draft-ietf-lamps-rfc5750-bis-08.txt   rfc8550.txt 
LAMPS J. Schaad Internet Engineering Task Force (IETF) J. Schaad
Internet-Draft August Cellars Request for Comments: 8550 August Cellars
Obsoletes: 5750 (if approved) B. Ramsdell Obsoletes: 5750 B. Ramsdell
Intended status: Standards Track Brute Squad Labs, Inc. Category: Standards Track Brute Squad Labs, Inc.
Expires: March 8, 2019 S. Turner ISSN: 2070-1721 S. Turner
sn3rd sn3rd
September 4, 2018 April 2019
Secure/Multipurpose Internet Mail Extensions (S/ MIME) Version 4.0 Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Certificate Handling Certificate Handling
draft-ietf-lamps-rfc5750-bis-08
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) v4.0 agents. Secure/Multipurpose Internet Mail Extensions (S/MIME) v4.0 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
Internet X.509 Public Key Infrastructure Certificate and CRL Profile. ("Internet X.509 Public Key Infrastructure Certificate and
S/MIME agents must meet the certificate processing requirements in Certificate Revocation List (CRL) Profile"). S/MIME agents must meet
this document as well as those in RFC 5280. This document obsoletes the certificate-processing requirements in this document as well as
RFC 5750. those in RFC 5280. This document obsoletes RFC 5750.
Contributing to this document
The source for this draft is being maintained in GitHub. Suggested
changes should be submitted as pull requests at <https://github.com/
lamps-wg/smime>. Instructions are on that page as well. Editorial
changes can be managed in GitHub, but any substantial issues need to
be discussed on the LAMPS mailing list.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering This document is a product of the Internet Engineering Task Force
Task Force (IETF). Note that other groups may also distribute (IETF). It represents the consensus of the IETF community. It has
working documents as Internet-Drafts. The list of current Internet- received public review and has been approved for publication by the
Drafts is at https://datatracker.ietf.org/drafts/current/. Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Internet-Drafts are draft documents valid for a maximum of six months Information about the current status of this document, any errata,
and may be updated, replaced, or obsoleted by other documents at any and how to provide feedback on it may be obtained at
time. It is inappropriate to use Internet-Drafts as reference https://www.rfc-editor.org/info/rfc8550.
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 8, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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than English. than English.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Conventions Used in This Document . . . . . . . . . . . . 4 1.2. Conventions Used in This Document . . . . . . . . . . . . 5
1.3. Compatibility with Prior Practice S/MIME . . . . . . . . 5 1.3. Compatibility with Prior Practice of S/MIME . . . . . . . 6
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 . . . . . . . . . . 6
1.5. Changes from S/MIME v3.1 to S/MIME v3.2 . . . . . . . . . 6 1.5. Changes from S/MIME v3.1 to S/MIME v3.2 . . . . . . . . . 7
1.6. Changes since S/MIME 3.2 . . . . . . . . . . . . . . . . 7 1.6. Changes since S/MIME 3.2 . . . . . . . . . . . . . . . . 8
2. CMS Options . . . . . . . . . . . . . . . . . . . . . . . . . 7 2. CMS Options . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1. Certificate Revocation Lists . . . . . . . . . . . . . . 7 2.1. Certificate Revocation Lists . . . . . . . . . . . . . . 9
2.2. Certificate Choices . . . . . . . . . . . . . . . . . . . 8 2.2. Certificate Choices . . . . . . . . . . . . . . . . . . . 9
2.2.1. Historical Note about CMS Certificates . . . . . . . 8 2.2.1. Historical Note about CMS Certificates . . . . . . . 9
2.3. CertificateSet . . . . . . . . . . . . . . . . . . . . . 8 2.3. Included Certificates . . . . . . . . . . . . . . . . . . 10
3. Using Distinguished Names for Internet Mail . . . . . . . . . 9 3. Using Distinguished Names for Internet Mail . . . . . . . . . 11
4. Certificate Processing . . . . . . . . . . . . . . . . . . . 10 4. Certificate Processing . . . . . . . . . . . . . . . . . . . 12
4.1. Certificate Revocation Lists . . . . . . . . . . . . . . 11 4.1. Certificate Revocation Lists . . . . . . . . . . . . . . 13
4.2. Certificate Path Validation . . . . . . . . . . . . . . . 12 4.2. Certificate Path Validation . . . . . . . . . . . . . . . 13
4.3. Certificate and CRL Signing Algorithms and Key Sizes . . 13 4.3. Certificate and CRL Signing Algorithms, and Key Sizes . . 14
4.4. PKIX Certificate Extensions . . . . . . . . . . . . . . . 14 4.4. PKIX Certificate Extensions . . . . . . . . . . . . . . . 15
4.4.1. Basic Constraints . . . . . . . . . . . . . . . . . . 14 4.4.1. Basic Constraints . . . . . . . . . . . . . . . . . . 16
4.4.2. Key Usage Certificate Extension . . . . . . . . . . . 15 4.4.2. Key Usage Extension . . . . . . . . . . . . . . . . . 16
4.4.3. Subject Alternative Name . . . . . . . . . . . . . . 15 4.4.3. Subject Alternative Name . . . . . . . . . . . . . . 17
4.4.4. Extended Key Usage Extension . . . . . . . . . . . . 16 4.4.4. Extended Key Usage Extension . . . . . . . . . . . . 17
5. IANA Considertions . . . . . . . . . . . . . . . . . . . . . 16 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
6. Security Considerations . . . . . . . . . . . . . . . . . . . 16 6. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.1. Normative References . . . . . . . . . . . . . . . . . . 18 7.1. Reference Conventions . . . . . . . . . . . . . . . . . . 20
7.2. Informational References . . . . . . . . . . . . . . . . 21 7.1. Normative References . . . . . . . . . . . . . . . . . . 20
Appendix A. Historic Considerations . . . . . . . . . . . . . . 24 7.2. Informative References . . . . . . . . . . . . . . . . . 23
A.1. Signature Algorithms and Key Sizes . . . . . . . . . . . 24 Appendix A. Historic Considerations . . . . . . . . . . . . . . 26
A.1. Signature Algorithms and Key Sizes . . . . . . . . . . . 26
Appendix B. Moving S/MIME v2 Certificate Handling to Historic Appendix B. Moving S/MIME v2 Certificate Handling to Historic
Status . . . . . . . . . . . . . . . . . . . . . . . 25 Status . . . . . . . . . . . . . . . . . . . . . . . 27
Appendix C. Acknowledgments . . . . . . . . . . . . . . . . . . 25 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction 1. Introduction
S/MIME (Secure/Multipurpose Internet Mail Extensions) v4.0, described S/MIME (Secure/Multipurpose Internet Mail Extensions) v4.0, described
in [I-D.ietf-lamps-rfc5751-bis], provides a method to send and in [RFC8551], provides a method to send and receive secure MIME
receive secure MIME messages. Before using a public key to provide messages. Before using a public key to provide security services,
security services, the S/MIME agent MUST verify that the public key the S/MIME agent MUST verify that the public key is valid. S/MIME
is valid. S/MIME agents MUST use PKIX certificates to validate agents MUST use PKIX certificates to validate public keys as
public keys as described in the Internet X.509 Public Key described in [RFC5280] ("Internet X.509 Public Key Infrastructure
Infrastructure (PKIX) Certificate and CRL Profile [RFC5280]. S/MIME Certificate and Certificate Revocation List (CRL) Profile"). S/MIME
agents MUST meet the certificate processing requirements documented agents MUST meet the certificate-processing requirements specified in
in this document in addition to those stated in [RFC5280]. this document in addition to those stated in [RFC5280].
This specification is compatible with the Cryptographic Message This specification is compatible with the Cryptographic Message
Syntax (CMS) RFC 5652 [RFC5652] in that it uses the data types Syntax (CMS) [RFC5652] in that it uses the data types defined by CMS.
defined by CMS. It also inherits all the varieties of architectures It also inherits all the varieties of architectures for certificate-
for certificate-based key management supported by CMS. based key management supported by CMS.
