, same functionality and flexibility as was intended in the PKCS #6.
sending and receiving agents MUST NOT use PKCS #6 extended Thus, sending and receiving agents MUST NOT use PKCS #6 extended
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 sending
a message to recipients that may not have access to the sender's 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 public key through any other means or when sending a signed message
a new recipient. The inclusion of certificates in outgoing messages to a new recipient. The inclusion of certificates in outgoing
can be omitted if S/MIME objects are sent within a group of messages can be omitted if S/MIME objects are sent within a group of
correspondents that has established access to each other's 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
SHOULD be able to handle an arbitrarily large number of certificates SHOULD be able to handle an arbitrarily large number of certificates
and chains. and chains.
Agents MAY send CA certificates, that is, certificates that are self- Agents MAY send CA certificates, that is, certificates that are
signed and can be considered the "root" of other chains. Note that self-signed and can be considered the "root" of other chains. Note
receiving agents SHOULD NOT simply trust any self-signed certificates that receiving agents SHOULD NOT simply trust any self-signed
as valid CAs, but SHOULD use some other mechanism to determine if this certificates as valid CAs, but SHOULD use some other mechanism to
is a CA that should be trusted. Also note that in the case of DSA determine if this is a CA that should be trusted. Also note that in
certificates the parameters may be located in the root certificate. the case of DSA certificates the parameters may be located in the
This would require that the recipient possess the root certificate in root certificate. This would require that the recipient possess the
order to perform a signature verification, and is a valid example of a root certificate in order to perform a signature verification, and is
case where transmitting the root certificate may be required. a valid 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
but are not currently recommended. supported but are not currently recommended.
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. scope of this specification.
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-822]. The address must be an "addr-spec" as defined described in [RFC-822]. The address must be an "addr-spec" as defined
in Section 6.1 of that specification. The email address SHOULD be in in Section 6.1 of that specification. The email address SHOULD be in
the subjectAltName extension, and SHOULD NOT be in the subject the subjectAltName extension, and SHOULD NOT be in the subject
distinguished name. distinguished name.
Receiving agents MUST recognize email addresses in the subjectAltName Receiving agents MUST recognize email addresses in the subjectAltName
field. Receiving agents MUST recognize email addresses in the field. Receiving agents MUST recognize email addresses in the
Distinguished Name field in the PKCS #9 emailAddress attribute. Distinguished Name field in the PKCS #9 emailAddress attribute.
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 From
or Sender header of a mail message matches an Internet mail address in or Sender header of a mail message matches an Internet mail address
the signer's certificate, if mail addresses are present in the in the signer's certificate, if mail addresses are present in the
certificate. A receiving agent SHOULD provide some explicit alternate certificate. A receiving agent SHOULD provide some explicit alternate
processing of the message if this comparison fails, which may be to processing of the message if this comparison fails, which may be to
display a message that shows the recipient the addresses in the display a message that shows the recipient the addresses in the
certificate or other certificate details. certificate or other certificate details.
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 PKIX certificates, except that the SEQUENCE) in S/MIME-compliant PKIX certificates, except that the
subject DN in a user's (i.e. end-entity) certificate MAY be an empty subject DN in a user's (i.e. end-entity) certificate MAY be an empty
SEQUENCE in which case the subjectAltName extension will include the SEQUENCE in which case the subjectAltName extension will include the
subject's identifier and MUST be marked as critical. 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 example
of a certificate retrieval-only mechanism that is compatible with of a certificate retrieval-only mechanism that is compatible with
classic X.500 Distinguished Names. The PKIX Working Group is classic X.500 Distinguished Names. The PKIX Working Group is
investigating other mechanisms such as directory servers. Another investigating other mechanisms such as directory servers. Another
method under consideration by the IETF is to provide certificate method under consideration by the IETF is to provide certificate
retrieval services as part of the existing Domain Name System (DNS). retrieval services as part of the existing Domain Name System (DNS).
