RFC 8463

RFC 8463: A New Cryptographic Signature Method for DomainKeys Identified Mail (DKIM) 💌✨

Okay bestie, here's the title block tea 🫖. This is RFC 8463, written by J. Levine of Taughannock Networks, dated September 2018. It's on the Standards Track 🔥 (ISSN 2070-1721), and it updates RFC 6376, the main DKIM Signatures spec. It doesn't obsolete anything. 🦋

Abstract ✨

This doc adds a brand new signing algorithm called Ed25519-SHA256 to "DomainKeys Identified Mail (DKIM) Signatures" (RFC 6376) 💅. DKIM verifiers are required to implement this algorithm, so it's not optional for them 🫶.

Status of This Memo 📜

This is an Internet Standards Track document 🌸.

It's a product of the Internet Engineering Task Force (IETF) and represents the consensus of the IETF community. It got public review and was approved for publication by the Internet Engineering Steering Group (IESG). If you want more on Internet Standards, check out Section 2 of RFC 7841 💖.

Info about the current status, any errata, and how to send feedback lives at https://www.rfc-editor.org/info/rfc8463 🧚.

Copyright Notice 🎀

Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved 💜.

This doc is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) as they stood on the date it was published. Please actually read those, because they spell out your rights and restrictions for this doc 🙈. Code Components pulled out of this document must include Simplified BSD License text, as described in Section 4.e of the Trust Legal Provisions, and they come without warranty, as described in the Simplified BSD License ⚠️.

Table of Contents 🗺️

1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
2.  Conventions Used in This Document . . . . . . . . . . . . . .   2
3.  Ed25519-SHA256 Signing Algorithm  . . . . . . . . . . . . . .   3
4.  Signature and Key Syntax  . . . . . . . . . . . . . . . . . .   3
  4.1.  Signature Syntax  . . . . . . . . . . . . . . . . . . . .   3
  4.2.  Key Syntax  . . . . . . . . . . . . . . . . . . . . . . .   3
5.  Choice and Strength of Keys and Algorithms  . . . . . . . . .   4
6.  Transition Considerations . . . . . . . . . . . . . . . . . .   4
7.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
  8.1.  "DKIM Key Type" Registry  . . . . . . . . . . . . . . . .   4
9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
  9.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
  9.2.  Informative References  . . . . . . . . . . . . . . . . .   5
Appendix A.  Example of a Signed Message  . . . . . . . . . . . .   6
  A.1.  Secret Keys . . . . . . . . . . . . . . . . . . . . . . .   6
  A.2.  Public Key DNS Records  . . . . . . . . . . . . . . . . .   6
  A.3.  Signed Message  . . . . . . . . . . . . . . . . . . . . .   7
Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1. Introduction 👋

So DKIM [RFC6376] signs emails by hashing the selected message header fields and the body, then signing that header hash with a digital signature 💌. Recipients grab the verification key from the DNS 🔑. The defining docs only specify one signing algorithm, RSA [RFC3447], which has since been obsoleted by [RFC8017] 🙃.

This doc adds a new, stronger signing algorithm: the Edwards-Curve Digital Signature Algorithm using the Curve25519 curve (Ed25519) 💪. It has way shorter keys than RSA for similar levels of security, which is honestly iconic ✨.

2. Conventions Used in This Document 📖

Here's the rule for the key words. When "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" show up in all capitals like that, they get read as described in BCP 14 [RFC2119] [RFC8174], and only then 🧐.

Syntax descriptions use Augmented BNF (ABNF) [RFC5234]. The ABNF tokens sig-a-tag-k and key-k-tag-type are imported from [RFC6376] 🧩.

3. Ed25519-SHA256 Signing Algorithm 🔐

The Ed25519-SHA256 signing algorithm computes a message hash as defined in Section 3 of [RFC6376], using SHA-256 [FIPS-180-4-2015] as the hash-alg 🌈. It then signs that hash with the PureEdDSA variant Ed25519, as defined in RFC 8032, Section 5.1 [RFC8032] ✨. The example keys and signatures in Appendix A are based on the test vectors in RFC 8032, Section 7.1 [RFC8032] 🍓.

The DNS record for the verification public key has a k=ed25519 tag, which tells you the key is an Ed25519 key and not an RSA one 🗝️.

This is an extra DKIM signature algorithm, added to Section 3.3 of [RFC6376] the way Section 3.3.4 of that doc envisioned 🦋.

Note: Ed25519 public keys are 256 bits long, so the base64-encoded key is only 44 octets. That means the DNS key record data will usually fit in a single 255-byte TXT string 🎀. It works even with DNS provisioning software that can't handle multistring TXT records, which is a big win 🙌.

4. Signature and Key Syntax 🧾

The syntax of DKIM signatures and DKIM keys gets updated as follows 💜.

4.1. Signature Syntax ✍️

The syntax of DKIM algorithm tags in Section 3.5 of [RFC6376] gets updated by adding this rule to the existing rule for sig-a-tag-k:

ABNF:

    sig-a-tag-k =/ "ed25519"

4.2. Key Syntax 🔑

The syntax of DKIM key tags in Section 3.6.1 of [RFC6376] gets updated by adding this rule to the existing rule for key-k-tag-type:

ABNF:

    key-k-tag-type  =/ "ed25519"

The p= value in the key record is the Ed25519 public key encoded in base64 💖. Since the key is 256 bits long, the base64 text comes out to 44 octets. For a sample key record that uses the public key from [RFC8032], Section 7.1, Test 1, check out Appendix A.2 🧚.

---

Part 1 done, bestie! 🌸 Part 2 picks up at Section 5, where the algorithm choices, the transition stuff, and the security considerations live 🔥.

A.3. Signed Message ✉️

okay so this is the finished product, bestie 💌 the actual signed email! the vibe rule here: every line starts at the very first position, EXCEPT the continuation lines on the DKIM-Signature header fields, which start with one single space 🎀 and there's a blank line right after the "Joe." line 🫶

DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed;
 d=football.example.com; i=@football.example.com;
 q=dns/txt; s=brisbane; t=1528637909; h=from : to :
 subject : date : message-id : from : subject : date;
 bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
 b=/gCrinpcQOoIfuHNQIbq4pgh9kyIK3AQUdt9OdqQehSwhEIug4D11Bus
 Fa3bT3FY5OsU7ZbnKELq+eXdp1Q1Dw==
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
 d=football.example.com; i=@football.example.com;
 q=dns/txt; s=test; t=1528637909; h=from : to : subject :
 date : message-id : from : subject : date;
 bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
 b=F45dVWDfMbQDGHJFlXUNB2HKfbCeLRyhDXgFpEL8GwpsRe0IeIixNTe3
 DhCVlUrSjV4BwcVcOF6+FF3Zo9Rpo1tFOeS9mPYQTnGdaSGsgeefOsk2Jz
 dA+L10TeYt9BgDfQNZtKdN1WO//KgIqXP7OdEFE4LjFYNcUxZQ4FADY+8=
From: Joe SixPack <joe@football.example.com>
To: Suzie Q <suzie@shopping.example.net>
Subject: Is dinner ready?
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
Message-ID: <20030712040037.46341.5F8J@football.example.com>

Hi.

We lost the game.  Are you hungry yet?

Joe.

---

Author's Address 🦋

the author is John Levine, from Taughannock Networks, and here's how to reach him 💅

John Levine
Taughannock Networks
PO Box 727
Trumansburg, NY  14886
United States of America

Phone: +883.5100.01196712
Email: standards@taugh.com