This document obsoletes [RFC5750]. The most significant changes This document obsoletes [RFC5750]. The most significant changes
revolve around changes in recommendations around the cryptographic revolve around changes in recommendations around the cryptographic
algorithms used by the specification. More details can be found in algorithms used by the specification. More details can be found in
Section 1.6. Section 1.6.
This specification contains a number of references to documents that
have been obsoleted or replaced. This is intentional, as the updated
documents often do not have the same information or protocol
requirements in them.
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:
[X.680]. Abstract Syntax Notation One, as defined in ITU-T X.680 [X.680].
Attribute certificate (AC): An X.509 AC is a separate structure from Attribute certificate (AC):
a subject's public key X.509 certificate. A subject may have An X.509 AC is a separate structure from a subject's public key
multiple X.509 ACs associated with each of its public key X.509 X.509 certificate. A subject may have multiple X.509 ACs
certificates. Each X.509 AC binds one or more attributes with one of associated with each of its public key X.509 certificates. Each
the subject's public key X.509 certificates. The X.509 AC syntax is X.509 AC binds one or more attributes with one of the subject's
defined in [RFC5755]. public key X.509 certificates. The X.509 AC syntax is defined in
[RFC5755].
Certificate: A type that binds an entity's name to a public key with Certificate:
a digital signature. This type is defined in the Internet X.509 A type that binds an entity's name to a public key with a digital
Public Key Infrastructure (PKIX) Certificate and CRL Profile signature. This type is defined in [RFC5280]. This type also
[RFC5280]. This type also contains the distinguished name of the contains the distinguished name of the certificate issuer (the
certificate issuer (the signer), an issuer-specific serial number, signer), an issuer-specific serial number, the issuer's signature
the issuer's signature algorithm identifier, a validity period, and algorithm identifier, a validity period, and extensions also
extensions also defined in that document. defined in that document.
Certificate Revocation List (CRL): A type that contains information Certificate Revocation List (CRL):
about certificates whose validity an issuer has revoked. The A type that contains information about certificates whose validity
information consists of an issuer name, the time of issue, the next an issuer has revoked. The information consists of an issuer
scheduled time of issue, a list of certificate serial numbers and name, the time of issue, the next scheduled time of issue, a list
their associated revocation times, and extensions as defined in of certificate serial numbers and their associated revocation
[RFC5280]. The CRL is signed by the issuer. The type intended by times, and extensions as defined in [RFC5280]. The CRL is signed
this specification is the one defined in [RFC5280]. by the issuer. The type intended by this specification is the one
defined in [RFC5280].
Receiving agent: Software that interprets and processes S/MIME CMS Receiving agent:
objects, MIME body parts that contain CMS objects, or both. Software that interprets and processes S/MIME CMS objects, MIME
body parts that contain CMS objects, or both.
Sending agent: Software that creates S/MIME CMS objects, MIME body Sending agent:
parts that contain CMS objects, or both. Software that creates S/MIME CMS objects, MIME body parts that
contain CMS objects, or both.
S/MIME agent: User software that is a receiving agent, a sending S/MIME agent:
agent, or both. User software that is a receiving agent, a sending 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", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in
14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
We define the additional requirement levels: We define the additional requirement levels:
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 expect that a requirement marked as SHOULD+ will be
at 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 expect that a requirement marked as SHOULD- will be demoted
to a MAY in a future version of this document. 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 expect that this requirement will no longer be a MUST in a
future document. Although its status will be determined at a future document. Although its status will be determined at
later time, it is reasonable to expect that if a future a later time, it is reasonable to expect that if a future
revision of a document alters the status of a MUST- revision of a document alters the status of a MUST-
requirement, it will remain at least a SHOULD or a SHOULD-. requirement, it will remain at least a SHOULD or a SHOULD-.
The term RSA in this document almost always refers to the PKCS#1 v1.5 The term "RSA" in this document almost always refers to the
RSA signature algorithm even when not qualified as such. There are a PKCS #1 v1.5 RSA signature algorithm even when not qualified as such.
couple of places where it refers to the general RSA cryptographic There are a couple of places where it refers to the general RSA
operation; these can be determined from the context where it is used. cryptographic operation; these can be determined from the context
where it is used.
1.3. Compatibility with Prior Practice S/MIME 1.3. Compatibility with Prior Practice of S/MIME
S/MIME version 4.0 agents ought to attempt to have the greatest S/MIME version 4.0 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
[SMIMEv2], S/MIME version 3 is described in RFC 2630 through RFC 2634 inclusive [SMIMEv2].
inclusive and RFC 5035 [SMIMEv3], and S/MIME version 3.1 is described
in RFC 3850, RFC 3851, RFC 3852, RFC 2634, and RFC 5035 [SMIMEv3.1]. - S/MIME version 3 is described in RFC 2630 through RFC 2634
RFC 2311 also has historical information about the development of inclusive and RFC 5035 [SMIMEv3].
S/MIME.
- S/MIME version 3.1 is described in RFC 2634, RFC 3850, RFC 3851,
RFC 3852, and RFC 5035 [SMIMEv3.1].
- S/MIME version 3.2 is described in RFC 2634, RFC 5035, RFC 5652,
RFC 5750, and RFC 5751 [SMIMEv3.2].
- RFC 2311 also has historical information about the development of
S/MIME.
Appendix A contains information about algorithms that were used for Appendix A contains information about algorithms that were used for
prior versions of S/MIME but are no longer considered to meet modern prior versions of S/MIME but are no longer considered to meet modern
security standards. Support of these algorithms may be needed to security standards. Support of these algorithms may be needed to
support historic S/MIME artifacts such as messages or files, but support historic S/MIME artifacts such as messages or files but
SHOULD NOT be used for new artifacts. SHOULD NOT be used for new artifacts.
1.4. Changes from S/MIME v3 to S/MIME v3.1 1.4. Changes from S/MIME v3 to S/MIME v3.1
This section reflects the changes that were made when S/MIME v3.1 was This section reflects the changes that were made when S/MIME v3.1 was
released. The RFC2119 langauage may have superceeded in later released. The language of RFC 2119 ("MUST", "SHOULD", etc.) used for
versions. S/MIME v3 may have been superseded in later versions.
Version 1 and version 2 CRLs MUST be supported. - Version 1 and version 2 CRLs MUST be supported.
Multiple certification authority (CA) certificates with the same - Multiple certification authority (CA) certificates with the same
subject and public key, but with overlapping validity periods, MUST subject and public key, but with overlapping validity periods,
be supported. MUST be supported.
Version 2 attribute certificates SHOULD be supported, and version 1 - Version 2 ACs SHOULD be supported, and version 1 ACs MUST NOT be
attributes certificates MUST NOT be used. 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 was added. discouraged, and security language was added.
Clarified use of email address use in certificates. Certificates - Clarified email address use in certificates. Certificates that do
that do not contain an email address have no requirements for not contain an email address have no requirements for verifying
verifying the email address associated with the certificate. 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
that does not have at least one of the the digitalSignature or certificate that does not have at least one of the
nonRepudiation bits set. digitalSignature or nonRepudiation bits set.
Clarifications for the interpretation of the key usage and extended - Added clarifications for the interpretation of the key usage and
key usage extensions. extended key usage extensions.
1.5. Changes from S/MIME v3.1 to S/MIME v3.2 1.5. Changes from S/MIME v3.1 to S/MIME v3.2
This section reflects the changes that were made when S/MIME v3.2 was This section reflects the changes that were made when S/MIME v3.2 was
released. The RFC2119 langauage may have superceeded in later released. The language of RFC 2119 ("MUST", "SHOULD", etc.) used for
versions. S/MIME v3.1 may have been superseded in later versions.
Conventions Used in This Document: Moved to Section 1.2. Added Note that the section numbers listed here (e.g., "Section 6") are
definitions for SHOULD+, SHOULD-, and MUST-. from [RFC5750].
