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


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.

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Table of Contents

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].

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.


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

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

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.

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.

DKIM-Signature Tag Specifications additions
!fs (this document) active

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 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.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008.
[RFC6376] Crocker, D., Hansen, T. and M. Kucherawy, "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, September 2011.

Author's Address

John Levine Taughannock Networks PO Box 727 Trumansburg, NY 14886 Phone: +1 831 480 2300 EMail: URI: