draft-ietf-smime-3850bis-11.txt   rfc5750.txt 
S/MIME WG B. Ramsdell
Internet Draft Brute Squad Labs
Intended Status: Standard Track S. Turner
Obsoletes: 3850 (once approved) IECA
Expires: November 12, 2009 May 12, 2009
Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2
Certificate Handling
draft-ietf-smime-3850bis-11.txt
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. This document may contain material
from IETF Documents or IETF Contributions published or made publicly
available before November 10, 2008. The person(s) controlling the
copyright in some of this material may not have granted the IETF
Trust the right to allow modifications of such material outside the
IETF Standards Process. Without obtaining an adequate license from
the person(s) controlling the copyright in such materials, this
document may not be modified outside the IETF Standards Process, and
derivative works of it may not be created outside the IETF Standards
Process, except to format it for publication as an RFC or to
translate it into languages other than English.
Internet-Drafts are working documents of the Internet Engineering
Task 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
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on November 12, 2009. Internet Engineering Task Force (IETF) B. Ramsdell
Request for Comments: 5750 Brute Squad Labs
Copyright Notice Obsoletes: 3850 S. Turner
Category: Standards Track IECA
Copyright (c) 2009 IETF Trust and the persons identified as the ISSN: 2070-1721 January 2010
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2
Provisions Relating to IETF Documents in effect on the date of Certificate Handling
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract Abstract
This document specifies conventions for X.509 certificate usage by This document specifies conventions for X.509 certificate usage by
Secure/Multipurpose Internet Mail Extensions (S/MIME) v3.2 agents. Secure/Multipurpose Internet Mail Extensions (S/MIME) v3.2 agents.
S/MIME provides a method to send and receive secure MIME messages, S/MIME provides a method to send and receive secure MIME messages,
and certificates are an integral part of S/MIME agent processing. and certificates are an integral part of S/MIME agent processing.
S/MIME agents validate certificates as described in RFC 5280, the S/MIME agents validate certificates as described in RFC 5280, the
Internet X.509 Public Key Infrastructure Certificate and CRL Profile. Internet X.509 Public Key Infrastructure Certificate and CRL Profile.
S/MIME agents must meet the certificate processing requirements in S/MIME agents must meet the certificate processing requirements in
this document as well as those in RFC 5280. This document obsoletes this document as well as those in RFC 5280. This document obsoletes
RFC 3850. RFC 3850.
Discussion Status of This Memo
This draft is being discussed on the 'ietf-smime' mailing list. To This is an Internet Standards Track document.
subscribe, send a message to ietf-smime-request@imc.org with the
single word subscribe in the body of the message. There is a Web site This document is a product of the Internet Engineering Task Force
for the mailing list at <http://www.imc.org/ietf-smime/>. (IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by
the Internet Engineering Steering Group (IESG). Further
information on Internet Standards is available in Section 2 of
RFC 5741.
Information about the current status of this document, any
errata, and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5750.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents Table of Contents
1. Introduction...................................................3 1. Introduction ....................................................3
1.1. Definitions...............................................3 1.1. Definitions ................................................3
1.2. Conventions used in this document.........................4 1.2. Conventions Used in This Document ..........................4
1.3. Compatibility with Prior Practice S/MIME..................4 1.3. Compatibility with Prior Practice S/MIME ...................4
1.4. Changes From S/MIME v3 to S/MIME v3.1.....................5 1.4. Changes from S/MIME v3 to S/MIME v3.1 ......................5
1.5. Changes Since S/MIME v3.1.................................5 1.5. Changes since S/MIME v3.1 ..................................5
2. CMS Options....................................................6 2. CMS Options .....................................................6
2.1. Certificate Revocation Lists..............................6 2.1. Certificate Revocation Lists ...............................6
2.2. Certificate Choices.......................................7 2.2. Certificate Choices ........................................6
2.2.1. Historical Note About CMS Certificates...............7 2.3. CertificateSet .............................................7
2.3. CertificateSet............................................7 3. Using Distinguished Names for Internet Mail .....................8
3. Using Distinguished Names For Internet Mail....................8 4. Certificate Processing ..........................................9
4. Certificate Processing.........................................9 4.1. Certificate Revocation Lists ..............................10
4.1. Certificate Revocation Lists.............................10 4.2. Certificate Path Validation ...............................11
4.2. Certificate Path Validation..............................11 4.3. Certificate and CRL Signing Algorithms and Key Sizes ......11
4.3. Certificate and CRL Signing Algorithms and Key Sizes.....12 4.4. PKIX Certificate Extensions ...............................12
4.4. PKIX Certificate Extensions..............................12 5. Security Considerations ........................................15
5. IANA Considerations...........................................15 6. References .....................................................17
6. Security Considerations.......................................15 6.1. Reference Conventions .....................................17
7. References....................................................18 6.2. Normative References ......................................17
7.1. Normative References.....................................18 6.3. Informative References ....................................19
7.2. Informative References...................................19
Appendix A. Moving S/MIME v2 Certificate Handling to Historic Appendix A. Moving S/MIME v2 Certificate Handling to Historic
Status...............................................22 Status.................................................21
Appendix B. Acknowledgements.....................................22 Appendix B. Acknowledgments........................................21
1. Introduction 1. Introduction
S/MIME (Secure/Multipurpose Internet Mail Extensions) v3.2, described S/MIME (Secure/Multipurpose Internet Mail Extensions) v3.2, described
in [SMIME-MSG], provides a method to send and receive secure MIME in [SMIME-MSG], provides a method to send and receive secure MIME
messages. Before using a public key to provide security services, messages. Before using a public key to provide security services,
the S/MIME agent MUST verify that the public key is valid. S/MIME the S/MIME agent MUST verify that the public key is valid. S/MIME
agents MUST use PKIX certificates to validate public keys as agents MUST use PKIX certificates to validate public keys as
described in the Internet X.509 Public Key Infrastructure (PKIX) described in the Internet X.509 Public Key Infrastructure (PKIX)
Certificate and CRL Profile [KEYM]. S/MIME agents MUST meet the Certificate and CRL Profile [KEYM]. S/MIME agents MUST meet the
certificate processing requirements documented in this document in certificate processing requirements documented in this document in
addition to those stated in [KEYM]. addition to those stated in [KEYM].