Section 1.1: Updated ASN.1 definition and reference. - Moved "Conventions Used in This Document" to Section 1.2. Added
definitions for SHOULD+, SHOULD-, and MUST-.
Section 1.3: Added text about v3.1 RFCs. - Section 1.1: Updated ASN.1 definition and reference.
Section 3: Aligned email address text with RFC 5280. Updated note - Section 1.3: Added text about v3.1 RFCs.
to indicate emailAddress IA5String upper bound is 255
characters. Added text about matching email addresses.
Section 4.2: Added text to indicate how S/MIME agents locate the - Section 3: Aligned email address text with RFC 5280. Updated note
correct user certificate. to indicate that the emailAddress IA5String upper bound is
255 characters. Added text about matching email addresses.
Section 4.3: RSA with SHA-256 (PKCS #1 v1.5) added as MUST; DSA with - Section 4.2: Added text to indicate how S/MIME agents locate the
SHA-256 added as SHOULD+; RSA with SHA-1, DSA with SHA-1, correct user certificate.
and RSA with MD5 changed to SHOULD-; and RSASSA-PSS with
SHA-256 added as SHOULD+. Updated key sizes and changed
pointer to PKIX RFCs.
Section 4.4.1: Aligned with PKIX on use of basic constraints - Section 4.3: RSA with SHA-256 (PKCS #1 v1.5) added as MUST; DSA
extension in CA certificates. Clarified which extension with SHA-256 added as SHOULD+; RSA with SHA-1, DSA with SHA-1, and
is used to constrain end entities from using their keys RSA with MD5 changed to SHOULD-; and RSASSA-PSS with SHA-256 added
to perform issuing authority operations. as SHOULD+. Updated key sizes and changed pointer to PKIX RFCs.
Section 5: Updated security considerations. - Section 4.4.1: Aligned with PKIX on the use of a basicConstraints
extension in CA certificates. Clarified which extension is used
to constrain end entities from using their keys to perform
issuing-authority operations.
Section 7: Moved references from Appendix B to Section 6. Updated - Section 5: Updated security considerations.
the references.
Appendix A: Moved Appendix A to Appendix B. Added Appendix A to move - Section 6: Moved references from Appendix A of RFC 3850 to this
S/MIME v2 Certificate Handling to Historic Status. section. Updated the references.
- Appendix A: Added Appendix A to move S/MIME v2 Certificate
Handling to Historic status.
1.6. Changes since S/MIME 3.2 1.6. Changes since S/MIME 3.2
This section reflects the changes that were made when S/MIME v4.0 was This section reflects the changes that were made when S/MIME v4.0 was
released. The RFC2119 langauage may have superceeded in later released. The language of RFC 2119 ("MUST", "SHOULD", etc.) used for
versions. S/MIME v3.2 may have been superseded by S/MIME v4.0 and may be
superseded by future versions.
Section 3: Require support for internationalized email addresses. - Section 3: Support for internationalized email addresses is
required.
Section 4.3: Mandated support for ECDSA with P-256 and Ed25519. - Section 4.3: Mandated support for the Elliptic Curve Digital
Moved algorithms with SHA-1 and MD5 to historical status. Signature Algorithm (ECDSA) with P-256 and the Edwards-curve
Moved DSA support to historical status. Increased lower Digital Signature Algorithm (EdDSA) with curve25519 [RFC8410].
bounds on RSA key sizes. SHA-1 and MD5 algorithms are marked as historical, as they are no
longer considered secure. As the Digital Signature Algorithm
(DSA) has been replaced by elliptic curve versions, support for
DSA is now considered historical. Increased lower bounds on RSA
key sizes.
Appendix A: Add a new appendix for algorithms that are now considered - Appendix A: Added Appendix A for algorithms that are now
to be historical. considered to be historical.
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 the CMS format for S/MIME messages is defined in [RFC8551].
[I-D.ietf-lamps-rfc5751-bis].
2.1. Certificate Revocation Lists 2.1. Certificate Revocation Lists
Receiving agents MUST support the Certificate Revocation List (CRL) Receiving agents MUST support the CRL format defined in [RFC5280].
format defined in [RFC5280]. If sending agents include CRLs in If sending agents include CRLs in outgoing messages, the CRL format
outgoing messages, the CRL format defined in [RFC5280] MUST be used. defined in [RFC5280] MUST be used. Receiving agents MUST support
Receiving agents MUST support both v1 and v2 CRLs. both v1 and v2 CRLs.
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 [RFC5280]. All agents MUST perform revocation status as specified in [RFC5280]. All agents MUST perform revocation status
checking in accordance with [RFC5280]. Receiving agents MUST checking in accordance with [RFC5280]. Receiving agents MUST
recognize CRLs in received S/MIME messages. recognize 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 [RFC5280]. End-entity certificates MAY include an profiled in [RFC5280]. End-entity certificates MAY include an
Internet mail address, as described in Section 3. Internet mail address, as described in Section 3.
Receiving agents SHOULD support X.509 version 2 attribute Receiving agents SHOULD support X.509 version 2 ACs. See [RFC5755]
certificates. See [RFC5755] for details about the profile for for details about the profile for ACs.
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 public key content types: PKIX, PKCS #6 extended certificates
[PKCS6], and PKIX attribute certificates. [PKCS6], and PKIX ACs.
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 certificate extensions.
receiving agents MUST NOT use PKCS #6 extended certificates. Thus, sending and receiving agents MUST NOT use PKCS #6 extended
Receiving agents MUST be able to parse and process a message certificates. Receiving agents MUST be able to parse and process a
containing PKCS #6 extended certificates although ignoring those message containing PKCS #6 extended certificates, although ignoring
certificates is expected behavior. those certificates is expected behavior.
X.509 version 1 attribute certificates are also not widely X.509 version 1 ACs are also not widely implemented and have
implemented, and have been superseded with version 2 attribute been superseded by version 2 ACs. Sending agents MUST NOT send
certificates. Sending agents MUST NOT send version 1 attribute version 1 ACs.
certificates.
2.3. CertificateSet 2.3. Included Certificates
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
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 originator's public key(s). This is especially important when
sending a message to recipients that may not have access to the sending a message to recipients that may not have access to the
sender's public key through any other means or when sending a signed 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 have 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 by using a database or directory handle messages without certificates by using a database or directory
lookup scheme to find them. lookup scheme to find them.
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 certification authority (CA) that it to, but not including, a CA that it believes that the recipient may
believes that the recipient may trust as authoritative. A receiving trust as authoritative. A receiving agent MUST be able to handle an
agent MUST be able to handle an arbitrarily large number of arbitrarily large number of certificates and chains.
certificates and chains.
Agents MAY send CA certificates, that is, cross-certificates, self- Agents MAY send CA certificates -- that is, cross-certificates,
issued certificates, and self-signed certificates. Note that self-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
this is a CA that should be trusted. Also note that when is a CA that should be trusted. Also note that when certificates
certificates contain Digital Signature Algorithm (DSA) public keys contain DSA public keys the parameters may be located in the root
the parameters may be located in the root certificate. This would certificate. This would require that the recipient possess both the
require that the recipient possess both the end-entity certificate end-entity certificate and the root certificate to perform a
and the root certificate to perform a signature verification, and is signature verification, and is a valid example of a case where
a valid example of a case where transmitting the root certificate may transmitting the root certificate may be required.
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 ACs included in
certificates included in CMS objects. All other issues regarding the CMS objects. All other issues regarding the generation and use of
generation and use of X.509 attribute certificates are outside of the X.509 ACs are outside the scope of this specification. One
scope of this specification. One specification that addresses specification that addresses AC use is defined in [RFC3114].
attribute certificate use is defined in [RFC3114].
