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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.
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.
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 20183. Ed25519-SHA256 Signing Algorithm
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.
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 20185. 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.
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.
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.
[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>.
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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.
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
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