This specification is compatible with the Cryptographic Message This specification is compatible with the Cryptographic Message
Syntax (CMS) RFC 3852 and RFC 4853 [CMS] in that it uses the data Syntax (CMS) RFC 5652 [CMS] in that it uses the data types defined by
types defined by CMS. It also inherits all the varieties of CMS. It also inherits all the varieties of architectures for
architectures for certificate-based key management supported by CMS. certificate-based key 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.680 ASN.1: Abstract Syntax Notation One, as defined in ITU-T X.680
[X.680]. [X.680].
Attribute Certificate (AC): An X.509 AC is a separate structure from Attribute certificate (AC): An X.509 AC is a separate structure from
a subject's public key X.509 Certificate. A subject may have 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 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 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 the subject's public key X.509 certificates. The X.509 AC syntax is
defined in [ACAUTH]. defined in [ACAUTH].
Certificate: A type that binds an entity's name to a public key with Certificate: A type that binds an entity's name to a public key with
a digital signature. This type is defined in the Internet X.509 a digital signature. This type is defined in the Internet X.509
Public Key Infrastructure (PKIX) Certificate and CRL Profile [KEYM]. Public Key Infrastructure (PKIX) Certificate and CRL Profile [KEYM].
This type also contains the distinguished name of the certificate This type also contains the distinguished name of the certificate
issuer (the signer), an issuer-specific serial number, the issuer's issuer (the signer), an issuer-specific serial number, the issuer's
signature algorithm identifier, a validity period, and extensions signature algorithm identifier, a validity period, and extensions
also defined in that document. also defined in that document.
skipping to change at page 4, line 24 skipping to change at page 4, line 14
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. Conventions used in this document 1.2. Conventions Used in This Document
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].
We define some additional terms here: We define some additional terms here:
SHOULD+ This term means the same as SHOULD. However, the authors SHOULD+ This term means the same as SHOULD. However, the authors
expect that a requirement marked as SHOULD+ will be promoted at expect that a requirement marked as SHOULD+ will be
some future time to be a MUST. promoted at some future time to be a MUST.
SHOULD- This term means the same as SHOULD. However, the authors SHOULD- This term means the same as SHOULD. However, the authors
expect that a requirement marked as SHOULD- will be demoted to a expect that a requirement marked as SHOULD- will be
MAY in a future version of this document. demoted to a MAY in a future version of this document.
MUST- This term means the same as MUST. However, the authors MUST- This term means the same as MUST. However, the authors
expect that this requirement will no longer be a MUST in a future expect that this requirement will no longer be a MUST in a
document. Although its status will be determined at a later future document. Although its status will be determined
time, it is reasonable to expect that if a future revision of a at a later time, it is reasonable to expect that if a
document alters the status of a MUST- requirement, it will remain future revision of a document alters the status of a MUST-
at least a SHOULD or a SHOULD-. requirement, it will remain at least a SHOULD or a
SHOULD-.
1.3. Compatibility with Prior Practice S/MIME 1.3. Compatibility with Prior Practice S/MIME
S/MIME version 3.2 agents ought to attempt to have the greatest S/MIME version 3.2 agents ought to 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
[SMIMEv2], S/MIME version 3 is described in RFC 2630 through RFC 2634 [SMIMEv2], S/MIME version 3 is described in RFC 2630 through RFC 2634
inclusive and RFC 5035 [SMIMEv3], and S/MIME version 3.1 is described inclusive and RFC 5035 [SMIMEv3], and S/MIME version 3.1 is described
in RFC 3850, RFC 3851, RFC 3852, RFC 2634, RFC4853, and RFC 5035 in RFC 3850, RFC 3851, RFC 3852, RFC 2634, and RFC 5035 [SMIMEv3.1].
[SMIMEv3.1]. RFC 2311 also has historical information about the RFC 2311 also has historical information about the development of
development of S/MIME. S/MIME.
1.4. Changes From S/MIME v3 To S/MIME v3.1 1.4. Changes from S/MIME v3 to S/MIME v3.1
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 certification authority (CA) certificates with the same
with overlapping validity periods, MUST be supported. subject and public key, but 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 was added.
Clarified use of email address use in certificates. Certificates Clarified use of email address use in certificates. Certificates
that do not contain an email address have no requirements for that do not contain an email address have no requirements for
verifying the 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.
1.5. Changes Since S/MIME v3.1 1.5. Changes since S/MIME v3.1
Conventions Used in This Document: Moved to section 1.2. Added Conventions Used in This Document: Moved to Section 1.2. Added
definitions for SHOULD+, SHOULD-, and MUST-. definitions for SHOULD+, SHOULD-, and MUST-.
Sec 1.1: Updated ASN.1 definition and reference. Section 1.1: Updated ASN.1 definition and reference.
Sec 1.3: Added text about v3.1 RFCs. Section 1.3: Added text about v3.1 RFCs.
Sec 3: Aligned email address text with RFC 5280. Updated note to Section 3: Aligned email address text with RFC 5280. Updated
indicate emailAddress IA5String upper bound is 255 characters. Added note to indicate emailAddress IA5String upper bound is
text about matching email addresses. 255 characters. Added text about matching email
addresses.
Sec 4.2: Added text to indicate how S/MIME agents locate the correct Section 4.2: Added text to indicate how S/MIME agents locate the
user certificate. correct user certificate.
Sec 4.3: RSA with SHA-256 (PKCS #1 v1.5) added as MUST, DSA with SHA- Section 4.3: RSA with SHA-256 (PKCS #1 v1.5) added as MUST; DSA
256 added as SHOULD+, RSA with SHA-1, DSA with SHA-1, and RSA with with SHA-256 added as SHOULD+; RSA with SHA-1, DSA
MD5 changed to SHOULD-, and RSASSA-PSS with SHA-256 added as SHOULD+. with SHA-1, and RSA with MD5 changed to SHOULD-; and
Updated key sizes and changed pointer to PKIX RFCs. RSASSA-PSS with SHA-256 added as SHOULD+. Updated key
sizes and changed pointer to PKIX RFCs.
Sec 4.4.1: Aligned with PKIX on use of basic constraints extension in Section 4.4.1: Aligned with PKIX on use of basic constraints
CA certificates. Clarified which extension is used to constrain EEs extension in CA certificates. Clarified which
from using their keys to perform issuing authority operations. extension is used to constrain end entities from using
their keys to perform issuing authority operations.
Sec 6: Updated security considerations. Section 5: Updated security considerations.
Sec 7: Moved references from Appendix B to section 7. Updated the Section 7: Moved references from Appendix B to Section 6.
references. Updated the references.