3. Using Distinguished Names for Internet Mail 3. Using Distinguished Names for Internet Mail
End-entity certificates MAY contain an Internet mail address. Email End-entity certificates MAY contain an Internet mail address.
addresses restricted to 7-bit ASCII characters use the pkcs-9-at- Email addresses restricted to 7-bit ASCII characters use the
emailAddress OID (see below) and are encoded as described in pkcs-9-at-emailAddress object identifier (OID) (see below) and are
Section 4.2.1.6 of [RFC5280]. Internationalized Email address names encoded as described in Section 4.2.1.6 of [RFC5280].
use the OID defined in [I-D.ietf-lamps-eai-addresses] and are encoded Internationalized email address names use the OID defined in
as described there. The email address SHOULD be in the [RFC8398] and are encoded as described therein. The email address
subjectAltName extension, and SHOULD NOT be in the subject SHOULD be in the subjectAltName extension and SHOULD NOT be in the
distinguished name. subject distinguished name.
Receiving agents MUST recognize and accept certificates that contain Receiving agents MUST recognize and accept certificates that contain
no email address. 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
both ASCII and internationalized email addresses in the both ASCII and internationalized email addresses in the
subjectAltName field. Receiving agents MUST recognize email subjectAltName extension. Receiving agents MUST recognize email
addresses in the Distinguished Name field in the PKCS #9 [RFC2985] addresses in the distinguished name field in the PKCS #9 [RFC2985]
emailAddress attribute: emailAddress attribute:
pkcs-9-at-emailAddress OBJECT IDENTIFIER ::= pkcs-9-at-emailAddress OBJECT IDENTIFIER ::=
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1 } { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1 }
Note that this attribute MUST be encoded as IA5String and has an Note that this attribute MUST be encoded as IA5String and has an
upper bound of 255 characters. Comparing of email addresses is upper bound of 255 characters. The comparing of email addresses is
fraught with peril. [I-D.ietf-lamps-eai-addresses] defines the fraught with peril. [RFC8398] defines the procedure for doing the
procedure for doing comparison of Internationalized email addresses. comparison of internationalized email addresses. For ASCII email
For ASCII email addresses the domain component (right-hand side of addresses, the domain component (right-hand side of the '@') MUST be
the '@') MUST be compared using a case-insensitive function. The compared using a case-insensitive function. The local name
local name component (left-hand side of the '@') SHOULD be compared component (left-hand side of the '@') SHOULD be compared using a
using a case-insensitive function. Some localities may perform other case-insensitive function. Some localities may perform other
transformations on the local name component before doing the transformations on the local name component before doing the
comparison, however an S/MIME client cannot know what specific comparison; however, an S/MIME client cannot know what specific
localities do. localities do.
Sending agents SHOULD make the address in the From or Sender header Sending agents SHOULD make the address in the From or Sender header
in 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 certificate. Receiving agents MUST check that the address in the
From or Sender header of a mail message matches an Internet mail From or Sender header of a mail message matches an Internet mail
address in the signer's certificate, if mail addresses are present in address in the signer's certificate, if mail addresses are present in
the certificate. A receiving agent SHOULD provide some explicit the certificate. A receiving agent SHOULD provide some explicit
alternate processing of the message if this comparison fails; this alternate processing of the message if this comparison fails; this
might be done by displaying or logging a message that shows the might be done by displaying or logging a message that shows the
recipient the mail addresses in the certificate or other certificate recipient the mail addresses in the certificate or other certificate
details. 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 distinguished name (DN) in a user's (i.e., end-entity) subject distinguished name in a user's (i.e., an end entity's)
certificate MAY be an empty SEQUENCE in which case the subjectAltName certificate MAY be an empty SEQUENCE, in which case the
extension will include the subject's identifier and MUST be marked as subjectAltName extension will include the subject's identifier and
critical. 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. The IETF has published [RFC8162] which X.500 distinguished names. The IETF has published [RFC8162], which
describes an experimental protocol to retrieve certificates from the describes an experimental protocol to retrieve certificates from the
Domain Name System (DNS). Until such mechanisms are widely used, Domain Name System (DNS). Until such mechanisms are widely used,
their utility may be limited by the small number of the their utility may be limited by the small number of the
correspondent's certificates that can be retrieved. At a minimum, correspondent's certificates that can be retrieved. At a minimum,
for initial S/MIME deployment, a user agent could automatically for initial S/MIME deployment, a user agent could automatically
generate a message to an intended recipient requesting the generate a message to an intended recipient requesting the
recipient's 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 as to guarantee their later retrieval. In many environments,
environments, it may be desirable to link the certificate retrieval/ it may be desirable to link the certificate retrieval/storage
storage mechanisms together in some sort of certificate database. In mechanisms together in some sort of certificate database. In its
its simplest form, a certificate database would be local to a simplest form, a certificate database would be local to a particular
particular user and would function in a similar way as an "address user and would function in a way similar to an "address book" that
book" that stores a user's frequent correspondents. In this way, the stores a user's frequent correspondents. In this way, the
certificate retrieval mechanism would be limited to the certificates certificate retrieval mechanism would be limited to the certificates
that a user has stored (presumably from incoming messages). A that a user has stored (presumably from incoming messages). A
comprehensive certificate retrieval/storage solution might combine comprehensive certificate retrieval/storage solution might combine
two or more mechanisms to allow the greatest flexibility and utility two or more mechanisms to allow the greatest flexibility and utility
to the user. For instance, a secure Internet mail agent might resort to the user. For instance, a secure Internet mail agent might resort
to checking a centralized certificate retrieval mechanism for a to checking a centralized certificate retrieval mechanism for a
certificate if it cannot be found in a user's local certificate certificate if it cannot be found in a user's local 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 opposed to just a single certificate. This is described in
[RFC5751]. [RFC8551].
Agents MUST handle multiple valid certification authority (CA) Agents MUST handle multiple valid CA certificates containing the same
certificates containing the same subject name and the same public subject name and the same public keys but with overlapping validity
keys but with overlapping validity intervals. 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 in an incoming messages. A from a CA than to get information stored in an incoming message. A
receiving agent SHOULD have access to some CRL retrieval mechanism in receiving agent SHOULD have access to some CRL retrieval mechanism in
order to gain access to certificate revocation information when order to gain access to certificate revocation information when
validating certification paths. A receiving or sending agent SHOULD validating certification paths. A receiving or sending agent SHOULD
also provide a mechanism to allow a user to store incoming also provide a mechanism to allow a user to store incoming
certificate revocation information for correspondents in such a way certificate revocation information for correspondents in such a way
so as to guarantee its later retrieval. 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 [RFC5280]. All agents MUST perform revocation status as specified in [RFC5280]. All agents MUST perform revocation status
checking in accordance with [RFC5280]. Receiving agents MUST checking in accordance with [RFC5280]. Receiving agents MUST
recognize CRLs in received S/MIME messages. recognize CRLs in received S/MIME messages.
skipping to change at page 12, line 34 skipping to change at page 14, line 6
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 [RFC5280] when validating a validation MUST be performed as per [RFC5280] 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). (although in most cases they will be related to the sender).
Incoming certificates and CRLs SHOULD be cached for use in path Incoming certificates and CRLs SHOULD be cached for use in path
validation and optionally stored for later use. This temporary validation and optionally stored for later use. This temporary
certificate and CRL cache SHOULD be used to augment any other certificate and CRL cache SHOULD be used to augment any other
certificate and CRL retrieval mechanisms for path validation on certificate and CRL retrieval mechanisms for path validation on
incoming signed messages. 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, using either (1) the issuerAndSerialNumber, which identifies
signer's certificate by the issuer's distinguished name and the the signer's certificate by the issuer's distinguished name and the
certificate serial number, or the subjectKeyIdentifier, which certificate serial number or (2) 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 an
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 [RFC5652] for the CMS fields that found in OriginatorInfo. See [RFC5652] for the CMS fields that
reference the originator's and recipient's certificates. reference 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 CRLs are signed by the certificate issuer.
the certificate issuer. Receiving agents: Receiving agents:
- MUST support ECDSA with curve P-256 with SHA-256. - MUST support ECDSA with curve P-256 with SHA-256.
