Cryptographic Update to DKIM
draft-ietf-dcrup-dkim-crypto-01
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (dcrup WG) | |
|---|---|---|---|
| Author | John R. Levine | ||
| Last updated | 2017-06-12 | ||
| Stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-dcrup-dkim-crypto-01
Network Working Group J. Levine
Internet-Draft Taughannock Networks
Updates: 6376 (if approved) June 11, 2017
Intended status: Standards Track
Expires: December 13, 2017
Cryptographic Update to DKIM
draft-ietf-dcrup-dkim-crypto-01
Abstract
DKIM was designed to allow new cryptographic algorithms to be added.
This document adds a new algorithm and a new way to represent
signature validation keys, and deprecates obsolete signing
algorithms.
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 13, 2017.
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
(http://trustee.ietf.org/license-info) in effect on the date of
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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. EdDSA-SHA256 Signing Algorithm . . . . . . . . . . . . . . . 3
4. Public key fingerprints . . . . . . . . . . . . . . . . . . . 3
5. Key and algorithm choice and strength . . . . . . . . . . . . 4
6. Transition Considerations . . . . . . . . . . . . . . . . . . 4
7. Security Considerations . . . . . . . . . . . . . . . . . . . 4
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
8.1. DKIM Signature Tag Registry . . . . . . . . . . . . . . . 4
8.2. DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 5
8.3. DKIM Key Type registry . . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
9.1. Normative References . . . . . . . . . . . . . . . . . . 5
9.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
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 content 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 recommends key sizes of 1024 to 2048 bits. While 1024
bit signatures are common, stronger signatures are not. Widely used
DNS configuration software places a practical limit on key sizes,
because the software only handles a single 256 octet string in a TXT
record, and RSA keys longer than 1156 bits don't fit in 256 octets.
This document adds a new signing algorithm, Edwards-Curve Digital
Signature Algorithm (EdDSA), which has much shorter keys than RSA for
similar levels of security. It also adds a new key representation,
with the key itself in the signature and a shorter key fingerprint in
the DNS.
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].
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Syntax descriptions use Augmented BNF (ABNF) [RFC5234]. The ABNF
tokens sig-a-tag-k, key-k-tag-type, and base64tring are imported from
[RFC6376].
3. EdDSA-SHA256 Signing Algorithm
The eddsa-sha256 signing algorithm computes a message hash as defined
in section 3 of [RFC6376], and signs it using the EdDSA algorithm
defined in [RFC8032]. The DNS record for the verification public key
MUST have a "k=ecdh" or "k=ecdhfp" tag to indicate that the key is an
ECDH rather than RSA key.
The syntax of DKIM signature tags is updated as follows:
ABNF:
sig-a-tag-k = "rsa" / "rsafp" / "eddsa" / "eddsafp" x-sig-a-tag-k
The syntax of DKIM key tags is updated as follows:
ABNF:
key-k-tag-type = "rsa" / "rsafp" / "eddsa" / "eddsafp"
/ x-key-k-tag-type
4. Public key fingerprints
Rather than using a public key stored in the DNS, a signature MAY
include the corresponding public key, with a fingerprint in the DNS.
For an RSA signature, the Signing Algorithm is rsafp-sha256, or for
an EdDSA signature the Signing Algorithm is eddsafp-sha256. The
corresponding public key is included in the signature as a base64
encoded string with a k= tag. The key in the tag is the same one
that would be published in a non-fingerprint key record.
The DNS record contains a sha-256 hash of the public key, stored in
base64 in the p= tag. The key type tag MUST be present and contains
k=rsafp or k=ecdhfp.
k= The public key (base64; REQUIRED). Whitespace is ignored in
this value and MUST be ignored when reassembling the original
key.
ABNF:
sig-k-tag = %x6b [FWS] "=" [FWS] sig-k-tag-data
sig-k-tag-data = base64string
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5. Key and algorithm choice and strength
[[ splice in text from Scott K's draft to taste here ]]
Section 3.3 of [RFC6376] describes DKIM's hash and signature
algorithms. It is updated as follows:
Signers MUST NOT implement and verifiers SHOULD NOT implement the
rsa-sha1 algorithm. Signers SHOULD implement and verifiers MUST
implement the rsafp-256, ecdh-sha256, and ecdhfp-sha256 algorithms.
Signers that use rsa-sha256 signatures MUST use keys at least 1024
bits long and SHOULD use keys 2048 bits long. Verifiers MUST NOT
accept rsa-sha256 signatures with keys less than 1024 bits long.
[[ is there any reason to allow or require RSA keys longer than 2048
? ]]
6. Transition Considerations
For backward compatibility, signers MAY add multiple signatures that
use old and new signing algorithms or key representations. 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
EdDSA and key fingerprints are widely used cryptographic techniques,
so the security of DKIM signatures using new signing algorithms
should be at least as good as those using old algorithms. Since key
fingerprints make it possible to publish verification records for RSA
keys of any length, rsafp signatures SHOULD use key lengths of 1536
or 2048 bits.
8. IANA Considerations
IANA is requested to update registries as follows.
8.1. DKIM Signature Tag Registry
The following value is added to the DKIM Signature Tag Registry
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+------+-----------------+--------+
| TYPE | REFERENCE | STATUS |
+------+-----------------+--------+
| k | (this document) | active |
+------+-----------------+--------+
Table 1: DKIM Signature Tag Registry Added Value
8.2. DKIM Hash Algorithms Registry
The following value is updated in the DKIM Hash Algorithms Registry
+------+-------------------+----------+
| TYPE | REFERENCE | STATUS |
+------+-------------------+----------+
| sha1 | [FIPS-180-3-2008] | historic |
+------+-------------------+----------+
Table 2: DKIM Hash Algorithms Registry Updated Value
8.3. DKIM Key Type registry
The following values is added to the DKIM Key Type Registry
+---------+-----------+--------+
| TYPE | REFERENCE | STATUS |
+---------+-----------+--------+
| rsafp | [RFC3447] | active |
| eddsa | [RFC8032] | active |
| eddsafp | [RFC8032] | active |
+---------+-----------+--------+
Table 3: 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,
<http://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, <http://www.rfc-editor.org/info/rfc3447>.
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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,
<http://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,
<http://www.rfc-editor.org/info/rfc8032>.
9.2. URIs
[1] mailto:dcrup@ietf.org
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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