A new cryptographic signature method for DKIM
draft-ietf-dcrup-dkim-crypto-12
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8463.
|
|
|---|---|---|---|
| Author | John R. Levine | ||
| Last updated | 2018-06-12 (Latest revision 2018-06-06) | ||
| Replaces | draft-levine-dcrup-dkim-crypto | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Last Call review
by Pete Resnick
Ready w/nits
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Jim Fenton | ||
| Shepherd write-up | Show Last changed 2018-05-22 | ||
| IESG | IESG state | Became RFC 8463 (Proposed Standard) | |
| Consensus boilerplate | Yes | ||
| Telechat date |
(None)
Needs a YES. Needs 9 more YES or NO OBJECTION positions to pass. |
||
| Responsible AD | Alexey Melnikov | ||
| Send notices to | (None) | ||
| IANA | IANA review state | IANA OK - Actions Needed |
draft-ietf-dcrup-dkim-crypto-12
Network Working Group J. Levine
Internet-Draft Taughannock Networks
Updates: 6376 (if approved) June 6, 2018
Intended status: Standards Track
Expires: December 8, 2018
A new cryptographic signature method for DKIM
draft-ietf-dcrup-dkim-crypto-12
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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 8, 2018.
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.
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 5
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 keys . . . . . . . . . . . . . . . . . . . . . . . 6
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
[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 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, so the security of
DKIM signatures using new signing algorithms should be at least as
good as those using old algorithms.
All of the security advice in [RFC6376] continues to apply except
that the advice 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
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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>.
[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 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.
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A.1. Secret keys
Ed25519 secret key in base64.
fL+5V9EquCZAovKik3pA6Lk9zwCzoEtjIuIqK9ZXHHA=
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
brisbane._domainkey.football.example.com. IN TXT (
"v=DKIM1; k=ed25519; p=yi50DjK5O9pqbFpNHklsv9lqaS0ArSYu02qp1S0DW1Y=")
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.
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DKIM-Signature: v=1; a=ed25519-sha256; c=simple/simple;
d=football.example.com; i=@football.example.com;
q=dns/txt; s=brisbane; t=1518460054; h=from : to :
subject : date : message-id : from : subject : date;
bh=4bLNXImK9drULnmePzZNEBleUanJCX5PIsDIFoH4KTQ=;
b=9/dsDChY0YMTtD5Eyw3wx7x22BlSJP7M5ECbJ7GWrR45nXlTCGb8l0YB
o0wBLR++X5LqmsxXaOYLLJe46l10AQ==
DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple;
d=football.example.com; i=@football.example.com;
q=dns/txt; s=test; t=1527915362; h=from : to : subject :
date : message-id : from : subject : date;
bh=4bLNXImK9drULnmePzZNEBleUanJCX5PIsDIFoH4KTQ=;
b=icKcLSEZYXJ95flvWE8FT6hl5iqd8MC/LEKYH0QjsqYy6MO/4pgVNCZH
l/RAXAuADxE/40Fg7uTlxwwD1hjN2Ple6J//cJfslBdDOq6zTVbne1dqtl
NOat7iamJ1AfRqyG+ja7a2AZsrpUuJ7VA6O+0zRYPqpwMEkEFIzI9i/Xk=
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
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.
04 to 05: Remove deprecation cruft and inconsistent key advice. Fix
p= and k= text.
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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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