- MUST support EdDSA with curve 25519 using PureEdDSA mode. - MUST support EdDSA with curve25519 using PureEdDSA mode.
- MUST- support RSA PKCS#1 v1.5 with SHA-256. - MUST- support RSA PKCS #1 v1.5 with SHA-256.
- SHOULD support RSASSA-PSS with SHA-256. - SHOULD support the RSA Probabilistic Signature Scheme (RSASSA-PSS)
with SHA-256.
Implementations SHOULD use deterministic generation for the parameter Implementations SHOULD use deterministic generation for the parameter
'k' for ECDSA as outlined in [RFC6979]. EdDSA is defined to generate 'k' for ECDSA as outlined in [RFC6979]. EdDSA is defined to generate
this parameter deterministically. this parameter deterministically.
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 <= 2047 : SHOULD NOT (see Historic Considerations) key size <= 2047 : SHOULD NOT (see Appendix A)
2048 <= key size <= 4096 : MUST (see Security Considerations) 2048 <= key size <= 4096 : MUST (see Security Considerations)
4096 < key size : MAY (see Security Considerations) 4096 < key size : MAY (see Security Considerations)
The signature algorithm object identifiers for RSA PKCS#1 v1.5 and The signature algorithm OIDs for RSA PKCS #1 v1.5 and RSASSA-PSS with
RSASSA-PSS with SHA-256 using 1024-bit through 3072-bit public keys SHA-256 using 1024-bit through 3072-bit public keys are specified in
are specified in [RFC4055] and the signature algorithm definition is [RFC4055], and the signature algorithm definition is found in
found in [FIPS186-2] with Change Notice 1. [FIPS186-2] with Change Notice 1.
The signature algorithm object identifiers for RSA PKCS#1 v1.5 and The signature algorithm OIDs for RSA PKCS #1 v1.5 and RSASSA-PSS with
RSASSA-PSS with SHA-256 using 4096-bit public keys are specified in SHA-256 using 4096-bit public keys are specified in [RFC4055], and
[RFC4055] and the signature algorithm definition is found in the signature algorithm definition is found in [RFC3447].
[RFC3447].
For RSASSA-PSS with SHA-256 see [RFC4056]. For RSASSA-PSS with SHA-256, see [RFC4056].
For ECDSA see [RFC5758] and [RFC6090]. The first reference provides For ECDSA, see [RFC5758] and [RFC6090]. The first reference provides
the signature algorithm's object identifier and the second provides the signature algorithm's OID, and the second provides the signature
the signature algorithm's definition. Curves other than curve P-256 algorithm's definition. Curves other than curve P-256 MAY be used as
MAY be used as well. well.
For EdDSA see [I-D.ietf-curdle-pkix] and [RFC8032]. The first For EdDSA, see [RFC8032] and [RFC8410]. The first reference provides
reference provides the signature algorithm's object identifier and the signature algorithm's OID, and the second provides the signature
the second provides the signature algorithm's definition. Other algorithm's definition. Curves other than curve25519 MAY be used as
curves than curve 25519 MAY be used as well. well.
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 LAMPS Working Group has ongoing efforts to identify and create The LAMPS Working Group has ongoing efforts to identify and create
extensions that 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 that 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 [RFC5280]. extensions are defined in [RFC5280].
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 name certificate extensions when
they appear in end-entity and CA certificates. Some mechanism SHOULD they appear in end-entity and CA certificates. Some mechanism SHOULD
exist to gracefully handle other certificate extensions when they exist to gracefully handle other certificate extensions when they
appear in end-entity or CA certificates. appear in end-entity or CA certificates.
Certificates issued for the S/MIME environment SHOULD NOT contain any Certificates issued for the S/MIME environment SHOULD NOT contain any
critical extensions (extensions that have the critical field set to critical extensions (extensions that have the critical field set to
TRUE) other than those listed here. These extensions SHOULD be 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
deemed critical to the correct interpretation of the associated is 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 [RFC5280], Interpretation and syntax for all extensions MUST follow [RFC5280],
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 basicConstraints 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 constraints extension that identifies them as issuing authority basicConstraints extension that identifies them as issuing-authority
certificates. End-entity certificates contain the key usage certificates. End-entity certificates contain the key usage
extension that restrains end-entities from using the key when extension, which restrains end entities from using the key when
performing issuing authority operations (see Section 4.4.2). performing issuing-authority operations (see Section 4.4.2).
As per [RFC5280], certificates MUST contain a basicConstraints As per [RFC5280], certificates MUST contain a basicConstraints
extension in CA certificates, and SHOULD NOT contain that extension extension in CA certificates and SHOULD NOT contain that extension in
in end-entity certificates. end-entity certificates.
4.4.2. Key Usage Certificate Extension 4.4.2. Key Usage 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. which a public key listed in a valid certificate may be used.
Issuing 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, CRLs, and other data.
lists, and other data.
For example, a certification authority may create subordinate issuer For example, a CA may create subordinate issuer certificates that
certificates that contain a key usage extension that specifies that contain a key usage extension that specifies that the corresponding
the corresponding public key can be used to sign end user public key can be used to sign end-user certificates and 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 that contains the key usage it was verified using a certificate that contains a key usage
extension without at least one of the digitalSignature or extension without at least one of the digitalSignature or
nonRepudiation bits set. Sometimes S/MIME is used as a secure nonRepudiation bits set. Sometimes S/MIME is used as a secure
message transport for applications beyond interpersonal messaging; in message transport for applications beyond interpersonal messaging; in
such cases, the S/MIME-enabled application can specify additional such cases, the S/MIME-enabled application can specify additional
requirements concerning the digitalSignature or nonRepudiation bits requirements concerning the digitalSignature or nonRepudiation bits
within this extension. within this 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 both the digitalSignature and nonRepudiation bits are presume that both the digitalSignature and nonRepudiation bits
set. 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 or addresses that
the entity for this certificate. If the local portion of the email correspond to the entity for this certificate. If the local portion
address is ASCII, it MUST be encoded using the rfc822Name CHOICE of of the email address is ASCII, it MUST be encoded using the
the GeneralName type as described in [RFC5280], Section 4.2.1.6. If rfc822Name CHOICE of the GeneralName type as described in [RFC5280],
the local portion of the email address is not ASCII, it MUST be Section 4.2.1.6. If the local portion of the email address is not
encoded using the otherName CHOICE of the GeneralName type as ASCII, it MUST be encoded using the otherName CHOICE of the
described in [I-D.ietf-lamps-eai-addresses], Section 3. Since the GeneralName type as described in [RFC8398], Section 3. Since the
SubjectAltName type is a SEQUENCE OF GeneralName, multiple email SubjectAltName type is a SEQUENCE OF GeneralName, multiple email
addresses MAY be present. addresses MAY be present.
4.4.4. Extended Key Usage Extension 4.4.4. Extended Key Usage Extension
The extended key usage extension also serves to limit the technical The extended key usage extension also serves to limit the technical
purposes for which a public key listed in a valid certificate may be purposes for which a public key listed in a valid certificate may be
used. The set of technical purposes for the certificate therefore 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 that indicating a digital signature and an extended key usage extension
includes the email protection OID, then the certificate may be used that includes the id-kp-emailProtection OID, then the certificate may
for signing but not encrypting S/MIME messages. If the certificate be used for signing but not encrypting S/MIME messages. If the
contains a key usage extension indicating digital signature but no certificate contains a key usage extension indicating a digital
extended key usage extension, then the certificate may also be used signature but no extended key usage extension, then the certificate
to sign but not encrypt S/MIME messages. may also be used to sign but not encrypt S/MIME messages.
