A new cryptographic signature method for DKIM
draft-ietf-dcrup-dkim-crypto-10
Network Working Group J. Levine
Internet-Draft Taughannock Networks
Updates: 6376 (if approved) May 29, 2018
Intended status: Standards Track
Expires: November 30, 2018
A new cryptographic signature method for DKIM
draft-ietf-dcrup-dkim-crypto-10
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 November 30, 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. Informative References . . . . . . . . . . . . . . . . . 5
9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Appendix A. Example of a signed message . . . . . . . . . . . . 5
A.1. Secret key . . . . . . . . . . . . . . . . . . . . . . . 5
A.2. Public key DNS record . . . . . . . . . . . . . . . . . . 6
A.3. Signed Message . . . . . . . . . . . . . . . . . . . . . 6
Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
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] 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 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
[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>.
[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>.
[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. 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 an ed25519-rsa DKIM signature.
A.1. Secret key
Ed25519 secret key in base64.
fL+5V9EquCZAovKik3pA6Lk9zwCzoEtjIuIqK9ZXHHA=
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A.2. Public key DNS record
test._domainkey.example.com IN TXT
"k=ed25519; p=yi50DjK5O9pqbFpNHklsv9lqaS0ArSYu02qp1S0DW1Y="
A.3. Signed Message
The text in each line of the message start at the first position
except for the five continuation lines on the DKIM-Signature which
start with a single space.
DKIM-Signature: v=1; a=ed25519; c=relaxed/simple; d=example.com;
i=@example.com; q=dns/txt; s=test; t=5; h=message-id :
date : from : to : subject : date : from : subject;
bh=wE7NXSkgnx9PGiavN4OZhJztvkqPDlemV3OGuEnLwNo=;
b=wt7P+9DoBwcln1RKE3LN7069ZEEiSyVE/NH1YXnqnJy4JcrSCZUbeIEh
vXssPHelX4yNSXG9eTGTwwk5NxYqBw==
Received: from localhost
Message-ID: <example@example.com>
Date: Mon, 01 Jan 2011 01:02:03 +0400
From: Test User <test@example.com>
To: somebody@example.com
Subject: Testing
This is a test message.
Appendix B. Change log
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.
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: +883.5100.01196712
Email: standards@taugh.com
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