Appendix A: Moved Appendix A to Appendix B. Added Appendix A to move Appendix A: Moved Appendix A to Appendix B. Added Appendix A to
S/MIME v2 Certificate Handling to Historic Status. move S/MIME v2 Certificate Handling to Historic
Status.
2. CMS Options 2. CMS Options
The CMS message format allows for a wide variety of options in The CMS message format allows for a wide variety of options in
content and algorithm support. This section puts forth a number of content and algorithm support. This section puts forth a number of
support requirements and recommendations in order to achieve a base support requirements and recommendations in order to achieve a base
level of interoperability among all S/MIME implementations. Most of level of interoperability among all S/MIME implementations. Most of
the CMS format for S/MIME messages is defined in [SMIME-MSG]. the CMS format for S/MIME messages is defined in [SMIME-MSG].
2.1. Certificate Revocation Lists 2.1. Certificate Revocation Lists
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 messages, the CRL format defined in [KEYM] MUST be used. In all
cases, 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. Certificate Choices 2.2. Certificate Choices
Receiving agents MUST support v1 X.509 and v3 X.509 certificates as Receiving agents MUST support v1 X.509 and v3 X.509 certificates as
profiled in [KEYM]. End entity certificates MAY include an Internet profiled in [KEYM]. End-entity certificates MAY include an Internet
mail address, as described in section 3. 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 certificates. See [ACAUTH] for details about the profile for
attribute 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
and PKIX Attribute Certificates. [PKCS6], 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 situations where the certificates in a signed message may be
unrelated 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
skipping to change at page 8, line 8 skipping to change at page 8, line 6
sender's public key through any other means or when sending a signed sender's public key through any other means or when sending a signed
message to a new recipient. The inclusion of certificates in message to a new recipient. The inclusion of certificates in
outgoing messages can be omitted if S/MIME objects are sent within a outgoing messages can be omitted if S/MIME objects are sent within a
group of 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 certification authority (CA) that it
that the recipient may trust as authoritative. A receiving agent believes that the recipient may trust as authoritative. A receiving
MUST be able to handle an arbitrarily large number of certificates agent MUST be able to handle an arbitrarily large number of
and chains. certificates and chains.
Agents MAY send CA certificates, that is, cross-certificates, self- Agents MAY send CA certificates, that is, cross-certificates, self-
issued certificates, and self-signed certificates. Note that issued certificates, and self-signed certificates. Note that
receiving agents SHOULD NOT simply trust any self-signed certificates receiving agents SHOULD NOT simply trust any self-signed certificates
as valid CAs, but SHOULD use some other mechanism to determine if as valid CAs, but SHOULD use some other mechanism to determine if
this is a CA that should be trusted. Also note that when this is a CA that should be trusted. Also note that when
certificates contain DSA public keys the parameters may be located in certificates contain Digital Signature Algorithm (DSA) public keys
the root certificate. This would require that the recipient possess the parameters may be located in the root certificate. This would
both the end-entity certificate as well as the root certificate to require that the recipient possess both the end-entity certificate
perform a signature verification, and is a valid example of a case and the root certificate to perform a signature verification, and is
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 Receiving agents MUST support chaining based on the distinguished
name fields. Other methods of building certificate chains MAY be name fields. Other methods of building certificate chains MAY be
supported. 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 [KEYM] Section 4.2.1.6. The email address SHOULD be in described in [KEYM], Section 4.2.1.6. 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 and accept certificates that contain Receiving agents MUST recognize and accept certificates that contain
no email address. Agents are allowed to provide an alternative no email address. Agents are allowed to provide an alternative
mechanism for associating an email address with a certificate that mechanism for associating an email address with a certificate that
does not contain an email address, such as through the use of the does not contain an email address, such as through the use of the
agent's address book, if available. Receiving agents MUST recognize agent's address book, if available. Receiving agents MUST recognize
email addresses in the subjectAltName field. Receiving agents MUST email addresses in the subjectAltName field. Receiving agents MUST
recognize email addresses in the Distinguished Name field in the PKCS recognize email addresses in the Distinguished Name field in the PKCS
skipping to change at page 9, line 28 skipping to change at page 9, line 25
some explicit alternate processing of the message if this comparison some explicit alternate processing of the message if this comparison
fails, which may be to display a message that shows the recipient the fails, which may be to display a message that shows the recipient the
addresses in 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 certificates, except that the SEQUENCE) in S/MIME-compliant X.509 certificates, except that the
subject DN in a user's (i.e., end-entity) certificate MAY be an empty subject distinguished name (DN) in a user's (i.e., end-entity)
SEQUENCE in which case the subjectAltName extension will include the certificate MAY be an empty SEQUENCE in which case the subjectAltName
subject's identifier and MUST be marked as critical. extension will include the subject's identifier and MUST be marked as
critical.
4. Certificate Processing 4. Certificate Processing
S/MIME agents need to provide some certificate retrieval mechanism in S/MIME agents need to provide some certificate retrieval mechanism in
order to gain access to certificates for recipients of digital order to gain access to certificates for recipients of digital
envelopes. There are many ways to implement certificate retrieval envelopes. There are many ways to implement certificate retrieval
mechanisms. [X.500] directory service is an excellent example of a mechanisms. [X.500] directory service is an excellent example of a
certificate retrieval-only mechanism that is compatible with classic certificate retrieval-only mechanism that is compatible with classic
X.500 Distinguished Names. Another method under consideration by the X.500 Distinguished Names. Another method under consideration by the
IETF is to provide certificate retrieval services as part of the IETF is to provide certificate retrieval services as part of the
existing Domain Name System (DNS). Until such mechanisms are widely existing Domain Name System (DNS). Until such mechanisms are widely
used, their utility may be limited by the small number of the used, their utility may be limited by the small number of the
correspondent's certificates that can be retrieved. At a minimum, for correspondent's certificates that can be retrieved. At a minimum,
initial S/MIME deployment, a user agent could automatically generate for initial S/MIME deployment, a user agent could automatically
a message to an intended recipient requesting the recipient's generate a message to an intended recipient requesting the
certificate in a signed return message. 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 a way so as to guarantee their later retrieval. In many
environments, it may be desirable to link the certificate environments, it may be desirable to link the certificate
retrieval/storage mechanisms together in some sort of certificate retrieval/storage mechanisms together in some sort of certificate
database. In its simplest form, a certificate database would be database. In its simplest form, a certificate database would be
local to a particular user and would function in a similar way as an local to a particular user and would function in a similar way as an
"address book" that stores a user's frequent correspondents. In this "address book" that stores a user's frequent correspondents. In this
way, the certificate retrieval mechanism would be limited to the way, the certificate retrieval mechanism would be limited to the
certificates that a user has stored (presumably from incoming certificates that a user has stored (presumably from incoming
messages). A comprehensive certificate retrieval/storage solution messages). A comprehensive certificate retrieval/storage solution
may combine two or more mechanisms to allow the greatest flexibility may combine two or more mechanisms to allow the greatest flexibility
and utility to the user. For instance, a secure Internet mail agent and utility to the user. For instance, a secure Internet mail agent
may resort to checking a centralized certificate retrieval mechanism may resort to checking a centralized certificate retrieval mechanism
for a certificate if it can not be found in a user's local for a certificate if it cannot be found in a user's local certificate
certificate storage/retrieval database. storage/retrieval database.