If the extended key usage extension is present in the certificate, 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 id-kp-emailProtection OID or the
defined in [RFC5280]. S/MIME uses other than interpersonal messaging anyExtendedKeyUsage OID as defined in [RFC5280]. S/MIME uses other
MAY require the explicit presence of the extended key usage extension than interpersonal messaging MAY require the explicit presence of the
or other OIDs to be present in the extension or both. extended key usage extension, the presence of other OIDs in the
extension, or both.
5. IANA Considertions 5. IANA Considerations
This document has no new IANA considerations. This document has no IANA actions.
6. Security Considerations 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 an 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 some other program, there is no single
to handle such failures. Just because the methods to handle the way to handle such failures. Just because the methods to handle the
failures have 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 the following:
- no Internet mail addresses in a certificate match the sender of a - no Internet mail addresses in a certificate match the sender of 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 certificate chain leads to a trusted CA
- no ability to check the CRL for a certificate - no ability to check the CRL for a certificate is implemented
- an invalid CRL was received - an invalid CRL was received
- the CRL being checked is expired - the CRL being checked is expired
- the certificate is expired - the certificate is expired
- the certificate has been revoked - the certificate has been revoked
There are certainly other instances where a certificate may be There are certainly other instances where a certificate may be
invalid, and it is the responsibility of the processing software to invalid, and it is the responsibility of the processing software to
check them all thoroughly, and to decide what to do if the check check them all thoroughly and 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
recently revoked certificates, which might lead an agent to accept a recently revoked certificates; this scenario might lead an agent to
certificate that has been revoked in a subsequent CRL. accept a certificate that has been revoked in a subsequent CRL.
When determining the time for a certificate validity check, agents When determining the time for a certificate validity check, agents
have to be careful to use a reliable time. In most cases the time have to be careful to use a reliable time. In most cases, the time
used SHOULD be the current time, some exceptions to this would be: used SHOULD be the current time. Some exceptions to this would be as
follows:
- The time the message was received is stored in a secure manner and - The time the message was received is stored in a secure manner and
is used at a later time to validate the message. is used at a later time to validate the message.
- The time in a SigningTime attribute found in a counter signature - The time in a SigningTime attribute is found in a countersignature
attribute which has been successfully validated. attribute [RFC5652] that has been successfully validated.
The SigningTime attribute could be deliberately set to direct the The signingTime attribute could be deliberately set to a time where
receiving agent to check a CRL that could have out-of-date revocation the receiving agent would (1) use a CRL that does not contain a
status for a certificate, or cause an improper result when checking revocation for the signing certificate or (2) use a certificate that
the Validity field of a certificate. This could be done either by has expired or is not yet valid. This could be done by either
the sender of the message, or an attacker which has compromised the (1) the sender of the message or (2) an attacker that has compromised
key of the sender. the key of the sender.
In addition to the Security Considerations identified in [RFC5280], In addition to the security considerations identified in [RFC5280],
caution should be taken when processing certificates that 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 attacks or other attacks. For example, keys selected to
excessive cryptographic processing, or extensive lists of CRL require excessive cryptographic processing, or extensive lists of CRL
Distribution Point (CDP) and/or Authority Information Access (AIA) Distribution Point (CDP) and/or Authority Information Access (AIA)
addresses in the certificate, could be used to mount denial-of- addresses in the certificate, could be used to mount denial-of-
service attacks. Similarly, attacker-specified CDP and/or AIA service attacks. Similarly, attacker-specified CDP and/or AIA
addresses could be included in fake certificates to allow the addresses could be included in fake certificates to allow the
originator to detect receipt of the message even if signature originator to detect receipt of the message even if signature
verification fails. verification fails.
RSA keys of less than 2048 bits are now considered by many experts to RSA keys of less than 2048 bits are now considered by many experts to
be cryptographically insecure (due to advances in computing power), be cryptographically insecure (due to advances in computing power)
and SHOULD no longer be used to sign certificates or CRLs. Such keys and SHOULD no longer be used to sign certificates or CRLs. Such keys
were previously considered secure, so processing previously received were previously considered secure, so processing previously received
signed and encrypted mail may require processing certificates or CRLs signed and encrypted mail may require processing certificates or CRLs
signed with weak keys. Implementations that wish to support previous signed with weak keys. Implementations that wish to support previous
versions of S/MIME or process old messages need to consider the versions of S/MIME or process old messages need to consider the
security risks that result from accepting certificates and CRLs with security risks that result from accepting certificates and CRLs with
smaller key sizes (e.g., spoofed certificates) versus the costs of smaller key sizes (e.g., spoofed certificates) versus the costs of
denial of service. If an implementation supports verification of denial of service. If an implementation supports verification of
certificates or CRLs generated with RSA and DSA keys of less than certificates or CRLs generated with RSA and DSA keys of less than
2048 bits, it MUST warn the user. Implementers should consider 2048 bits, it MUST warn the user. Implementers should consider
skipping to change at page 18, line 43 skipping to change at page 20, line 20
Server implementations (e.g., secure mail list servers) where user Server implementations (e.g., secure mail list servers) where user
warnings are not appropriate SHOULD reject messages with weak warnings are not appropriate SHOULD reject messages with weak
cryptography. cryptography.
If an implementation is concerned about compliance with National If an implementation is concerned about compliance with National
Institute of Standards and Technology (NIST) key size Institute of Standards and Technology (NIST) key size
recommendations, then see [SP800-57]. recommendations, then see [SP800-57].
7. References 7. References
7.1. Normative References 7.1. Reference Conventions
[ESS] refers to [RFC2634] and [RFC5035].
[SMIMEv2] refers to [RFC2311], [RFC2312], [RFC2313], [RFC2314], and
[RFC2315].
[SMIMEv3] refers to [RFC2630], [RFC2631], [RFC2632], [RFC2633],
[RFC2634], and [RFC5035].
[SMIMEv3.1] refers to [RFC2634], [RFC3850], [RFC3851], [RFC3852],
and [RFC5035].
[SMIMEv3.2] refers to [RFC2634], [RFC5035], [RFC5652], [RFC5750],
and [RFC5751].
[SMIMEv4] refers to [RFC2634], [RFC5035], [RFC5652], [RFC8551], and
this document.
7.2. Normative References
[FIPS186-2] [FIPS186-2]
National Institute of Standards and Technology (NIST), National Institute of Standards and Technology (NIST),
"Digital Signature Standard (DSS) [With Change Notice 1]", "Digital Signature Standard (DSS) (also with Change
Federal Information Processing Standards Notice 1)", Federal Information Processing Standards
Publication 186-2, January 2000. Publication 186-2, January 2000,
<https://csrc.nist.gov/publications/detail/fips/186/2/
archive/2000-01-27>.
[FIPS186-3] [FIPS186-3]
National Institute of Standards and Technology (NIST), National Institute of Standards and Technology (NIST),
"Digital Signature Standard (DSS)", Federal Information "Digital Signature Standard (DSS)", Federal Information
Processing Standards Publication 186-3, June 2009. Processing Standards Publication 186-3, June 2009,
<https://csrc.nist.gov/csrc/media/publications/fips/186/3/
[I-D.ietf-lamps-eai-addresses] archive/2009-06-25/documents/fips_186-3.pdf>.
Melnikov, A. and W. Chuang, "Internationalized Email
Addresses in X.509 certificates", draft-ietf-lamps-eai-
addresses-18 (work in progress), March 2018.
[I-D.ietf-lamps-rfc5751-bis]
Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Message Specification", draft-ietf-lamps-rfc5751-bis-11
(work in progress), July 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC2634] Hoffman, P., Ed., "Enhanced Security Services for S/MIME", [RFC2634] Hoffman, P., Ed., "Enhanced Security Services for S/MIME",
RFC 2634, DOI 10.17487/RFC2634, June 1999, RFC 2634, DOI 10.17487/RFC2634, June 1999,
<https://www.rfc-editor.org/info/rfc2634>. <https://www.rfc-editor.org/info/rfc2634>.
skipping to change at page 20, line 30 skipping to change at page 22, line 20
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009, RFC 5652, DOI 10.17487/RFC5652, September 2009,
<https://www.rfc-editor.org/info/rfc5652>. <https://www.rfc-editor.org/info/rfc5652>.
