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Network Working Group                                          J. Levine
Internet-Draft                                      Taughannock Networks
Intended status: Standards Track                      September 28, 2015
Expires: March 31, 2016


                   Mandatory Tags for DKIM Signatures
                    draft-levine-dkim-conditional-02

Abstract

   The DKIM protocol applies a cryptographic signature to an e-mail
   message.  This specification extends DKIM to allow new signature tags
   that validators are required to evaluate.  The first such tag
   specifies a second signature that must be present for a signature to
   be valid.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on March 31, 2016.

Copyright Notice

   Copyright (c) 2015 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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Mandatory DKIM header tags  . . . . . . . . . . . . . . . . .   3
     3.1.  Signature version numbers . . . . . . . . . . . . . . . .   3
     3.2.  Processing mandatory tags . . . . . . . . . . . . . . . .   3
     3.3.  Forward signature (!fs) tag . . . . . . . . . . . . . . .   3
   4.  Typical application scenario  . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   7.  Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  -01 to -02  . . . . . . . . . . . . . . . . . . . . . . .   5
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   DKIM [RFC6376] defines a cryptographic header field consisting of a
   series of tags and values.  The values include signed hashes of some
   of the header fields and part or all of the body of a message.  The
   signature contains a domain name that is responsible for the
   signature.  The signature is valid if the hashes in the signature
   match the corresponding hashes of the message at validation time, the
   signature is validated by a public key retrieved from that
   responsible domain's DNS, and it is before the expiration time in the
   signature header field.

   This specification defines the syntax for new tags in a signature
   header field that specify additional conditions that must be
   satisfied for a signature to be valid.  The first such condition
   requires the presence of an additional signature from a specified
   different domain.  It also defines a new version 2 of the DKIM
   protocol to support the new semantics of conditional signatures.

2.  Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   Syntax descriptions use Augmented BNF (ABNF)[RFC5234].

   The ABNF "ALPHA", "FWS", "tag-list" and "domain-name" are defined as
   in [RFC6376].






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3.  Mandatory DKIM header tags

   The current DKIM specification defines a set of header tags, some of
   which are required to appear in every signature and some of which are
   optional.  It also allows a signer to include private tags that don't
   conflict with the registered ones.  Since verifiers ignore tags that
   they don't understand, new tags can only provide new information
   about the message, or enable new verification schemes for signatures
   that would otherwise be considered invalid.

   A Mandatory Tag is a new kind of tag prefixed with an exclamation
   point.  Its syntax is otherwise identical to an ordinary tag.

       ABNF:

       tag-spec  =/  [FWS] "!" tag-name [FWS] "=" [FWS] tag-value [FWS]

3.1.  Signature version numbers

   Any DKIM signature with a mandatory tag MUST have version "2" in the
   signature's version tag.

   All valid DKIM version 1 signatures are also valid version 2
   signatures, with "v=1" replaced by "v=2" in the DKIM-Signature
   header.  Signatures without mandatory tags SHOULD continue to use
   version "1" for backward compatibility.

3.2.  Processing mandatory tags

   When a verifier encounters a mandatory tag in a signature, it MUST
   process the tag according to the tag's definition.  If the verifier
   is unable to process the tag the verifier MUST return PERMFAIL for
   that signature.  If there are multiple signatures on a message, the
   verifier continues to verify other signatures as usual.  It is valid
   to have a mixture of version "1" and version "2" signatures on a
   single message.

3.3.  Forward signature (!fs) tag

   The "!fs" mandatory tag means that the signature is only valid if an
   additional signature is present in the message.  The value of the !fs
   tag is a domain name that is the value of the d= tag of the
   additional signature.  The condition is satisfied if the message
   includes at least one valid DKIM signature header field with
   responsible domain (the d= tag) being one specified by the !fs tag.






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   Chained !fs tags are valid and may be useful in scenarios with
   multiple levels of forwarders.  DKIM verifiers SHOULD handle at least
   three levels of !fs chaining.

4.  Typical application scenario

   A sender that expects a message to be forwarded might put both a
   conventional DKIM signature and a signature with a !fs tag that
   refers to the domain name of the expected forwarder.  At the time the
   message is forwarded, the forwarder uses the conventional signature
   to assess the message, edits the message, and then signs the outgoing
   message with its own signature.  Subsequent recipients observe both
   the forwarder's signature and the signature with the !fs tag that
   matches the other signature, and use either or both to assess the
   message.  If a message arrives with signature containing a !fs but no
   forwarding signature, the recipient would ignore that signature.
   That signature would typically be a "weak" signature that covers the
   From, To, Date, and Message-ID headers but does not cover the Subject
   header or the message body, so that it would remain valid even if the
   forwarder makes changes typical of forwarders such as mailing lists.

5.  IANA Considerations

   IANA is requested to add this entry to the "DKIM-Signature Tag
   Specifications" registry.

                    +------+-----------------+--------+
                    | TYPE | REFERENCE       | STATUS |
                    +------+-----------------+--------+
                    | !fs  | (this document) | active |
                    +------+-----------------+--------+

           Table 1: DKIM-Signature Tag Specifications additions

6.  Security Considerations

   DKIM was designed to provide assurances that a message with a valid
   signature was received in essentially the same form that it was sent.
   The forwarding signature condition deliberately creates a loophole
   for messages intended to be forwarded by entities that edit the
   message.  It opens up a variety of obvious replay attacks that may or
   may not be important depending on both the selection of target
   domains for messages to be forwarded, and the behavior of forwarders
   that receive messages with conditional signatures.

   A sender can limit the conceptual size of the loophole by being
   selective about what other domains it allows in its !fs tags, and by




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   using the x= tag to limit the time during which forwarded signatures
   are valid.

7.  Change Log

   Please remove this section before publication.

7.1.  -01 to -02

   Change tag character from @ to ! per Murray.

   Add suggestions about limiting the forwarding loophole.

8.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/
              RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [RFC6376]  Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
              "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
              RFC 6376, DOI 10.17487/RFC6376, September 2011,
              <http://www.rfc-editor.org/info/rfc6376>.

Author's Address

   John Levine
   Taughannock Networks
   PO Box 727
   Trumansburg, NY  14886

   Phone: +1 831 480 2300
   Email: standards@taugh.com
   URI:   http://jl.ly











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