A new cryptographic signature method for DKIM
draft-ietf-dcrup-dkim-crypto-07
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (dcrup WG) | |
|---|---|---|---|
| Author | John R. Levine | ||
| Last updated | 2017-12-18 (Latest revision 2017-12-01) | ||
| Replaces | draft-levine-dcrup-dkim-crypto | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text xml htmlized pdfized bibtex | ||
| Reviews |
GENART Last Call review
(of
-12)
Ready with Nits
SECDIR Last Call review
(of
-12)
Has Nits
|
||
| Stream | WG state | Waiting for WG Chair Go-Ahead | |
| Document shepherd | Barry Leiba | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | Barry Leiba <barryleiba@computer.org> |
draft-ietf-dcrup-dkim-crypto-07
Network Working Group J. Levine
Internet-Draft Taughannock Networks
Updates: 6376 (if approved) December 1, 2017
Intended status: Standards Track
Expires: June 4, 2018
A new cryptographic signature method for DKIM
draft-ietf-dcrup-dkim-crypto-07
Abstract
DKIM was designed to allow new cryptographic algorithms to be added.
This document adds a new signing 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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material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 4, 2018.
Copyright Notice
Copyright (c) 2017 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
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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 . . . . . . . . . . . . 3
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 . . . . . . . . . . . . . . . . . . 4
9.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
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 where it is stored in a TXT record. The defining
documents specify a single signing algorithm, RSA [RFC3447], and
recommend key sizes of 1024 to 2048 bits.
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], and signs it with the Hash variant
of Ed25519, as defined in in RFC 8032 section 5.1 [RFC8032]. The
signing algorithm is HashEdDSA. (Even though the input to the
signing algorithm has already been hashed, the PureEdDSA which does
not do an additional hash is not widely implemented, and the extra
hash causes no problems other than an insignificant slowdown.) The
DNS record for the verification public key MUST have a "k=ed25519"
tag to indicate that the key is an Ed25519 rather than RSA key.
Note: since Ed25519 keys are 256 bits long, 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"
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.
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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
9.1. Normative References
[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>.
[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>.
[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>.
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[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. URIs
[1] mailto:dcrup@ietf.org
Appendix A. Change log
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.
Author's Address
John Levine
Taughannock Networks
PO Box 727
Trumansburg, NY 14886
Phone: +1 831 480 2300
Email: standards@taugh.com
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