Independent Submission A. Melnikov
Request for Comments: 7281 Isode Ltd
Category: Informational June 2014
ISSN: 2070-1721
Authentication-Results Registration for S/MIME Signature Verification
Abstract
RFC 7001 specifies the Authentication-Results header field for
conveying results of message authentication checks. This document
defines a new authentication method to be used in the Authentication-
Results header field for S/MIME-related signature checks.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7281.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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to this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 2
3. "smime" Authentication Method . . . . . . . . . . . . . . . . 2
3.1. S/MIME Results . . . . . . . . . . . . . . . . . . . . . 2
3.2. Email Authentication Parameters for S/MIME . . . . . . . 4
3.2.1. body.smime-part . . . . . . . . . . . . . . . . . . . 4
3.2.2. body.smime-identifier . . . . . . . . . . . . . . . . 4
3.2.3. body.smime-serial and body.smime-issuer . . . . . . . 4
3.3. Examples . . . . . . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Normative References . . . . . . . . . . . . . . . . . . 9
6.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11
1. Introduction
[RFC7001] specifies the Authentication-Results header field for
conveying results of message authentication checks. As S/MIME
signature verification (and alteration) is sometimes implemented in
border message transfer agents, guards, and gateways (for example,
see [RFC3183]), there is a need to convey signature verification
status to Mail User Agents (MUAs) and downstream filters. This
document defines a new authentication method to be used in the
Authentication-Results header field for S/MIME-related signature
checks.
2. Conventions Used in This Document
The formal syntax uses the Augmented Backus-Naur Form (ABNF)
[RFC5234] notation, including the core rules defined in Appendix B of
[RFC5234].
3. "smime" Authentication Method
S/MIME signature and countersignature verification is represented by
the "smime" method and is defined in [RFC5751].
3.1. S/MIME Results
The result values used by S/MIME [RFC5751] are as follows:
+-----------+-------------------------------------------------------+
| Result | Meaning |
| code | |
+-----------+-------------------------------------------------------+
| none | The message was not signed. |
| | |
| pass | The message was signed, the signature or signatures |
| | were acceptable to the verifier, and the signature(s) |
| | passed verification tests. |
| | |
| fail | The message was signed and the signature or |
| | signatures were acceptable to the verifier, but they |
| | failed the verification test(s). |
| | |
| policy | The message was signed and signature(s) passed |
| | verification tests, but the signature or signatures |
| | were not acceptable to the verifier. |
| | |
| neutral | The message was signed but the signature or |
| | signatures contained syntax errors or were not |
| | otherwise able to be processed. This result is also |
| | used for other failures not covered elsewhere in this |
| | list. |
| | |
| temperror | The message could not be verified due to some error |
| | that is likely transient in nature, such as a |
| | temporary inability to retrieve a certificate or |
| | Certificate Revocation List (CRL). A later attempt |
| | may produce a final result. |
| | |
| permerror | The message could not be verified due to some error |
| | that is unrecoverable, such as a required header |
| | field being absent or the signer's certificate not |
| | being available. A later attempt is unlikely to |
| | produce a final result. |
+-----------+-------------------------------------------------------+
A signature is "acceptable to the verifier" if it passes local policy
checks (or there are no specific local policy checks). For example,
a verifier might require that the domain in the rfc822Name
subjectAltName in the signing certificate matches the domain in the
address of the sender of the message (value of the Sender header
field, if present; value of the From header field otherwise), thus
making third-party signatures unacceptable. [RFC5751] advises that
if a message fails verification, it should be treated as an unsigned
message. A report of "fail" here permits the receiver of the report
to decide how to handle the failure. A report of "neutral" or "none"
preempts that choice, ensuring that the message will be treated as if
it had not been signed.
3.2. Email Authentication Parameters for S/MIME
This document defines several new authentication parameters for
conveying S/MIME-related information, such as the location of an
S/MIME signature and the identity associated with the entity that
signed the message or one of its body parts.
