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Internet Engineering Task Force (IETF)                         J. LevineRequest for Comments: 8463                          Taughannock NetworksUpdates:6376                                             September 2018Category: Standards TrackISSN: 2070-1721A New Cryptographic Signature Method forDomainKeys 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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://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 toBCP 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 2018Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Conventions Used in This Document . . . . . . . . . . . . . .23.  Ed25519-SHA256 Signing Algorithm  . . . . . . . . . . . . . .34.  Signature and Key Syntax  . . . . . . . . . . . . . . . . . .34.1.  Signature Syntax  . . . . . . . . . . . . . . . . . . . .34.2.  Key Syntax  . . . . . . . . . . . . . . . . . . . . . . .35.  Choice and Strength of Keys and Algorithms  . . . . . . . . .46.  Transition Considerations . . . . . . . . . . . . . . . . . .47.  Security Considerations . . . . . . . . . . . . . . . . . . .48.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .48.1.  "DKIM Key Type" Registry  . . . . . . . . . . . . . . . .49.  References  . . . . . . . . . . . . . . . . . . . . . . . . .59.1.  Normative References  . . . . . . . . . . . . . . . . . .59.2.  Informative References  . . . . . . . . . . . . . . . . .5Appendix A.  Example of a Signed Message  . . . . . . . . . . . .6A.1.  Secret Keys . . . . . . . . . . . . . . . . . . . . . . .6A.2.  Public Key DNS Records  . . . . . . . . . . . . . . . . .6A.3.  Signed Message  . . . . . . . . . . . . . . . . . . . . .7   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .71.  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 inBCP 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 20183.  Ed25519-SHA256 Signing Algorithm   The Ed25519-SHA256 signing algorithm computes a message hash as   defined inSection 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 inRFC 8032, Section 5.1 [RFC8032].  Example keys and   signatures inAppendix A are based on the test vectors inRFC 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 toSection 3.3   of [RFC6376] as envisioned inSection 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 inSection 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 inSection 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.  SeeAppendix 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 20185.  Choice and Strength of Keys and AlgorithmsSection 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 inAppendix 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 inSection 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" RegistryLevine                       Standards Track                    [Page 4]

RFC 8463                   DKIM Crypto Update             September 20189.  References9.1.  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 inRFC2119 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 2018Appendix 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 2018A.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.comLevine                       Standards Track                    [Page 7]