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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,
 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 an RSA key.

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

 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 work even with DNS provisioning
 software that doesn't handle multistring 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.

Levine Standards Track [Page 3] RFC 8463 DKIM Crypto Update September 2018

5. Choice and Strength of Keys and Algorithms

 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.

 The example message in Appendix A has two signatures with the same d=
 and i= identifiers but different a= algorithms and s= selectors.

7. Security Considerations

 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 has updated a registry as follows.

8.1. "DKIM Key Type" Registry

 The following value has been added to the "DKIM Key Type" registry:

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

         Table 1: Value Added to the "DKIM Key Type" Registry

Levine Standards Track [Page 4] RFC 8463 DKIM Crypto Update September 2018

9. References

9.1. Normative References

 [FIPS-180-4-2015]
            National Institute of Standards and Technology, "Secure
            Hash Standard (SHS)", FIPS PUB 180-4,
            DOI 10.6028/NIST.FIPS.180-4, August 2015,
            <http://nvlpubs.nist.gov/nistpubs/FIPS/
            NIST.FIPS.180-4.pdf>.

 [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>.

 [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>.

 [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
            "PKCS #1: RSA Cryptography Specifications Version 2.2",
            RFC 8017, DOI 10.17487/RFC8017, November 2016,
            <https://www.rfc-editor.org/info/rfc8017>.

 [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>.

Levine Standards Track [Page 5] RFC 8463 DKIM Crypto Update September 2018

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.

—-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")

Levine Standards Track [Page 6] RFC 8463 DKIM Crypto Update September 2018

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 header
 fields, which start with a single space.  A blank line follows 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

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

 Phone: +883.5100.01196712
 Email: standards@taugh.com

Levine Standards Track [Page 7]