Receiving and sending agents SHOULD provide a mechanism for the Receiving and sending agents SHOULD provide a mechanism for the
import and export of certificates, using a CMS certs-only message. import and export of certificates, using a CMS certs-only message.
This allows for import and export of full certificate chains as This allows for import and export of full certificate chains as
opposed to just a single certificate. This is described in [SMIME- opposed to just a single certificate. This is described in
MSG]. [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 certificates containing the same subject name and the same public
keys 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 In general, it is always better to get the latest CRL information
from a CA than to get information stored away from incoming messages. from a CA than to get information stored away from incoming messages.
A receiving agent SHOULD have access to some certificate revocation A receiving agent SHOULD have access to some CRL retrieval mechanism
list (CRL) retrieval mechanism in order to gain access to certificate in order to gain access to certificate revocation information when
revocation information when validating certification paths. A validating certification paths. A receiving or sending agent SHOULD
receiving or sending agent SHOULD also provide a mechanism to allow a also provide a mechanism to allow a user to store incoming
user to store incoming certificate revocation information for certificate revocation information for correspondents in such a way
correspondents in such a way so as to guarantee its later retrieval. 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 verification) access to the latest CRL information may be difficult
or impossible. The use of CRL information, therefore, may be or impossible. The use of CRL information, therefore, may be
dictated by the value of the information that is protected. The dictated by the value of the information that is protected. The
value of the CRL information in a particular context is beyond the value of the CRL information in a particular context is beyond the
scope of this specification but may be governed by the policies scope of this specification but may be governed by the policies
associated with 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. Certificate Path Validation 4.2. Certificate 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 (e.g., RSA); or forming a encrypting a content-encryption key (e.g., RSA), or forming a
pairwise symmetric key (e.g., Diffie-Hellman) to be used to encrypt pairwise symmetric key (e.g., Diffie-Hellman) to be used to encrypt
or decrypt a content-encryption key. or decrypt a 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.
When verifying a signature and the certificates that are included in When verifying a signature and the certificates that are included in
the message, if a signingCertificate attribute from RFC 2634 [ESS] or the message, if a signingCertificate attribute from RFC 2634 [ESS] or
a signingCertificateV2 attribute from RFC 5035 [ESS] is found in an a signingCertificateV2 attribute from RFC 5035 [ESS] is found in an
S/MIME message, it SHALL be used to identify the signer's S/MIME message, it SHALL be used to identify the signer's
certificate. Otherwise, the certificate is identified in an S/MIME certificate. Otherwise, the certificate is identified in an S/MIME
message, either using the issuerAndSerialNumber which identifies the message, either using the issuerAndSerialNumber, which identifies the
signer's certificate by the issuer's distinguished name and the signer's certificate by the issuer's distinguished name and the
certificate serial number, or the subjectKeyIdentifier which certificate serial number, or the subjectKeyIdentifier, which
identifies the signer's certificate by a key identifier. identifies the signer's certificate by a key identifier.
When decrypting an encrypted message, if a When decrypting an encrypted message, if a
SMIMEEncryptionKeyPreference attribute is found in an encapsulating SMIMEEncryptionKeyPreference attribute is found in an encapsulating
SignedData, it SHALL be used to identify the originator's certificate SignedData, it SHALL be used to identify the originator's certificate
found in OriginatorInfo. See [CMS] for the CMS fields that reference found in OriginatorInfo. See [CMS] for the CMS fields that reference
the originator's and recipient's certificates. the originator's and recipient's certificates.
4.3. Certificate and CRL Signing Algorithms and Key Sizes 4.3. Certificate and CRL Signing Algorithms and Key Sizes
Certificates and Certificate Revocation Lists (CRLs) are signed by Certificates and Certificate Revocation Lists (CRLs) are signed by
the certificate issuer. Receiving agents: the certificate issuer. Receiving agents:
- MUST support RSA with SHA-256 - MUST support RSA with SHA-256
- SHOULD+ support DSA with SHA-256 - SHOULD+ support DSA with SHA-256
- SHOULD+ support RSASSA-PSS with SHA-256 - SHOULD+ support RSASSA-PSS with SHA-256
- SHOULD- support RSA with SHA-1 - SHOULD- support RSA with SHA-1
- SHOULD- support DSA with SHA-1 - SHOULD- support DSA with SHA-1
- SHOULD- support RSA with MD5 - SHOULD- support RSA with MD5
The following are the RSA and RSASSA-PSS key size requirements for The following are the RSA and RSASSA-PSS key size requirements for
S/MIME receiving agents during certificate and CRL signature S/MIME receiving agents during certificate and CRL signature
skipping to change at page 12, line 26 skipping to change at page 12, line 16
- SHOULD- support RSA with SHA-1 - SHOULD- support RSA with SHA-1
- SHOULD- support DSA with SHA-1 - SHOULD- support DSA with SHA-1
- SHOULD- support RSA with MD5 - SHOULD- support RSA with MD5
The following are the RSA and RSASSA-PSS key size requirements for The following are the RSA and RSASSA-PSS key size requirements for
S/MIME receiving agents during certificate and CRL signature S/MIME receiving agents during certificate and CRL signature
verification: verification:
key size <= 1023 : MAY (see Section 6) key size <= 1023 : MAY (see Section 5)
1024 <= key size <= 4096 : MUST (see Section 6) 1024 <= key size <= 4096 : MUST (see Section 5)
4096 < key size : MAY (see Section 6) 4096 < key size : MAY (see Section 5)
The following are the DSA key size requirements for S/MIME receiving The following are the DSA key size requirements for S/MIME receiving
agents during certificate and CRL signature verification: agents during certificate and CRL signature verification:
key size <= 1023 : MAY (see Section 6) key size <= 1023 : MAY (see Section 5)
1024 <= key size <= 3072 : MUST (see Section 6) 1024 <= key size <= 3072 : MUST (see Section 5)
For 512-bit RSA with SHA-1 see [KEYMALG] and [FIPS186-2] without For 512-bit RSA with SHA-1 see [KEYMALG] and [FIPS186-2] without
Change Notice 1, for 512-bit RSA with SHA-256 see [RSAOAEP] and Change Notice 1, for 512-bit RSA with SHA-256 see [RSAOAEP] and
[FIPS186-2] without Change Notice 1, for 1024-bit through 3072-bit [FIPS186-2] without Change Notice 1, for 1024-bit through 3072-bit
RSA with SHA-256 see [RSAOAEP] and [FIPS186-2] with Change Notice 1, RSA with SHA-256 see [RSAOAEP] and [FIPS186-2] with Change Notice 1,
and for 4096-bit RSA with SHA-256 see [RSAOAEP] and [PKCS1]. In and for 4096-bit RSA with SHA-256 see [RSAOAEP] and [PKCS1]. In
either case, the first reference provides the signature algorithm's either case, the first reference provides the signature algorithm's
object identifier and the second provides the signature algorithm's object identifier and the second provides the signature algorithm's
definition. definition.
