A new cryptographic signature method for DKIM
draft-ietf-dcrup-dkim-crypto-13

Network Working Group                                          J. Levine
Internet-Draft                                      Taughannock Networks
Updates: 6376 (if approved)                                June 15, 2018
Intended status: Standards Track
Expires: December 17, 2018

             A new cryptographic signature method for DKIM
                    draft-ietf-dcrup-dkim-crypto-13

Abstract

   This document adds a new signing algorithm to DKIM, ed25519-sha256.
   DKIM verifiers are required to implement this algorithm.

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 December 17, 2018.

Copyright Notice

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

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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  . . . . . . . . . . . . . . . . . .   4
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   5
     9.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   Appendix A.  Example of a signed message  . . . . . . . . . . . .   5
     A.1.  Secret keys . . . . . . . . . . . . . . . . . . . . . . .   5
     A.2.  Public key DNS records  . . . . . . . . . . . . . . . . .   6
     A.3.  Signed Message  . . . . . . . . . . . . . . . . . . . . .   6
   Appendix B.  Change log . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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
   [RFC8174].

   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] 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 [RFC8032].  Example keys and
   signatures in Appendix A below 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
   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 public 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.  See Appendix A.2 for a sample key record using the public key
   in [RFC8032] Section 7.1, Test 1.

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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 can 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 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, All of the security
   advice in [RFC6376] continues to apply except that the security
   advice about ED25519 in Section 8 of [RFC8032] supplants the advice
   about RSA threats.

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

9.1.  Normative References

   [FIPS-180-4-2015]
              U.S. Department of Commerce, "Secure Hash Standard", FIPS
              PUB 180-4, August 2015,
              <http://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.180-4.pdf>.

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   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

   [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>.

9.3.  URIs

   [1] mailto:dcrup@ietf.org

Appendix A.  Example of a signed message

   This is a small message with both rsa-sha256 and ed25519-sha256 DKIM
   signatures.  The signatures are independent of each other, so either
   signature would be valid if the other were not present.

A.1.  Secret keys

   Ed25519 secret key in base64.  This is the secret key from [RFC8032]
   section 7.1 test 1, converted from hex to base64.

   nWGxne/9WmC6hEr0kuwsxERJxWl7MmkZcDusAxyuf2A=

   RSA secret key in PEM format.

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   -----BEGIN RSA PRIVATE KEY-----
   MIICXQIBAAKBgQDkHlOQoBTzWRiGs5V6NpP3idY6Wk08a5qhdR6wy5bdOKb2jLQi
   Y/J16JYi0Qvx/byYzCNb3W91y3FutACDfzwQ/BC/e/8uBsCR+yz1Lxj+PL6lHvqM
   KrM3rG4hstT5QjvHO9PzoxZyVYLzBfO2EeC3Ip3G+2kryOTIKT+l/K4w3QIDAQAB
   AoGAH0cxOhFZDgzXWhDhnAJDw5s4roOXN4OhjiXa8W7Y3rhX3FJqmJSPuC8N9vQm
   6SVbaLAE4SG5mLMueHlh4KXffEpuLEiNp9Ss3O4YfLiQpbRqE7Tm5SxKjvvQoZZe
   zHorimOaChRL2it47iuWxzxSiRMv4c+j70GiWdxXnxe4UoECQQDzJB/0U58W7RZy
   6enGVj2kWF732CoWFZWzi1FicudrBFoy63QwcowpoCazKtvZGMNlPWnC7x/6o8Gc
   uSe0ga2xAkEA8C7PipPm1/1fTRQvj1o/dDmZp243044ZNyxjg+/OPN0oWCbXIGxy
   WvmZbXriOWoSALJTjExEgraHEgnXssuk7QJBALl5ICsYMu6hMxO73gnfNayNgPxd
   WFV6Z7ULnKyV7HSVYF0hgYOHjeYe9gaMtiJYoo0zGN+L3AAtNP9huqkWlzECQE1a
   licIeVlo1e+qJ6Mgqr0Q7Aa7falZ448ccbSFYEPD6oFxiOl9Y9se9iYHZKKfIcst
   o7DUw1/hz2Ck4N5JrgUCQQCyKveNvjzkkd8HjYs0SwM0fPjK16//5qDZ2UiDGnOe
   uEzxBDAr518Z8VFbR41in3W4Y3yCDgQlLlcETrS+zYcL
   -----END RSA PRIVATE KEY-----

A.2.  Public key DNS records

   The public key p= value in the first record is the public key from
   [RFC8032] section 7.1 test 1, converted from hex to base64.

brisbane._domainkey.football.example.com. IN TXT (
 "v=DKIM1; k=ed25519; p=11qYAYKxCrfVS/7TyWQHOg7hcvPapiMlrwIaaPcHURo=")

test._domainkey.football.example.com. IN TXT (
 "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDkHlOQoBTzWR"
 "iGs5V6NpP3idY6Wk08a5qhdR6wy5bdOKb2jLQiY/J16JYi0Qvx/byYzCNb3W91y3FutAC"
 "DfzwQ/BC/e/8uBsCR+yz1Lxj+PL6lHvqMKrM3rG4hstT5QjvHO9PzoxZyVYLzBfO2EeC3"
 "Ip3G+2kryOTIKT+l/K4w3QIDAQAB")

A.3.  Signed Message

   The text in each line of the message starts at the first position
   except for the continuation lines on the DKIM-Signature headers which
   start with a single space.  A blank line follows the "Joe." line.

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   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.

Appendix B.  Change log

   12 to 13  Made example even less wrong.

   11 to 12  Made example less wrong.

   10 to 11  New example with both signatures, minor nits.

   09 to 10  Improve abstract, minor nits.

   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.

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   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

   John Levine
   Taughannock Networks
   PO Box 727
   Trumansburg, NY  14886

   Phone: +883.5100.01196712
   Email: standards@taugh.com

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