Until such mechanisms are widely used, their utility may be limited by Until such mechanisms are widely used, their utility may be limited
the small number of correspondent's certificates that can be by the small number of correspondent's certificates that can be
retrieved. At a minimum, for initial S/MIME deployment, a user agent retrieved. At a minimum, for initial S/MIME deployment, a user agent
could automatically generate a message to an intended recipient could automatically generate a message to an intended recipient
requesting that recipient's certificate in a signed return message. requesting 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 environments,
it may be desirable to link the certificate retrieval/storage it may be desirable to link the certificate retrieval/storage
mechanisms together in some sort of certificate database. In its mechanisms together in some sort of certificate database. In its
simplest form, a certificate database would be local to a particular simplest form, a certificate database would be local to a particular
user and would function in a similar way as a "address book" that user and would function in a similar way as a "address book" that
stores a user's frequent correspondents. In this way, the certificate stores a user's frequent correspondents. In this way, the certificate
retrieval mechanism would be limited to the certificates that a user retrieval mechanism would be limited to the certificates that a user
has stored (presumably from incoming messages). A comprehensive has stored (presumably from incoming messages). A comprehensive
certificate retrieval/storage solution may combine two or more certificate retrieval/storage solution may combine two or more
mechanisms to allow the greatest flexibility and utility to the user. mechanisms to allow the greatest flexibility and utility to the user.
For instance, a secure Internet mail agent may resort For instance, a secure Internet mail agent may resort to checking a
to checking a centralized certificate retrieval mechanism for a centralized certificate retrieval mechanism for a certificate if it
certificate if it can not be found in a user's local certificate can not be found in a user's local certificate storage/retrieval
storage/retrieval database. 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].
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
revocation information when validating certificate chains. A receiving certificate-revocation information when validating certificate
or sending agent SHOULD also provide a mechanism to allow a user to chains. A receiving or sending agent SHOULD also provide a mechanism
store incoming certificate-revocation information for correspondents to allow a user to store incoming certificate-revocation information
in such a way so as to guarantee its later retrieval. for 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
certificate chain validation even if the certificate was already certificate chain 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 dictated
the value of the information that is protected. The value of the CRL by the value of the information that is protected. The value of the
information in a particular context is beyond the scope of this draft CRL information in a particular context is beyond the scope of this
but may be governed by the policies associated with particular memo but may be governed by the policies associated with particular
certificate hierarchies. certificate hierarchies.
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 Certificate Chain Validation 4.2 Certificate Chain Validation
In creating a user agent for secure messaging, certificate, CRL, and In creating a user agent for secure messaging, certificate, CRL, and
certificate chain validation SHOULD be highly automated while still certificate chain validation SHOULD be highly automated while still
acting in the best interests of the user. Certificate, CRL, and chain acting in the best interests of the user. Certificate, CRL, and chain
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 chain validation Certificates and CRLs are made available to the chain 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). Incoming
certificates and CRLs SHOULD be cached for use in chain validation and certificates and CRLs SHOULD be cached for use in chain validation
optionally stored for later use. This temporary certificate and CRL and optionally stored for later use. This temporary certificate and
cache SHOULD be used to augment any other certificate and CRL CRL cache SHOULD be used to augment any other certificate and CRL
retrieval mechanisms for chain validation on incoming signed messages. retrieval mechanisms for chain 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 [DSS]. verifying the signatures on certificates and CRLs made with id-dsa-
with-sha1 [DSS].
A receiving agent SHOULD be capable of verifying the signatures on A receiving agent SHOULD be capable of verifying the signatures on
certificates and CRLs made with md2WithRSAEncryption, certificates and CRLs made with md2WithRSAEncryption,
md5WithRSAEncryption and sha-1WithRSAEncryption signature algorithms md5WithRSAEncryption and sha-1WithRSAEncryption signature algorithms
with key sizes from 512 bits to 2048 bits described in [PKCS#1V2]. with key sizes from 512 bits to 2048 bits described in [PKCS#1V2].