For 512-bit DSA with SHA-1 see [KEYMALG] and [FIPS186-2] without For 512-bit DSA with SHA-1 see [KEYMALG] and [FIPS186-2] without
Change Notice 1, for 512-bit DSA with SHA-256 see [KEYMALG2] and Change Notice 1, for 512-bit DSA with SHA-256 see [KEYMALG2] and
[FIPS186-2] without Change Notice 1, for 1024-bit DSA with SHA-1 see [FIPS186-2] without Change Notice 1, for 1024-bit DSA with SHA-1 see
[KEYMALG] and [FIPS186-2] with Change Notice 1, for 1024-bit through [KEYMALG] and [FIPS186-2] with Change Notice 1, for 1024-bit through
3072 DSA with SHA-256 see [KEYMALG2] and [FIPS186-3]. In either case, 3072 DSA with SHA-256 see [KEYMALG2] and [FIPS186-3]. In either
the first reference provides the signature algorithm's object case, the first reference provides the signature algorithm's object
identifier and the second provides the signature algorithm's identifier and the second provides the signature algorithm's
definition. definition.
For RSASSA-PSS with SHA-256 see [RSAPSS]. For RSASSA-PSS with SHA-256 see [RSAPSS].
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 describes how such extensions can be information can be extended and describes how such extensions can be
used to control the process of issuing and validating certificates. used to control the process of issuing and validating certificates.
The PKIX Working Group has ongoing efforts to identify and create The PKIX Working Group has ongoing efforts to identify and create
extensions which have value in particular certification environments. extensions that have value in particular certification environments.
Further, there are active efforts underway to issue PKIX certificates Further, there are active efforts underway to issue PKIX certificates
for business purposes. This document identifies the minimum required for business purposes. This document identifies the minimum required
set of certificate extensions which have the greatest value in the set of certificate extensions that have the greatest value in the
S/MIME environment. The syntax and semantics of all the identified S/MIME environment. The syntax and semantics of all the identified
extensions 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.
skipping to change at page 13, line 47 skipping to change at page 13, line 29
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 TRUE) other than those listed here. These extensions SHOULD be
marked as non-critical unless the proper handling of the extension is marked as non-critical unless the proper handling of the extension is
deemed critical to the correct interpretation of the associated deemed critical to the correct interpretation of the associated
certificate. Other extensions may be included, but those extensions certificate. Other extensions may be included, but those extensions
SHOULD NOT be 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 4.4.1. Basic Constraints
The basic constraints extension serves to delimit the role and The basic constraints extension serves to delimit the role and
position that an issuing authority or end-entity certificate plays in position that an issuing authority or end-entity certificate plays in
a 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 the key usage certificates. End-entity certificates contain the key usage
extension which restrains EEs from using the key when performing extension that restrains end entities from using the key when
issuing authority operations (see Section 4.4.2). performing issuing authority operations (see Section 4.4.2).
As per [KEYM], Certificates MUST contain a basicConstraints extension As per [KEYM], certificates MUST contain a basicConstraints extension
in CA certificates, and SHOULD NOT contain that extension in end in CA certificates, and SHOULD NOT contain that extension in end-
entity certificates. entity 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 that contain a key usage extension that specifies that
the corresponding public key can be used to sign end user the corresponding public key can be used to sign end user
certificates 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 that 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 4.4.3. Subject Alternative Name
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 email address(es) that correspond(s) to preferred means to convey the email address(es) that correspond(s) to
the entity for this certificate. Any email addresses present MUST be the entity for this certificate. Any email addresses present MUST be
encoded using the rfc822Name CHOICE of the GeneralName type as encoded using the rfc822Name CHOICE of the GeneralName type as
described in [KEYM] Section 4.2.1.6. Since the SubjectAltName type described in [KEYM], Section 4.2.1.6. Since the SubjectAltName type
is a SEQUENCE OF GeneralName, multiple email addresses MAY be is a SEQUENCE OF GeneralName, multiple email addresses MAY be
present. 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 used. The set of technical purposes for the certificate therefore
are the intersection of the uses indicated in the key usage and are the intersection of the uses indicated in the key usage and
extended 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 indicating digital signature and an extended key usage extension that
which includes the email protection OID, then the certificate may be includes the email protection OID, then the certificate may be used
used for signing but not encrypting S/MIME messages. If the for signing but not encrypting S/MIME messages. If the certificate
certificate contains a key usage extension indicating digital contains a key usage extension indicating digital signature but no
signature, but no extended key usage extension then the certificate extended key usage extension, then the certificate may also be used
may also be used to sign but not encrypt S/MIME messages. to sign but not encrypt S/MIME messages.
If the extended key usage extension is present in the certificate If the extended key usage extension is present in the certificate,
then interpersonal message S/MIME receiving agents MUST check that it then interpersonal message S/MIME receiving agents MUST check that it
contains either the emailProtection or the anyExtendedKeyUsage OID as contains either the emailProtection or the anyExtendedKeyUsage OID as
defined in [KEYM]. S/MIME uses other than interpersonal messaging defined in [KEYM]. S/MIME uses other than interpersonal messaging
MAY require the explicit presence of the extended key usage extension MAY require the explicit presence of the extended key usage extension
or other OIDs to be present in the extension or both. or other OIDs to be present in the extension or both.
