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

Document Type RFC - Proposed Standard (September 2018; No errata)
Updates RFC 6376
Last updated 2018-09-12
Replaces draft-levine-dcrup-dkim-crypto
Stream IETF
Formats plain text pdf html bibtex
Reviews GENART will not review this version
Stream WG state Submitted to IESG for Publication
Document shepherd Jim Fenton
Shepherd write-up Show (last changed 2018-05-22)
IESG IESG state RFC 8463 (Proposed Standard)
Consensus Boilerplate Yes
Telechat date
Responsible AD Alexey Melnikov
Send notices to (None)
IANA IANA review state Version Changed - Review Needed
IANA action state RFC-Ed-Ack
Internet Engineering Task Force (IETF)                         J. Levine
Request for Comments: 8463                          Taughannock Networks
Updates: 6376                                             September 2018
Category: Standards Track
ISSN: 2070-1721

                A New Cryptographic Signature Method for
                   DomainKeys Identified Mail (DKIM)

Abstract

   This document adds a new signing algorithm, Ed25519-SHA256, to
   "DomainKeys Identified Mail (DKIM) Signatures" (RFC 6376).  DKIM
   verifiers are required to implement this algorithm.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained 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 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.

Levine                       Standards Track                    [Page 1]
RFC 8463                   DKIM Crypto Update             September 2018

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

   DKIM [RFC6376] signs email messages by creating hashes of selected
   message header fields 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 [RFC3447] (which has since been
   obsoleted by [RFC8017]).

   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 key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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

Levine                       Standards Track                    [Page 2]
RFC 8463                   DKIM Crypto Update             September 2018

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 signs the hash with the PureEdDSA variant Ed25519,
   as defined in RFC 8032, Section 5.1 [RFC8032].  Example keys and
   signatures in Appendix A are based on the test vectors in RFC 8032,
Show full document text