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 minumum required set business purposes. This document identifies the minumum 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 Certificate Extension, the Key Usage Certificate Constraints Certificate Extension, the Key Usage Certificate
Extension, authorityKeyID, subjectKeyID, and the subjectAltNames when Extension, authorityKeyID, subjectKeyID, and the subjectAltNames when
they appear in end-user certificates. Some mechanism SHOULD exist to they appear in end-user certificates. Some mechanism SHOULD exist to
handle the defined certificate extensions when they appear in handle the defined certificate extensions when they appear in
intermediate or CA certificates. intermediate 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 marked
as non-critical unless the proper handling of the extension is deemed as non-critical unless the proper handling of the extension is deemed
critical to the correct interpretation of the associated certificate. critical to the correct interpretation of the associated certificate.
Other extensions may be included, but those extensions SHOULD NOT be Other extensions may be included, but those extensions SHOULD NOT be
marked as critical. 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 of an issuing authority or end-entity certificate plays in a
chain of certificates. chain of certificates.
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. Issuing
authority certificates may contain a key usage extension that 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 keyUsage extension which specifies that certificates which contain a keyUsage extension which specifies that
the corresponding public key can be used to sign end user certs and the corresponding public key can be used to sign end user certs and
sign CRLs. 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.
4.4.2.1 Key Usage in Diffie-Hellman Key Exchange Certificates 4.4.2.1 Key Usage in Diffie-Hellman Key Exchange Certificates
For Diffie-Hellman key exchange certificates (certificates in which For Diffie-Hellman key exchange certificates (certificates in which
the subject public key algorithm is dhpublicnumber), if the keyUsage the subject public key algorithm is dhpublicnumber), if the keyUsage
keyAgreement bit is set to 1 AND if the public key is to be used to keyAgreement bit is set to 1 AND if the public key is to be used to
form a pairwise key to decrypt data, then the S/MIME agent MUST only form a pairwise key to decrypt data, then the S/MIME agent MUST only
use the public key if the keyUsage encipherOnly bit is set to 0. If use the public key if the keyUsage encipherOnly bit is set to 0. If
the keyUsage keyAgreement bit is set to 1 AND if the key is to be used the keyUsage keyAgreement bit is set to 1 AND if the key is to be
to form a pairwise key to encrypt data, then the S/MIME agent MUST used to form a pairwise key to encrypt data, then the S/MIME agent
only use the public key if the keyUsage decipherOnly bit is set to 0. MUST only use the public key if the keyUsage decipherOnly bit is set
to 0.
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-822 email address(es) that preferred means to convey the RFC-822 email address(es) that
correspond to the entity for this certificate. Any RFC-822 email correspond to the entity for this certificate. Any RFC-822 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-822 email addresses MAY be present. GeneralName, multiple RFC-822 email addresses MAY be present.
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 noticable steps to inform the end software should take immediate and noticable 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 match the sender of a
message - no Internet mail addresses in a certificate match the sender
- no certificate chain leads to a trusted CA of a message
- no ability to check the CRL for a certificate - no certificate chain leads to a trusted CA
- an invalid CRL was received - no ability to check the CRL for a certificate
- the CRL being checked is expired - an invalid CRL was received
- the certificate is expired - the CRL being checked is expired
- the certificate has been revoked - the certificate is expired
There are certainly other instances where a certificate may be - the certificate has been revoked
invalid, and it is the responsibility of the processing software to
check them all thoroughly, and to decide what to do if the check There are certainly other instances where a certificate may be
fails. 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.
A. References A. References
[CERTV2] "S/MIME Version 2 Certificate Handling", RFC 2312 [CERTV2] Dusse, S., Hoffman, P. and B. Ramsdell,"S/MIME Version 2
Certificate Handling", RFC 2312, March 1998.
[CMS] "Cryptographic Message Syntax", Internet Draft draft-housley- [CMS] Housley, R., "Cryptographic Message Syntax", RFC 2630,
smime-cms June 1999.