5. IANA Considerations 5. Security Considerations
None: All identifiers are already registered. Please remove this
section prior to publication as an RFC.
6. Security Considerations
All of the security issues faced by any cryptographic application All of the security issues faced by any cryptographic application
must be faced by a S/MIME agent. Among these issues are protecting must be faced by a S/MIME agent. Among these issues are protecting
the user's private key, preventing various attacks, and helping the the user's private key, preventing various attacks, and helping the
user avoid mistakes such as inadvertently encrypting a message for user avoid mistakes such as inadvertently encrypting a message for
the wrong recipient. The entire list of security considerations is the wrong recipient. The entire list of security considerations is
beyond the scope of this document, but some significant concerns are beyond the scope of this document, but some significant concerns are
listed 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 agent, a security gateway, or other program, there is no single way
to handle such failures. Just because the methods to handle the to handle such failures. Just because the methods to handle the
failures has not been listed, however, the reader should not assume failures have not been listed, however, the reader should not assume
that they are not important. The opposite is true: if a certificate that they are not important. The opposite is true: if a certificate
is not 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 Some of the many places where signature and certificate checking
might fail include: might fail include:
- no Internet mail addresses in a certificate matches the sender of - no Internet mail addresses in a certificate match the sender of a
a message, if the certificate contains at least one mail address message, if the certificate contains at least one mail address
- no certificate chain leads to a trusted CA
- 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
- 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
fails. fails.
It is possible for there to be multiple unexpired CRLs for a CA. If It is possible for there to be multiple unexpired CRLs for a CA. If
an agent is consulting CRLs for certificate validation, it SHOULD an agent is consulting CRLs for certificate validation, it SHOULD
make sure that the most recently issued CRL for that CA is consulted, make sure that the most recently issued CRL for that CA is consulted,
since an S/MIME message sender could deliberately include an older since an S/MIME message sender could deliberately include an older
unexpired CRL in an S/MIME message. This older CRL might not include unexpired CRL in an S/MIME message. This older CRL might not include
skipping to change at page 16, line 46 skipping to change at page 16, line 27
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 have to be careful to use a reliable time. Unless it is from a
trusted agent, this time MUST NOT be the SigningTime attribute found trusted agent, this time MUST NOT be the SigningTime attribute found
in an S/MIME message. For most sending agents, the SigningTime in an S/MIME message. For most sending agents, the SigningTime
attribute could be deliberately set to direct the receiving agent to attribute could be deliberately set to direct the receiving agent to
check a CRL that could have out-of-date revocation status for a check a CRL that could have out-of-date revocation status for a
certificate, or cause an improper result when checking the Validity certificate, or cause an improper result when checking the Validity
field of a certificate. field of a certificate.
In addition to the Security Considerations identified in [KEYM], In addition to the Security Considerations identified in [KEYM],
caution should be taken when processing certificates which have not caution should be taken when processing certificates that have not
first been validated to a trust anchor. Certificates could be first been validated to a trust anchor. Certificates could be
manufactured by untrusted sources for the purpose of mounting denial manufactured by untrusted sources for the purpose of mounting denial
of service or other attacks. For example, keys selected to require of service or other attacks. For example, keys selected to require
excessive cryptographic processing, or extensive lists of CDP and/or excessive cryptographic processing, or extensive lists of CRL
AIA addresses in the certificate, could be used to mount denial of Distribution Point (CDP) and/or Authority Information Access (AIA)
service attacks. Similarly, attacker-specified CRL Distribution addresses in the certificate, could be used to mount denial-of-
Point (CRLDP) and/or Authority Information Access (AIA) addresses service attacks. Similarly, attacker-specified CDP and/or AIA
could be included in fake certificates to allow the originator to addresses could be included in fake certificates to allow the
detect receipt of the message even if signature verification fails. originator to detect receipt of the message even if signature
verification fails.
The 4096-bit RSA key size requirement for certificate and CRL The 4096-bit RSA key size requirement for certificate and CRL
verification is larger than the 2048-bit RSA key sizes for message verification is larger than the 2048-bit RSA key sizes for message
signature generation/verification or message encryption/decryption in signature generation/verification or message encryption/decryption in
[SMIME-MSG] because many Root CAs included in certificate stores have [SMIME-MSG] because many root CAs included in certificate stores have
already issued Root certificates with 4096-bit key. The standard already issued root certificates with the 4096-bit key. The standard
that defines comparable key sizes for DSA is not yet available. In that defines comparable key sizes for DSA is not yet available. In
particular, [FIPS186-2] without Change Notice 1 allowed DSA key sizes particular, [FIPS186-2] without Change Notice 1 allowed DSA key sizes
between 512 and 1024 bits, [FIPS186-2] with Change Notice 1 only between 512 and 1024 bits, [FIPS186-2] with Change Notice 1 only
allowed DSA key sizes of 1024 bits, and [FIPS186-3] allowed DSA key allowed DSA key sizes of 1024 bits, and [FIPS186-3] allowed DSA key
sizes from 1024 to 3072 bits. Further, 4096-bit keys are normally sizes from 1024 to 3072 bits. Further, 4096-bit keys are normally
only used by Root certificates and not by subordinate CA only used by Root certificates and not by subordinate CA
certificates; thereby, lengthening the Root CA certificate's validity certificates, thereby lengthening the root CA certificate's validity
period. period.
RSA and DSA keys of less than 1024 bits are now considered by many RSA and DSA keys of less than 1024 bits are now considered by many
experts to be cryptographically insecure (due to advances in experts to be cryptographically insecure (due to advances in
computing power), and should no longer be used to sign certificates computing power), and should no longer be used to sign certificates
or CRLs. Such keys were previously considered secure, so processing or CRLs. Such keys were previously considered secure, so processing
previously received signed and encrypted mail may require processing previously received signed and encrypted mail may require processing
certificates or CRLs signed with weak keys. Implementations that certificates or CRLs signed with weak keys. Implementations that
wish to support previous versions of S/MIME or process old messages wish to support previous versions of S/MIME or process old messages
need to consider the security risks that result from accepting need to consider the security risks that result from accepting
skipping to change at page 17, line 42 skipping to change at page 17, line 24
certificates) versus the costs of denial of service. If an certificates) versus the costs of denial of service. If an
implementation supports verification of certificates or CRLs implementation supports verification of certificates or CRLs
generated with RSA and DSA keys of less than 1024 bits, it MUST warn generated with RSA and DSA keys of less than 1024 bits, it MUST warn
the user. Implementers should consider providing a stronger warning the user. Implementers should consider providing a stronger warning
for weak signatures on certificates and CRLs associated with newly for weak signatures on certificates and CRLs associated with newly
received messages than the one provided for certificates and CRLs received messages than the one provided for certificates and CRLs
associated with previously stored messages. Server implementations associated with previously stored messages. Server implementations
(e.g., secure mail list servers) where user warnings are not (e.g., secure mail list servers) where user warnings are not
appropriate SHOULD reject messages with weak cryptography. appropriate SHOULD reject messages with weak cryptography.
