A new cryptographic signature method for DKIM
draft-ietf-dcrup-dkim-crypto-09

Abstract

This document adds a new signing algorithm to DKIM.

Status of This Memo

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

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Copyright Notice

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Table of Contents

• 1. Introduction
• 2. Conventions Used in This Document
• 3. Ed25519-SHA256 Signing Algorithm
• 4. Signature and key syntax
• 4.1. Signature syntax
• 4.2. Key syntax
• 5. Key and algorithm choice and strength
• 6. Transition Considerations
• 7. Security Considerations
• 8. IANA Considerations
• 8.1. DKIM Key Type registry
• 9. Normative References
• Appendix A. Example of a signed message
• A.1. Secret key
• A.2. Public key DNS record
• A.3. Signed Message
• Appendix B. Change log
• Author's Address

1. Introduction

Discussion Venue:

Discussion about this draft is directed to the dcrup@ietf.org mailing list.

DKIM signs e-mail messages, by creating hashes of the message headers and body and signing the header hash with a digital signature. Message recipients fetch the signature verification key from the DNS. The defining documents specify a single signing algorithm, RSA.

This document adds a new stronger signing algorithm, Edwards-Curve Digital Signature Algorithm using the Curve25519 curve (ed25519), which has much shorter keys than RSA for similar levels of security.

2. Conventions Used in This Document

The capitalized 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). 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, and signs it with the PureEDdSA variant Ed25519, as defined in in RFC 8032 section 5.1. Example keys and signatures in Appendix XX below are based on the test vectors in RFC 8032 section 7.1.

The DNS record for the verification public key has a "k=ed25519" tag to indicate that the key is an Ed25519 rather than RSA key.

This is an additional DKIM signature algorithm added to Section 3.3 of [RFC6376] as envisioned in Section 3.3.4 of [RFC6376].

Note: since Ed25519 keys are 256 bits long, the base64 encoded key is only 44 octets, so only DNS key record data will generally fit in a single 255 byte TXT string, and will work with DNS provisioning software that doesn't handle multi-string TXT records.

4. Signature and key syntax

The syntax of DKIM signatures and DKIM keys are updated as follows.

4.1. Signature syntax

The syntax of DKIM algorithm tags in section 3.5 of [RFC6376] is 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] is 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 is 44 octets long. For example, a key record using the public key in [RFC8032] Section 7.1, Test 1, mignt be:

s._domainkey.example TXT (
   "v=DKIM1; k=ed25519; p=11qYAYKxCrfVS/7TyWQHOg7hcvPapiMlrwIaaPcHURo="
   )

5. Key and algorithm choice and strength

Section 3.3 of [RFC6376] describes DKIM's hash and signature algorithms. It is updated as follows:

Signers SHOULD implement and verifiers MUST implement the ed25519-sha256 algorithm.

6. Transition Considerations

For backward compatibility, signers MAY add multiple signatures that use old and new signing algorithms. Since there can only be a single key record in the DNS for each selector, the signatures will have to use different selectors, although they can use the same d= and i= identifiers.

7. Security Considerations

Ed25519 is a widely used cryptographic technique, so the security of DKIM signatures using new signing algorithms should be at least as good as those using old algorithms.

8. IANA Considerations

IANA is requested to update registries as follows.

8.1. DKIM Key Type registry

The following value is added to the DKIM Key Type Registry

9. Normative References

Appendix A. Example of a signed message

This is a small message with an ed25519-rsa DKIM signature.

A.1. Secret key

Ed25519 secret key in base64.

fL+5V9EquCZAovKik3pA6Lk9zwCzoEtjIuIqK9ZXHHA=

A.2. Public key DNS record

test._domainkey.example.com IN TXT
             "k=ed25519; p=yi50DjK5O9pqbFpNHklsv9lqaS0ArSYu02qp1S0DW1Y="

A.3. Signed Message

The text in each line of the message start at the first position except for the five continuation lines on the DKIM-Signature which start with a single space.

DKIM-Signature: v=1; a=ed25519; c=relaxed/simple; d=example.com;
 i=@example.com; q=dns/txt; s=test; t=5; h=message-id :
 date : from : to : subject : date : from : subject;
 bh=wE7NXSkgnx9PGiavN4OZhJztvkqPDlemV3OGuEnLwNo=;
 b=wt7P+9DoBwcln1RKE3LN7069ZEEiSyVE/NH1YXnqnJy4JcrSCZUbeIEh
 vXssPHelX4yNSXG9eTGTwwk5NxYqBw==
Received: from localhost
Message-ID: <example@example.com>
Date: Mon, 01 Jan 2011 01:02:03 +0400
From: Test User <test@example.com>
To: somebody@example.com
Subject: Testing

This is a test message.

Appendix B. Change log

08 to 09

Specify sha-256 for the extremely literal minded. Take out the prehash stuff. Add example.

07 to 08

Specify base64 key records. Style edits per Dave C.

06 to 07:

Remove RSA fingerprints. Change Pure to hashed eddsa.

05 to 06:

Editorial changes only.

04 to 05:

Remove deprecation cruft and inconsistent key advice. Fix p= and k= text.

03 to 04:

Change eddsa to ed25519. Add Martin's key regeneration issue. Remove hashed ed25519 keys. Fix typos and clarify text. Move syntax updates to separate section. Take out SHA-1 stuff.

01 to 02:

Clarify EdDSA algorithm is ed25519 with Pure version of the signing. Make references to tags and fields consistent.

Author's Address