[RFC5750] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet [RFC5750] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Certificate Mail Extensions (S/MIME) Version 3.2 Certificate
Handling", RFC 5750, DOI 10.17487/RFC5750, January 2010, Handling", RFC 5750, DOI 10.17487/RFC5750, January 2010,
<https://www.rfc-editor.org/info/rfc5750>. <https://www.rfc-editor.org/info/rfc5750>.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, DOI 10.17487/RFC5751, January
2010, <https://www.rfc-editor.org/info/rfc5751>.
[RFC5755] Farrell, S., Housley, R., and S. Turner, "An Internet [RFC5755] Farrell, S., Housley, R., and S. Turner, "An Internet
Attribute Certificate Profile for Authorization", Attribute Certificate Profile for Authorization",
RFC 5755, DOI 10.17487/RFC5755, January 2010, RFC 5755, DOI 10.17487/RFC5755, January 2010,
<https://www.rfc-editor.org/info/rfc5755>. <https://www.rfc-editor.org/info/rfc5755>.
[RFC5758] Dang, Q., Santesson, S., Moriarty, K., Brown, D., and T. [RFC5758] Dang, Q., Santesson, S., Moriarty, K., Brown, D., and T.
Polk, "Internet X.509 Public Key Infrastructure: Polk, "Internet X.509 Public Key Infrastructure:
Additional Algorithms and Identifiers for DSA and ECDSA", Additional Algorithms and Identifiers for DSA and ECDSA",
RFC 5758, DOI 10.17487/RFC5758, January 2010, RFC 5758, DOI 10.17487/RFC5758, January 2010,
<https://www.rfc-editor.org/info/rfc5758>. <https://www.rfc-editor.org/info/rfc5758>.
[RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature [RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature
Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature
Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
2013, <https://www.rfc-editor.org/info/rfc6979>. 2013, <https://www.rfc-editor.org/info/rfc6979>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[SMIMEv3.2] [RFC8398] Melnikov, A., Ed. and W. Chuang, Ed., "Internationalized
"S/MIME version 3.2". Email Addresses in X.509 Certificates", RFC 8398,
DOI 10.17487/RFC8398, May 2018,
This group of documents represents S/MIME version 3.2. <https://www.rfc-editor.org/info/rfc8398>.
This set of documents are [RFC2634], [RFC5750], [[This
Document]], [RFC5652], and [RFC5035].
[SMIMEv4.0]
"S/MIME version 4.0".
This group of documents represents S/MIME version 4.0. [RFC8551] Schaad, J., Ramsdell, B., and S. Turner,
This set of documents are [RFC2634], "Secure/Multipurpose Internet Mail Extensions (S/MIME)
[I-D.ietf-lamps-rfc5751-bis], [[This Document]], Version 4.0 Message Specification", RFC 8551,
[RFC5652], and [RFC5035]. DOI 10.17487/RFC8551, April 2019,
<https://www.rfc-editor.org/info/rfc8551>.
[X.680] "Information Technology - Abstract Syntax Notation One [X.680] "Information Technology - Abstract Syntax Notation One
(ASN.1): Specification of basic notation. ITU-T (ASN.1): Specification of basic notation", ITU-T
Recommendation X.680 (2002) | ISO/IEC 8824-1:2002.". Recommendation X.680, ISO/IEC 8824-1:2015, August 2015,
<https://www.itu.int/rec/T-REC-X.680>.
7.2. Informational References
[ESS] "Enhanced Security Services for S/ MIME".
This is the set of documents dealing with enhanced
security services and refers to [RFC2634] and [RFC5035].
[I-D.ietf-curdle-pkix] 7.3 Informative References
Josefsson, S. and J. Schaad, "Algorithm Identifiers for
Ed25519, Ed448, X25519 and X448 for use in the Internet
X.509 Public Key Infrastructure", draft-ietf-curdle-
pkix-10 (work in progress), May 2018.
[PKCS6] RSA Laboratories, "PKCS #6: Extended-Certificate Syntax [PKCS6] RSA Laboratories, "PKCS #6: Extended-Certificate Syntax
Standard", November 1993. Standard", November 1993.
[RFC2311] 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", L. Repka, "S/MIME Version 2 Message Specification",
RFC 2311, DOI 10.17487/RFC2311, March 1998, RFC 2311, DOI 10.17487/RFC2311, March 1998,
<https://www.rfc-editor.org/info/rfc2311>. <https://www.rfc-editor.org/info/rfc2311>.
[RFC2312] Dusse, S., Hoffman, P., Ramsdell, B., and J. Weinstein, [RFC2312] Dusse, S., Hoffman, P., Ramsdell, B., and J. Weinstein,
skipping to change at page 23, line 5 skipping to change at page 24, line 24
[RFC3851] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail [RFC3851] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification", Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, DOI 10.17487/RFC3851, July 2004, RFC 3851, DOI 10.17487/RFC3851, July 2004,
<https://www.rfc-editor.org/info/rfc3851>. <https://www.rfc-editor.org/info/rfc3851>.
[RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)", [RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 3852, DOI 10.17487/RFC3852, July 2004, RFC 3852, DOI 10.17487/RFC3852, July 2004,
<https://www.rfc-editor.org/info/rfc3852>. <https://www.rfc-editor.org/info/rfc3852>.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, DOI 10.17487/RFC5751,
January 2010, <https://www.rfc-editor.org/info/rfc5751>.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic [RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090, Curve Cryptography Algorithms", RFC 6090,
DOI 10.17487/RFC6090, February 2011, DOI 10.17487/RFC6090, February 2011,
<https://www.rfc-editor.org/info/rfc6090>. <https://www.rfc-editor.org/info/rfc6090>.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations [RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms", for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC6151, March 2011, RFC 6151, DOI 10.17487/RFC6151, March 2011,
<https://www.rfc-editor.org/info/rfc6151>. <https://www.rfc-editor.org/info/rfc6151>.
skipping to change at page 23, line 30 skipping to change at page 25, line 5
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032, Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017, DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/info/rfc8032>. <https://www.rfc-editor.org/info/rfc8032>.
[RFC8162] Hoffman, P. and J. Schlyter, "Using Secure DNS to [RFC8162] Hoffman, P. and J. Schlyter, "Using Secure DNS to
Associate Certificates with Domain Names for S/MIME", Associate Certificates with Domain Names for S/MIME",
RFC 8162, DOI 10.17487/RFC8162, May 2017, RFC 8162, DOI 10.17487/RFC8162, May 2017,
<https://www.rfc-editor.org/info/rfc8162>. <https://www.rfc-editor.org/info/rfc8162>.
[SMIMEv2] "S/MIME version v2". [RFC8410] Josefsson, S. and J. Schaad, "Algorithm Identifiers for
Ed25519, Ed448, X25519, and X448 for Use in the Internet
This group of documents represents S/MIME version 2. This X.509 Public Key Infrastructure", RFC 8410,
set of documents are [RFC2311], [RFC2312], [RFC2313], DOI 10.17487/RFC8410, August 2018,
[RFC2314], and [RFC2315]. <https://www.rfc-editor.org/info/rfc8410>.
[SMIMEv3] "S/MIME version 3".
This group of documents represents S/MIME version 3. This
set of documents are [RFC2630], [RFC2631], [RFC2632],
[RFC2633], [RFC2634], and [RFC5035].
[SMIMEv3.1]
"S/MIME version 3.1".
This group of documents represents S/MIME version 3.1.
This set of documents are [RFC2634], [RFC3850], [RFC3851],
[RFC3852], and [RFC5035].
[SP800-57] [SP800-57] National Institute of Standards and Technology (NIST),
National Institute of Standards and Technology (NIST), "Recommendation for Key Management - Part 1: General",
"Special Publication 800-57: Recommendation for Key NIST Special Publication 800-57 Revision 4,
Management", August 2005. DOI 10.6028/NIST.SP.800-57pt1r4, January 2016,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-57pt1r4.pdf>.