If an implementation is concerned about compliance with NIST key size If an implementation is concerned about compliance with National
Institute of Standards and Technology (NIST) key size
recommendations, then see [SP800-57]. recommendations, then see [SP800-57].
7. References 6. References
7.1. Normative References 6.1. Reference Conventions
[ACAUTH] Farrell, S., Housley, R., and S. Turner, "An Internet [CMS] refers to [RFC5652].
Attribute Certificate Profile for Authorization",
draft-ietf-pkix-3281update-05.txt, work-in-progress.
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC [ESS] refers to [RFC2634] and [RFC5035].
3852, July 2004.
Housley, R., "Cryptographic Message Syntax (CMS) [SMIMEv2] refers to [RFC2311], [RFC2312], [RFC2313], [RFC2314], and
Multiple Signer Clarification", RFC 4853, April 2007. [RFC2315].
[ESS] Hoffman, P., "Enhanced Security Services for S/MIME", [SMIMEv3] refers to [RFC2630], [RFC2631], [RFC2632], [RFC2633],
RFC 2634, June 1999. [RFC2634], and [RFC5035].
Schaad, J., "ESS Update: Adding CertID Algorithm [SMIMv3.1] refers to [RFC2634], [RFC3850], [RFC3851], [RFC3852], and
Agility", RFC 5035, August 2007. [RFC5035].
6.2. Normative References
[ACAUTH] Farrell, S., Housley, R., and S. Turner, "An Internet
Attribute Certificate Profile for Authorization", RFC
5755, January 2010.
[RFC2634] Hoffman, P., Ed., "Enhanced Security Services for
S/MIME", RFC 2634, June 1999.
[RFC5035] Schaad, J., "Enhanced Security Services (ESS) Update:
Adding CertID Algorithm Agility", RFC 5035, August 2007.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
5652, September 2009.
[FIPS186-2] National Institute of Standards and Technology (NIST), [FIPS186-2] National Institute of Standards and Technology (NIST),
"Digital Signature Standard (DSS)", FIPS Publication "Digital Signature Standard (DSS)", FIPS Publication
186-3, January 2000. [With Change Notice 1] 186-3, January 2000. [With Change Notice 1]
[FIPS186-3] National Institute of Standards and Technology (NIST), [FIPS186-3] National Institute of Standards and Technology (NIST),
FIPS Publication 186-3: Digital Signature Standard, FIPS Publication 186-3: Digital Signature Standard, June
(draft) March 2006. 2009.
[KEYM] Cooper, D., Santesson, S., Farrell, S., Boeyen, S. [KEYM] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 5280, May 2008. List (CRL) Profile", RFC 5280, May 2008.
[KEYMALG] Bassham, L., Polk, W., and R. Housley, "Algorithms and [KEYMALG] Bassham, L., Polk, W., and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 3279, April 2002. List (CRL) Profile", RFC 3279, April 2002.
[KEYMALG2] Dang, Q., Santesson, S., Moriarty, K., Brown, D., and [KEYMALG2] Dang, Q., Santesson, S., Moriarty, K., Brown, D., and T.
T. Polk, "Internet X.509 Public Key Infrastructure: Polk, "Internet X.509 Public Key Infrastructure:
Additional Algorithms and Identifiers for DSA and Additional Algorithms and Identifiers for DSA and
ECDSA", draft-ietf-pkix-sha2-dsa-ecdsa-06.txt, work-in- ECDSA", RFC 5758, January 2010.
progress.
[MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate [MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[PKCS1] Jonsson, J. and B. Kaliki, "Public-Key Cryptography [PKCS1] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003. Version 2.1", RFC 3447, February 2003.
[PKCS9] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object [PKCS9] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
Classes and Attribute Types Version 2.0", RFC 2985, Classes and Attribute Types Version 2.0", RFC 2985,
November 2000. November 2000.
[RSAPSS] Schaad, J., "Use of RSASSA-PSS Signature Algorithm in [RSAPSS] Schaad, J., "Use of the RSASSA-PSS Signature Algorithm
Cryptographic Message Syntax (CMS)", RFC 4056, June in Cryptographic Message Syntax (CMS)", RFC 4056, June
2005. 2005.
[RSAOAEP] Schaad, J., Kaliski, B., and R. Housley, "Additional [RSAOAEP] Schaad, J., Kaliski, B., and R. Housley, "Additional
Algorithms and Identifiers for RSA Cryptography for use Algorithms and Identifiers for RSA Cryptography for use
in the Internet X.509 Public Key Infrastructure in the Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List (CRL) Certificate and Certificate Revocation List (CRL)
Profile", RFC 4055, June 2005. Profile", RFC 4055, June 2005.
[SMIME-MSG] Ramsdell, B., and S. Turner, "S/MIME Version 3.2 [SMIME-MSG] Ramsdell, B. and S. Turner, "Secure/Multipurpose
Message Specification", draft-ietf-smime-3851bis- Internet Mail Extensions (S/MIME) Version 3.2 Message
10.txt, work-in-progress. Specification", RFC 5751, January 2010.
[X.680] ITU-T Recommendation X.680 (2002) | ISO/IEC 8824- [X.680] ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002.
1:2002. Information Technology - Abstract Syntax Information Technology - Abstract Syntax Notation One
Notation One (ASN.1): Specification of basic notation. (ASN.1): Specification of basic notation.
7.2. Informative References 6.3. Informative References
[PKCS6] RSA Laboratories, "PKCS #6: Extended-Certificate Syntax [PKCS6] RSA Laboratories, "PKCS #6: Extended-Certificate Syntax
Standard", November 1993. Standard", November 1993.
