A new cryptographic signature method for DKIM
draft-ietf-dcrup-dkim-crypto-08

Network Working Group                                          J. Levine
Internet-Draft                                      Taughannock Networks
Updates: 6376 (if approved)                             January 22, 2018
Intended status: Standards Track
Expires: July 26, 2018

             A new cryptographic signature method for DKIM
                    draft-ietf-dcrup-dkim-crypto-08

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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   This Internet-Draft will expire on July 26, 2018.

Copyright Notice

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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.  Key and algorithm choice and strength . . . . . . . . . . . .   4
   6.  Transition Considerations . . . . . . . . . . . . . . . . . .   4
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
     8.1.  DKIM Key Type registry  . . . . . . . . . . . . . . . . .   4
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     9.2.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   Appendix A.  Change log . . . . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   Discussion Venue:    Discussion about this draft is directed to the
      dcrup@ietf.org [1] mailing list.

   DKIM [RFC6376] 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 [RFC3447].

   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) [RFC5234].  The ABNF
   tokens sig-a-tag-k and key-k-tag-type are imported from [RFC6376].

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3.  Ed25519-SHA256 Signing Algorithm

   The ed25519-sha256 signing algorithm computes a message hash as
   defined in section 3 of [RFC6376], and signs it with the Hash variant
   of Ed25519, as defined in in RFC 8032 section 5.1 [RFC8032].  The
   signing algorithm is HashEdDSA.

   Even though the input to the signing algorithm has already been
   hashed, the PureEdDSA version of the algorithm is not widely
   implemented, and the extra hash causes no problems beyond a minor
   computational slowdown.

   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.

   Note: since Ed25519 keys are 256 bits long, the base64 encoded key is
   only 44 octets, so DNS key record data will generally fit in a single
   255 byte TXT string, and will work even 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:

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 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

                     +---------+-----------+--------+
                     |   TYPE  | REFERENCE | STATUS |
                     +---------+-----------+--------+
                     | ed25519 | [RFC8032] | active |
                     +---------+-----------+--------+

               Table 1: DKIM Key Type Registry Added Values

9.  References

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9.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February
              2003, <https://www.rfc-editor.org/info/rfc3447>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC6376]  Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
              "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
              RFC 6376, DOI 10.17487/RFC6376, September 2011,
              <https://www.rfc-editor.org/info/rfc6376>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

9.2.  URIs

   [1] mailto:dcrup@ietf.org

Appendix A.  Change log

   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.

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Author's Address

   John Levine
   Taughannock Networks
   PO Box 727
   Trumansburg, NY  14886

   Phone: +1 831 480 2300
   Email: standards@taugh.com

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