[X.500] "ITU-T Recommendation X.500 (1997) | ISO/IEC 9594- 1:1997, [X.500] "Information technology - Open Systems Interconnection -
Information technology - Open Systems Interconnection - The Directory - Part 1: Overview of concepts, models and
The Directory: Overview of concepts, models and services", ITU-T Recommendation X.500,
services.". ISO/IEC 9594-1:2017.
Appendix A. Historic Considerations Appendix A. Historic Considerations
A.1. Signature Algorithms and Key Sizes A.1. Signature Algorithms and Key Sizes
There are a number of problems with validating certificates on There are a number of problems with validating certificates on
sufficiently historic messages. For this reason it is strongly sufficiently historic messages. For this reason, it is strongly
suggested that UAs treat these certificates differently from those on suggested that user agents treat these certificates differently from
current messages. These problems include: those on current messages. These problems include the following:
- CAs are not required to keep certificates on a CRL beyond one - CAs are not required to keep certificates on a CRL beyond one
update after a certificate has expired. This means that unless update after a certificate has expired. This means that unless
CRLs are cached as part of the message it is not always possible CRLs are cached as part of the message it is not always possible
to check if a certificate has been revoked. The same problems to check to see if a certificate has been revoked. The same
exist with OCSP responses as they may be based on a CRL rather problems exist with Online Certificate Status Protocol (OCSP)
than on the certificate database. responses, as they may be based on a CRL rather than on the
certificate database.
- RSA and DSA keys of less than 2048 bits are now considered by many - RSA and DSA keys of less than 2048 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). Such keys were previously considered secure, so computing power). Such keys were previously considered secure, so
processing of historic certificates will often result in the use the processing of historic certificates will often result in the
of weak keys. Implementations that wish to support previous use of weak keys. Implementations that wish to support previous
versions of S/MIME or process old messages need to consider the versions of S/MIME or process old messages need to consider the
security risks that result from smaller key sizes (e.g., spoofed security risks that result from smaller key sizes (e.g., spoofed
messages) versus the costs of denial of service. messages) versus the costs of denial of service.
[SMIMEv3.1] set the lower limit on suggested key sizes for [SMIMEv3.2] set the lower limit on suggested key sizes for
creating and validation at 1024 bits. Prior to that the lower creating and validation at 1024 bits. [SMIMEv3.1] set the lower
bound on key sizes was 512 bits. limit at 768 bits. Prior to that, the lower bound on key sizes
was 512 bits.
- Hash functions used to validate signatures on historic messages - Hash functions used to validate signatures on historic messages
may no longer be considered to be secure (see below). While there may no longer be considered to be secure (see below). While there
are not currently any known practical pre-image or second pre- are not currently any known practical pre-image or second
image attacks against MD5 or SHA-1, the fact they are no longer pre-image attacks against MD5 or SHA-1, the fact that they are no
considered to be collision resistant implies that the security longer considered to be collision resistant implies that the
level of any signature that is created with that these hash security level of any signature that is created with these hash
algorithms should also be considered as suspect. algorithms should also be considered as suspect.
The following algorithms have been called out for some level of The following algorithms have been called out for some level of
support by previous S/MIME specifications: support by previous S/MIME specifications:
- RSA with MD5 was dropped in [SMIMEv4.0]. MD5 is no longer - RSA with MD5 was dropped in [SMIMEv4]. MD5 is no longer
considered to be secure as it is no longer collision-resistant. considered to be secure, as it is no longer collision resistant.
Details can be found in [RFC6151]. Details can be found in [RFC6151].
- RSA and DSA with SHA-1 were dropped in [SMIMEv4.0]. SHA-1 is no - RSA and DSA with SHA-1 were dropped in [SMIMEv4]. SHA-1 is no
longer considered to be secure as it is no longer collision- longer considered to be secure, as it is no longer collision
resistant. The IETF statement on SHA-1 can be found in [RFC6194] resistant. The IETF statement on SHA-1 can be found in [RFC6194],
but it is out-of-date relative to the most recent advances. but it is out of date relative to the most recent advances.
- DSA with SHA-256 support was dropped in [SMIMEv4.0]. DSA was - DSA with SHA-256 support was dropped in [SMIMEv4]. DSA was
dropped as part of a general movement from finite fields to dropped as part of a general movement from finite fields to
elliptic curves. Issues have come up dealing with non- elliptic curves. Issues related to dealing with non-deterministic
deterministic generation of the parameter 'k' (see [RFC6979]). generation of the parameter 'k' have come up (see [RFC6979]).
For 512-bit RSA with SHA-1 see [RFC3279] and [FIPS186-2] without For 512-bit RSA with SHA-1, see [RFC3279] and [FIPS186-2] without
Change Notice 1, for 512-bit RSA with SHA-256 see [RFC4055] and Change Notice 1; for 512-bit RSA with SHA-256, see [RFC4055] and
[FIPS186-2] without Change Notice 1. [FIPS186-2] without Change Notice 1. The first reference provides
the signature algorithm's OID, and the second provides the signature
algorithm's definition.
For 512-bit DSA with SHA-1 see [RFC3279] and [FIPS186-2] without For 512-bit DSA with SHA-1, see [RFC3279] and [FIPS186-2] without
Change Notice 1, for 512-bit DSA with SHA-256 see [RFC5758] and Change Notice 1; for 512-bit DSA with SHA-256, see [RFC5758] 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
[RFC3279] and [FIPS186-2] with Change Notice 1, for 1024-bit through [RFC3279] and [FIPS186-2] with Change Notice 1; and for 1024-bit
3072 DSA with SHA-256 see [RFC5758] and [FIPS186-3]. In either case, through 3072-bit DSA with SHA-256, see [RFC5758] and [FIPS186-3].
the first reference provides the signature algorithm's object The first reference provides the signature algorithm's OID, and the
identifier and the second provides the signature algorithm's second provides the signature algorithm's definition.
definition.
Appendix B. Moving S/MIME v2 Certificate Handling to Historic Status Appendix B. Moving S/MIME v2 Certificate Handling to Historic Status
The S/MIME v3 [SMIMEv3], v3.1 [SMIMEv3.1], v3.2 [SMIMEv3.2], and v4.0 The S/MIME v3 [SMIMEv3], v3.1 [SMIMEv3.1], v3.2 [SMIMEv3.2], and v4.0
(this document) are backward compatible with the S/MIME v2 (this document) specifications are backward compatible with the
Certificate Handling Specification [SMIMEv2], with the exception of S/MIME v2 Certificate Handling Specification [SMIMEv2], with the
the algorithms (dropped RC2/40 requirement and added DSA and RSASSA- exception of the algorithms (dropped RC2/40 requirement, and added
PSS requirements). Therefore, RFC 2312 [SMIMEv2] was moved to DSA and RSASSA-PSS requirements). Therefore, RFC 2312 [SMIMEv2] was
Historic status. moved to Historic status.
Appendix C. Acknowledgments Acknowledgements
Many thanks go out to the other authors of the S/MIME v2 RFC: Steve Many thanks go out to the other authors of the S/MIME v2 Certificate
Dusse, Paul Hoffman, and Jeff Weinstein. Without v2, there wouldn't Handling RFC: Steve Dusse, Paul Hoffman, and Jeff Weinstein. Without
be a v3, v3.1, v3.2 or v4.0. v2, there wouldn't be a v3, v3.1, v3.2, or v4.0.
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 because they made direct the following people stand out in my mind because they 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,
and Denis Pinkas. and Denis Pinkas.
The version 4 update to the S/MIME documents was done under the The version 4 update to the S/MIME documents was done under the
auspices of the LAMPS Working Group. auspices of the LAMPS Working Group.
Authors' Addresses Authors' Addresses
Jim Schaad Jim Schaad
August Cellars August Cellars
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