DKIM Ed25519: A New Cryptographic Signature Method for DomainKeys Identified Mail (DKIM)
Adds Ed25519-SHA256 to DKIM: a signature scheme with much shorter keys than RSA, which every verifier must support.
Levine, J., "A New Cryptographic Signature Method for DomainKeys Identified Mail (DKIM)", RFC 8463, DOI 10.17487/RFC8463, September 2018, <https://www.rfc-editor.org/info/rfc8463>.
@misc{rfc8463,
series = {Request for Comments},
number = 8463,
howpublished = {RFC 8463},
publisher = {RFC Editor},
doi = {10.17487/RFC8463},
url = {https://www.rfc-editor.org/info/rfc8463},
author = {J. Levine},
title = {{A New Cryptographic Signature Method for DomainKeys Identified Mail (DKIM)}},
pagetotal = 7,
year = 2018,
month = sep,
} RFC text
Contents
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.
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. Choice and Strength of Keys and Algorithms . . . . . . . . . 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
Appendix A. Example of a Signed Message . . . . . . . . . . . . 6
A.1. Secret Keys . . . . . . . . . . . . . . . . . . . . . . . 6
A.2. Public Key DNS Records . . . . . . . . . . . . . . . . . 6
A.3. Signed Message . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. 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 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].
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. It signs the hash with the PureEdDSA variant Ed25519,
as defined in RFC 8032, Section 5.1 [RFC8032]. Example keys and
signatures in Appendix A 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 an RSA key.
This is an additional DKIM signature algorithm added to Section 3.3
of [RFC6376] as envisioned in Section 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 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. See Appendix A.2 for a sample key record using the public key
in [RFC8032], Section 7.1, Test 1.
5. 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.
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 in Appendix 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 in Section 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" Registry
9. References
9.1. 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 in RFC
2119 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>.
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.
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-----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-----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")
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 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.com
AI ELI5
On this page
§1Why DKIM Needed a Second Algorithm
DKIM (RFC 6376) signs email by hashing selected header fields and the body, then signing the header hash with a digital signature. Recipients fetch the key needed to check the signature from the DNS.
The original DKIM documents specify a single signing algorithm, RSA. This document adds a second, stronger one: the Edwards-Curve Digital Signature Algorithm using the Curve25519 curve, called Ed25519. The RFC notes that it has much shorter keys than RSA for similar levels of security.
DKIM is a seal on a letter that shows who wrote it. There was only one kind of seal, and it was big. Now there is a second, smaller kind.
§3The Ed25519-SHA256 Algorithm
The new algorithm computes the message hash exactly as Section 3 of RFC 6376 says, using SHA-256 as the hash. It then signs that hash with PureEdDSA, specifically the Ed25519 variant defined in Section 5.1 of RFC 8032.
The DNS record that holds the public key carries the tag k=ed25519, which tells a verifier that the key is an Ed25519 key and not an RSA key. This is the additional signature algorithm that Section 3.3.4 of RFC 6376 had anticipated.
A practical benefit: an Ed25519 public key is 256 bits long, so its base64 form is only 44 octets. The key record will generally fit in a single 255-byte TXT string and work even with DNS provisioning software that does not handle multistring TXT records.
The message is boiled down to a short fingerprint, and the fingerprint is signed with the new pen. A note in the phone book says which kind of pen was used.
§4Signature and Key Syntax
Two small grammar changes make the new algorithm legal in DKIM. In the signature, the algorithm tag (a=) may now be ed25519: the RFC adds sig-a-tag-k =/ "ed25519" to the ABNF in Section 3.5 of RFC 6376.
In the key record, the key type tag (k=) may now be ed25519, by adding key-k-tag-type =/ "ed25519" to Section 3.6.1 of RFC 6376. The p= value is then the Ed25519 public key encoded in base64, which is 44 octets of text.
The rule book for writing signatures gets one new word it is allowed to use, and so does the rule book for writing keys.
§5Who Must Implement It
This RFC updates Section 3.3 of RFC 6376 with a single requirement: signers SHOULD implement, and verifiers MUST implement, the Ed25519-SHA256 algorithm.
The asymmetry is deliberate. A verifier has to be able to check any signature it meets, while a signer is only recommended to start producing the new kind.
Everyone who checks the seals must learn to read the new one. People who make seals are only encouraged to use it.
§6Transition: Signing Twice
For backward compatibility, a signer can add several signatures to one message, some using the old algorithm and some the new one. There can be only a single key record in the DNS for each selector, so the two signatures have to use different selectors, although they can use the same d= and i= identifiers.
The example message in the appendix does exactly this: two signatures with the same d= and i= but different a= algorithms and s= selectors.
While people switch pens, you can sign the letter twice, once with each pen. Each pen needs its own name tag so the phone book can tell them apart.
| Signature | a= algorithm | s= selector | k= in key record |
|---|---|---|---|
| First THIS RFC | ed25519-sha256 | brisbane | ed25519 |
| Second | rsa-sha256 | test | rsa |
§7Security and IANA
All the security advice in RFC 6376 continues to apply. The one exception is that the advice about Ed25519 in Section 8 of RFC 8032 replaces the advice about RSA threats.
IANA added one value to the "DKIM Key Type" registry: type ed25519, referencing RFC 8032, with status active.
The old safety rules still count, except that the worries about the old pen are replaced by the worries about the new pen. The new pen's name is written in the official list of pens.
§7A Signed Example Message
The appendix shows a small message from football.example.com carrying both an RSA-SHA256 and an Ed25519-SHA256 signature. The two signatures are independent, so either would be valid if the other were absent. The Ed25519 keys are the test keys from Section 7.1, Test 1, of RFC 8032, converted from hex to base64.
The Ed25519 key record lives at brisbane._domainkey.football.example.com and the RSA one at test._domainkey.football.example.com, so the two selectors are brisbane and test.
The appendix shows a real-looking letter with two seals on it, each with its own name tag, so you can see how it is done.