    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
       carefully, as they describe your rights and restrictions with respect
       to this document.

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    Table of Contents

       1. Introduction ....................................................2
       2. Conventions Used in This Document ...............................2
       3. "smime" Authentication Method ...................................2
          3.1. S/MIME Results .............................................3
          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 .............5
          3.3. Examples ...................................................5
       4. IANA Considerations .............................................7
       5. Security Considerations .........................................9
       6. References .....................................................10
          6.1. Normative References ......................................10
          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].

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    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

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       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.

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    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"

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     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

     MIIDdwYJKoZIhvcNAQcCoIIDaDCCA2QCAQExCTAHBgUrDgMCGjALBgkqhkiG9w0BBwGgggL
     gMIIC3DCCApugAwIBAgICAMgwCQYHKoZIzjgEAzASMRAwDgYDVQQDEwdDYXJsRFNTMB4XDT
     k5MDgxNzAxMTA0OVoXDTM5MTIzMTIzNTk1OVowEzERMA8GA1UEAxMIQWxpY2VEU1MwggG2M
     IIBKwYHKoZIzjgEATCCAR4CgYEAgY3N7YPqCp45PsJIKKPkR5PdDteoDuxTxauECE//lOFz
     SH4M1vNESNH+n6+koYkv4dkwyDbeP5u/t0zcX2mK5HXQNwyRCJWb3qde+fz0ny/dQ6iLVPE
     /sAcIR01diMPDtbPjVQh11Tl2EMR4vf+dsISXN/LkURu15AmWXPN+W9sCFQDiR6YaRWa4E8
     baj7g3IStii/eTzQKBgCY40BSJMqo5+z5t2UtZakx2IzkEAjVc8ssaMMMeUF3dm1nizaoFP
     VjAe6I2uG4Hr32KQiWn9HXPSgheSz6Q+G3qnMkhijt2FOnOLl2jB80jhbgvMAF8bUmJEYk2
     RL34yJVKU1a14vlz7BphNh8Rf8K97dFQ/5h0wtGBSmA5ujY5A4GEAAKBgFzjuVp1FJYLqXr
     d4z+p7Kxe3L23ExE0phaJKBEj2TSGZ3V1ExI9Q1tv5VG/+onyohs+JH09B41bY8i7RaWgSu
     OF1s4GgD/oI34a8iSrUxq4Jw0e7wi/ZhSAXGKsZfoVi/G7NNTSljf2YUeyxDKE8H5BQP1Gp
     2NOM/Kl4vTyg+W4o4GBMH8wDAYDVR0TAQH/BAIwADAOBgNVHQ8BAf8EBAMCBsAwHwYDVR0j
     BBgwFoAUcEQ+gi5vh95K03XjPSC8QyuT8R8wHQYDVR0OBBYEFL5sobPjwfftQ3CkzhMB4v3
     jl/7NMB8GA1UdEQQYMBaBFEFsaWNlRFNTQGV4YW1wbGUuY29tMAkGByqGSM44BAMDMAAwLQ
     IUVQykGR9CK4lxIjONg2q1PWdrv0UCFQCfYVNSVAtcst3a53Yd4hBSW0NevTFjMGECAQEwG
     DASMRAwDgYDVQQDEwdDYXJsRFNTAgIAyDAHBgUrDgMCGjAJBgcqhkjOOAQDBC4wLAIUM/mG
     f6gkgp9Z0XtRdGimJeB/BxUCFGFFJqwYRt1WYcIOQoGiaowqGzVI

     ------=_NextBoundary____Fri,_06_Sep_2002_00:25:21--

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    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.     |       |      |
    |      |          |       |            |                |       |      |

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    | 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 |
       +-----------+-----------+----------+-----------------------+--------+

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    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.

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    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.

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    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
       14 Castle Mews
       Hampton, Middlesex  TW12 2NP
       United Kingdom

       EMail: Alexey.Melnikov@isode.com

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