[SECLABEL] Nicolls, W., "Implementing Company Classification [SECLABEL] Nicolls, W., "Implementing Company Classification Policy
Policy with the S/MIME Security Label", RFC 3114, May with the S/MIME Security Label", RFC 3114, May 2002.
2002.
[SMIMEv2] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L. and [RFC2311] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., and
L. Repka, "S/MIME Version 2 Message Specification", RFC L. Repka, "S/MIME Version 2 Message Specification", RFC
2311, March 1998. 2311, March 1998.
Dusse, S., Hoffman, P., Ramsdell, B., and J. Weinstein, [RFC2312] Dusse, S., Hoffman, P., Ramsdell, B., and J. Weinstein,
"S/MIME Version 2 Certificate Handling", RFC 2312, "S/MIME Version 2 Certificate Handling", RFC 2312, March
March 1998. 1998.
Kaliski, B., "PKCS #1: RSA Encryption Version 1.5", RFC [RFC2313] Kaliski, B., "PKCS #1: RSA Encryption Version 1.5", RFC
2313, March 1998. 2313, March 1998.
Kaliski, B., "PKCS #10: Certificate Request Syntax [RFC2314] Kaliski, B., "PKCS #10: Certification Request Syntax
Version 1.5", RFC 2314, March 1998. Version 1.5", RFC 2314, March 1998.
Kaliski, B., "PKCS #7: Certificate Message Syntax [RFC2315] Kaliski, B., "PKCS #7: Cryptographic Message Syntax
Version 1.5", RFC 2315, March 1998. Version 1.5", RFC 2315, March 1998.
[SMIMEv3] Housley, R., "Cryptographic Message Syntax", RFC 2630, [RFC2630] Housley, R., "Cryptographic Message Syntax", RFC 2630,
June 1999. June 1999.
Rescorla, E., "Diffie-Hellman Key Agreement Method", [RFC2631] Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC
RFC 2631, June 1999. 2631, June 1999.
Ramsdell, B., "S/MIME Version 3 Certificate Handling",
RFC 2632, June 1999.
Ramsdell, B., "S/MIME Version 3 Message Specification",
RFC 2633, June 1999.
Hoffman, P., "Enhanced Security Services for S/MIME",
RFC 2634, June 1999.
Schaad, J., "ESS Update: Adding CertID Algorithm
Agility", RFC 5035, August 2007.
[SMIMEv3.1] Housley, R., "Cryptographic Message Syntax", RFC 3852,
July 2004.
Housley, R., "Cryptographic Message Syntax (CMS) [RFC2632] Ramsdell, B., Ed., "S/MIME Version 3 Certificate
Multiple Signer Clarification", RFC 4853, April 2007. Handling", RFC 2632, June 1999.
Ramsdell, B., "S/MIME Version 3.1 Certificate [RFC2633] Ramsdell, B., Ed., "S/MIME Version 3 Message
Handling", RFC 3850, July 2004. Specification", RFC 2633, June 1999.
Ramsdell, B., "S/MIME Version 3.1 Message [RFC3850] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
Specification", RFC 3851, July 2004. Extensions (S/MIME) Version 3.1 Certificate Handling",
RFC 3850, July 2004.
Hoffman, P., "Enhanced Security Services for S/MIME", [RFC3851] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
RFC 2634, June 1999. Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004.
Schaad, J., "ESS Update: Adding CertID Algorithm [RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
Agility", RFC 5035, August 2007. 3852, July 2004.
[SP800-57] National Institute of Standards and Technology (NIST), [SP800-57] National Institute of Standards and Technology (NIST),
Special Publication 800-57: Recommendation for Key Special Publication 800-57: Recommendation for Key
Management, August 2005. Management, August 2005.
[X.500] ITU-T Recommendation X.500 (1997) | ISO/IEC 9594- [X.500] ITU-T Recommendation X.500 (1997) | ISO/IEC 9594-
1:1997, Information technology - Open Systems 1:1997, Information technology - Open Systems
Interconnection - The Directory: Overview of concepts, Interconnection - The Directory: Overview of concepts,
models and services. models and services.
Appendix A. Moving S/MIME v2 Certificate Handling to Historic Status Appendix A. Moving S/MIME v2 Certificate Handling to Historic Status
The S/MIME v3 [SMIMEv3], v3.1 [SMIMEv3.1], and v3.2 (this document) The S/MIME v3 [SMIMEv3], v3.1 [SMIMEv3.1], and v3.2 (this document)
are backwards compatible with the S/MIME v2 Certificate Handling are backwards compatible with the S/MIME v2 Certificate Handling
Specification [SMIMEv2], with the exception of the algorithms Specification [SMIMEv2], with the exception of the algorithms
(dropped RC2/40 requirement and added DSA and RSASSA-PSS (dropped RC2/40 requirement and added DSA and RSASSA-PSS
requirements). Therefore, it is recommended that RFC 2312 [SMIMEv2] requirements). Therefore, it is recommended that RFC 2312 [SMIMEv2]
be moved to Historic status. be moved to Historic status.
Appendix B. Acknowledgments Appendix B. Acknowledgments
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 Dusse, Paul Hoffman, and Jeff Weinstein. Without v2, there wouldn't
be a v3, v3.1 or v3.2. be a v3, v3.1, or v3.2.
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 because they made direct
made direct contributions to this document. contributions to this document.
Bill Flanigan, Trevor Freeman, Elliott Ginsburg, Alfred Hoenes, Paul Bill Flanigan, Trevor Freeman, Elliott Ginsburg, Alfred Hoenes, Paul
Hoffman, Russ Housley, David P. Kemp, Michael Myers, John Pawling, Hoffman, Russ Housley, David P. Kemp, Michael Myers, John Pawling,
Denis Pinkas, and Jim Schaad. Denis Pinkas, and Jim Schaad.
Authors' Addresses Authors' Addresses
Blake Ramsdell Blake Ramsdell
Brute Squad Labs, Inc. Brute Squad Labs, Inc.
EMail: blaker@gmail.com EMail: blaker@gmail.com
Sean Turner Sean Turner
IECA, Inc. IECA, Inc.
3057 Nutley Street, Suite 106 3057 Nutley Street, Suite 106
Fairfax, VA 22031 Fairfax, VA 22031
USA USA
EMail: turners@ieca.com EMail: turners@ieca.com
 End of changes. 132 change blocks. 
325 lines changed or deleted 314 lines changed or added

This html diff was produced by rfcdiff 1.37c. The latest version is available from http://tools.ietf.org/tools/rfcdiff/