[DSS] NIST FIPS PUB 186, "Digital Signature Standard", 18 May 1994. [DSS] NIST FIPS PUB 186, "Digital Signature Standard", 18 May
1994.
[KEYM] "Internet X.509 Public Key Infrastructure Certificate and CRL [KEYM] Housley, R., Ford, W., Polk, W. and D. Solo, "Internet
Profile", Internet-Draft draft-ietf-pkix-ipki-part1 X.509 Public Key Infrastructure Certificate and CRL
Profile", RFC 2459, January 1999.
[MUSTSHOULD] "Key words for use in RFCs to Indicate Requirement [MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate
Levels", RFC 2119 Requirement Levels", BCP 14, RFC 2119, March 1997.
[PKCS#1V2], "PKCS #1: RSA Cryptography Specifications Version 2.0", [PKCS#1V2] Kaliski, B., "PKCS #1: RSA Cryptography Specifications
RFC 2437 Version 2.0", RFC 2437, October 1998.
[RFC-822], "Standard For The Format Of ARPA Internet Text Messages", [RFC-822] Crocker, D., "Standard For The Format Of ARPA Internet
RFC 822. Text Messages", STD 11, RFC 822, August 1982.
[SMIME-MSG] "S/MIME Version 3 Message Specification ", Internet Draft [SMIME-MSG] Ramsdell, B., Editor, "S/MIME Version 3 Message
draft-ietf-smime-msg Specification", RFC 2633, June 1999.
[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.
B. Acknowledgements B. Acknowledgements
<TBD> Many thanks go out to the other authors of the S/MIME v2 RFC: Steve
Dusse, Paul Hoffman and Jeff Weinstein. Without v2, there wouldn't be
a v3.
C. Changes from last draft A number of the members of the S/MIME Working Group have also worked
very hard and contributed to this document. Any list of people is
doomed to omission and for that I apologize. In alphabetical order,
the following people stand out in my mind due to the fact that they
made direct contributions to this document.
Changed "I-D" to "document" in section 1 (Russ Housley) Bill Flanigan Elliott Ginsburg Paul Hoffman Russ Housley Michael
Added clarification to section 3.1 regarding emailAddress attribute Myers John Pawling Denis Pinkas Jim Schaad
from PKCS #9 (Russ Housley)
Redid 4.4.2.1 regarding Diffie-Hellman to clarify "doneness" (Russ
Housley)
Clarified 4.4 regarding certificate extensions and profiling efforts
(Russ Housley)
Clarified 2.3 regarding DSA parameters being scattered all over the
certificate chain, necessitating the transmission of the root
certificate (Jim Schaad)
Clarified 4.4.1 regarding basic constraints in end entity certificates
(Jim Schaad)
Changed an errant reference to [KEYM] to [SMIME-MSG] (Jim Schaad)
Removed certificate request language from section 1 (Jim Schaad)
Removed [CRMF] reference from section A (Jim Schaad)
Added reference to PKCS #1 v2 and DSS for signature algorithms in
section 4.3 (Jim Schaad)
Fixed some language in 4.4 regarding syntax and semantics of
extensions are defined in [KEYM] (Jim Schaad)
Changed back the errant reference to [KEYM] (John Pawling)
Promoted section 3.1 to section 3 (Paul Hoffman)
Added CRL processing clarification to 2.1 and 4.1 (WG Consensus "after
much exciting debate" initiated by Denis Pinkas)
D. Editors address Editor's Address
Blake Ramsdell Blake Ramsdell
Worldtalk Worldtalk
13122 NE 20th St., Suite C 17720 NE 65th St Ste 201
Bellevue, WA 98005 Redmond, WA 98052
(425) 882-8861
blaker@deming.com Phone: +1 425 376 0225
EMail: blaker@deming.com
Full Copyright Statement
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and distributed, in whole or in part, without restriction of any
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included on all such copies and derivative works. However, this
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The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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