Mandatory Tags for DKIM Signatures
draft-levine-dkim-conditional-04
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John Levine
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2020-08-30
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Network Working Group J. Levine
Internet-Draft Taughannock Networks
Intended status: Standards Track 30 August 2020
Expires: 3 March 2021
Mandatory Tags for DKIM Signatures
draft-levine-dkim-conditional-04
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
Task Force (IETF). Note that other groups may also distribute
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 3 March 2021.
Copyright Notice
Copyright (c) 2020 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 (https://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. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of 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 verification features . . . . . . . . . . . . . 3
3.2. Processing mandatory tags . . . . . . . . . . . . . . . . 4
3.3. Forward signature (!fs) tag . . . . . . . . . . . . . . . 4
4. Typical application scenarios . . . . . . . . . . . . . . . . 4
4.1. Sender use . . . . . . . . . . . . . . . . . . . . . . . 4
4.2. Forwarder use . . . . . . . . . . . . . . . . . . . . . . 5
4.3. Recipient use . . . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. -03 to -04 . . . . . . . . . . . . . . . . . . . . . . . 6
7.2. -02 to -03 . . . . . . . . . . . . . . . . . . . . . . . 7
7.3. -01 to -02 . . . . . . . . . . . . . . . . . . . . . . . 7
8. Normative References . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
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 changes the DKIM version tag to a
verification features tag to allow the new semantics of conditional
signatures.
2. Definitions
The upper case 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].
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Syntax descriptions use Augmented BNF (ABNF)[RFC5234].
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