3.2.1. body.smime-part
body.smime-part contains the MIME body part reference that contains
the S/MIME signature. The syntax of this property is described by
the smime-part ABNF production below. application/pkcs7-signature or
application/pkcs7-mime (containing SignedData) media type body parts
are referenced using the <section> syntax (see Section 6.4.5 of
[RFC3501]). If the signature being verified is encapsulated by
another Cryptographic Message Syntax (CMS) content type (e.g.,
application/pkcs7-mime containing EnvelopedData, which contains
SignedData), such an inner signature body part can be referenced
using "section[/section..." syntax.
smime-part = section ["/" smime-subpart]
smime-subpart = smime-part
section = <Defined in Section 6.4.5 of [RFC3501]>
3.2.2. body.smime-identifier
body.smime-identifier contains the email address [RFC5322] associated
with the S/MIME signature referenced in the corresponding body.smime-
part. The email address can be specified explicitly in the signer's
X.509 certificate or derived from the identity of the signer. Note
that this email address can correspond to a countersignature.
3.2.3. body.smime-serial and body.smime-issuer
body.smime-serial contains the serialNumber of the X.509 certificate
associated with the S/MIME signature (see Section 4.1.2.2 of
[RFC5280]) referenced in the corresponding body.smime-part.
body.smime-issuer contains the issuer name DN (distinguished name)
(e.g., "CN=CA1,ST=BC,c=CA") of the X.509 certificate associated with
the S/MIME signature (see Section 4.1.2.4 of [RFC5280]) referenced in
the corresponding body.smime-part.
Either both or neither of body.smime-serial and body.smime-issuer
should be present in an Authentication-Results header field.
body.smime-serial and body.smime-issuer are used for cases when
body.smime-identifier (email address) can't be derived by the entity
adding the corresponding Authentication-Results header field. For
example, this can be used when gatewaying from X.400.
3.3. Examples
Return-Path: <aliceDss@example.com>
Authentication-Results: example.net;
smime=fail (certificate is revoked by CRL)
body.smime-identifier=aliceDss@example.com
body.smime-part=2
Received: from ietfa.example.com (localhost [IPv6:::1])
by ietfa.example.com (Postfix) with ESMTP id 2875111E81A0;
Fri, 06 Sep 2002 00:35:14 -0700 (PDT)
MIME-Version: 1.0
To: User2@example.com
From: aliceDss@example.com
Subject: Example 4.8
Message-Id: <020906002550300.249@example.com>
Date: Fri, 06 Sep 2002 00:25:21 -0700
Content-Type: multipart/signed;
micalg=SHA1;
boundary="----=_NextBoundary____Fri,_06_Sep_2002_00:25:21";
protocol="application/pkcs7-signature"
This is a multi-part message in MIME format.
------=_NextBoundary____Fri,_06_Sep_2002_00:25:21
This is some sample content.
------=_NextBoundary____Fri,_06_Sep_2002_00:25:21
Content-Type: application/pkcs7-signature; name=smime.p7s
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7s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------=_NextBoundary____Fri,_06_Sep_2002_00:25:21--
4. IANA Considerations
IANA has added the following entries to the "Email Authentication
Methods" sub-registry of the "Email Authentication Parameters"
registry:
+------+----------+-------+------------+----------------+-------+------+
|Method| Defined | ptype | Property | Value |Status | Ver- |
| | in | | | | | sion |
+------+----------+-------+------------+----------------+-------+------+
| smime| [RFC5751]| body | smime-part | A reference to |active | 1 |
| | | | | the MIME body | | |
| | | | | part that | | |
| | | | | contains the | | |
| | | | | signature, as | | |
| | | | | defined in | | |
| | | | | Section 3.2.1 | | |
| | | | | of [RFC7281]. | | |
| | | | | | | |
| smime| [RFC5751]| body | smime- | The email |active | 1 |
| | | | identifier | address | | |
| | | | | [RFC5322] | | |
| | | | | associated | | |
| | | | | with the | | |
| | | | | S/MIME | | |
| | | | | signature. | | |
| | | | | The email | | |
| | | | | address can be | | |
| | | | | specified | | |
| | | | | explicitly or | | |
| | | | | derived from | | |
| | | | | the identity | | |
| | | | | of the signer. | | |
| | | | | Note that this | | |
| | | | | email address | | |
| | | | | can correspond | | |
| | | | | to a counter- | | |
| | | | | signature. | | |
| | | | | | | |
| smime| [RFC5751]| body | smime- | serialNumber |active | 1 |
| | | | serial | of the | | |
| | | | | certificate | | |
| | | | | associated | | |
| | | | | with the | | |
| | | | | S/MIME | | |
| | | | | signature (see | | |
| | | | | Section | | |
| | | | | 4.1.2.2 of | | |
| | | | | [RFC5280]. | | |
| | | | | | | |
| smime| [RFC5751]| body | smime- | Issuer name DN |active | 1 |
| | | | issuer | (e.g., "CN=CA1,| | |
| | | | | ST=BC,c=CA") | | |
| | | | | of the | | |
| | | | | certificate | | |
| | | | | associated | | |
| | | | | with the | | |
| | | | | S/MIME | | |
| | | | | signature (see | | |
| | | | | Section | | |
| | | | | 4.1.2.4 of | | |
| | | | | [RFC5280]. | | |
+------+----------+-------+------------+----------------+-------+------+
IANA has added the following entries to the "Email Authentication
Result Names" sub-registry of the "Email Authentication Parameters"
registry:
+-----------+-----------+----------+-----------------------+--------+
| Code | Defined | Auth | Meaning | Status |
| | | Method | | |
+-----------+-----------+----------+-----------------------+--------+
| none | [RFC7281] | smime | [RFC7281] Section 3.1 | active |
| | | | | |
| pass | [RFC7281] | smime | [RFC7281] Section 3.1 | active |
| | | | | |
| fail | [RFC7281] | smime | [RFC7281] Section 3.1 | active |
| | | | | |
| policy | [RFC7281] | smime | [RFC7281] Section 3.1 | active |
| | | | | |
| neutral | [RFC7281] | smime | [RFC7281] Section 3.1 | active |
| | | | | |
| temperror | [RFC7281] | smime | [RFC7281] Section 3.1 | active |
| | | | | |
| permerror | [RFC7281] | smime | [RFC7281] Section 3.1 | active |
+-----------+-----------+----------+-----------------------+--------+
5. Security Considerations
This document doesn't add new security considerations not already
covered by [RFC7001] and [RFC5751]. In particular, security
considerations related to the use of weak cryptography over
plaintext, weakening and breaking of cryptographic algorithms over
time, and changing the behavior of message processing based on
presence of a signature specified in [RFC5751] are relevant to this
document. Similarly, the following security considerations specified
in [RFC7001] are particularly relevant to this document: Forged
Header Fields, Misleading Results, Internal Mail Transfer Agent (MTA)
Lists, and Compromised Internal Hosts.
To repeat something already mentioned in RFC 7001, Section 7.1:
An MUA or filter that accesses a mailbox whose messages are
handled by a non-conformant MTA, and understands Authentication-
Results header fields, could potentially make false conclusions
based on forged header fields. A malicious user or agent could
forge a header field using the DNS domain of a receiving ADMD as
the authserv-id token in the value of the header field and, with
the rest of the value, claim that the message was properly
authenticated. The non-conformant MTA would fail to strip the
forged header field, and the MUA could inappropriately trust it.
For this reason, it is best not to have processing of the
Authentication-Results header field enabled by default; instead,
it should be ignored, at least for the purposes of enacting
filtering decisions, unless specifically enabled by the user or
administrator after verifying that the border MTA is compliant.
It is acceptable to have an MUA aware of this specification but
have an explicit list of hostnames whose Authentication-Results
header fields are trustworthy; however, this list should initially
be empty.
So, to emphasize this point: whenever possible, MUAs should implement
their own S/MIME signature verification instead of implementing this
specification.
Note that agents adding Authentication-Results header fields
containing S/MIME authentication method might be unable to verify
S/MIME signatures inside encrypted CMS content types such as
EnvelopedData [RFC5652]. So, agents processing Authentication-
Results header fields can't treat the lack of an Authentication-
Results header field with S/MIME authentication method as an
indication that the corresponding S/MIME signature is missing,
invalid, or valid.
6. References
6.1. Normative References
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[RFC7001] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 7001, September 2013.
6.2. Informative References
[RFC3183] Dean, T. and W. Ottaway, "Domain Security Services using
S/MIME", RFC 3183, October 2001.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
Appendix A. Acknowledgements
Thank you to Murray S. Kucherawy, David Wilson, Jim Schaad, SM, and
Steve Kille for comments and corrections on this document.
Author's Address
Alexey Melnikov
Isode Ltd
5
14 Castle Business Village
36 Station Road Mews
Hampton, Middlesex TW12 2BX
UK 2NP
United Kingdom
EMail: Alexey.Melnikov@isode.com