Cryptographic Update to DKIM
draft-ietf-dcrup-dkim-crypto-03
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (dcrup WG) | |
|---|---|---|---|
| Author | John R. Levine | ||
| Last updated | 2017-07-07 (Latest revision 2017-07-01) | ||
| 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 | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-dcrup-dkim-crypto-03
Network Working Group J. Levine
Internet-Draft Taughannock Networks
Updates: 6376 (if approved) July 1, 2017
Intended status: Standards Track
Expires: January 2, 2018
Cryptographic Update to DKIM
draft-ietf-dcrup-dkim-crypto-03
Abstract
DKIM was designed to allow new cryptographic algorithms to be added.
This document adds a new signing algorithm and a new way to represent
signature validation keys, and deprecates an obsolete signing
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 http://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 January 2, 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
(http://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.
Levine Expires January 2, 2018 [Page 1]
Internet-Draft DKIM Crypto Update July 2017
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. EdDSA-SHA256 Signing Algorithm . . . . . . . . . . . . . . . 3
4. Public key fingerprints . . . . . . . . . . . . . . . . . . . 3
5. Signature and key syntax . . . . . . . . . . . . . . . . . . 4
5.1. Signature syntax . . . . . . . . . . . . . . . . . . . . 4
5.2. Key syntax . . . . . . . . . . . . . . . . . . . . . . . 4
6. Key and algorithm choice and strength . . . . . . . . . . . . 5
7. Transition Considerations . . . . . . . . . . . . . . . . . . 5
8. Security Considerations . . . . . . . . . . . . . . . . . . . 5
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
9.1. DKIM Signature Tag Registry . . . . . . . . . . . . . . . 5
9.2. DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 6
9.3. DKIM Key Type registry . . . . . . . . . . . . . . . . . 6
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
10.1. Normative References . . . . . . . . . . . . . . . . . . 6
10.2. Informative References . . . . . . . . . . . . . . . . . 7
10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 7
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 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
for RSA keys, with the key itself in the signature and a shorter key
fingerprint that fits in 256 octets in the DNS.
Levine Expires January 2, 2018 [Page 2]
Internet-Draft DKIM Crypto Update July 2017
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, key-k-tag-type, and base64string 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 with Ed25519, the EdDSA
algorithm using the edwards25519 curve, as defined in in RFC 8032
section 5.1 [RFC8032]. The signing algorithm is PureEdDSA as defined
in RFC 8032 section 4, since the input to the signing algorithm has
already been hashed. The DNS record for the verification public key
MUST have a "k=eddsa" tag to indicate that the key is an EdDSA rather
than RSA key.
4. Public key fingerprints
Rather than using a public key stored in the DNS, an RSA signature
MAY include the corresponding public key, with a verification
fingerprint in the DNS. For an RSA signature with a key fingerprint,
the Signing Algorithm is rsafp-sha256. 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.
Note: since Ed25516 keys are 256 bits long, a SHA-256 hash of a key
is the same size as the key itself, so there would be no benefit to
storing eddsa key fingerprints in the key record rather than the keys
themselves.
Section 5.5 of [RFC6376], on computing the message hash and
signature, is modified as follows: When creating a signature with a
signing algorithm that uses a key fingerprint, the signer includes
the public key 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.
Section 3.7 of [RFC6376], on computing the message hashes, is not
modified. Since the key in the k= tag is known in advance, it
included in the signature in the same manner as all of the other
signature fields other than b=.
Levine Expires January 2, 2018 [Page 3]
Internet-Draft DKIM Crypto Update July 2017
Section 6.1.3 of [RFC6376], to compute the verification, is modified
as follows: In item 4, if the signing algorithm uses a key
fingerprint, extract the verification key from the k= tag. If there
is no such tag, the signature does not validate. Extract the key
hash from the p= tag of the key record. If there is no such tag or
the tag is empty, the signature does not validate. Compute the
SHA-256 hash of the verification key, and compare it to the value of
the key hash. If they are not the same, the signature does not
validate. Otherwise proceed to verify the signature using the
validation key and the algorithm described in the "a=" tag.
5. Signature and key syntax
The syntax of DKIM signatures and DKIM keys are updated as follows.
5.1. Signature syntax
The syntax of DKIM algorithm tags in section 3.5 of [RFC6376] s
updated as follows, where this rule replaces the existing rule for
sig-a-tag-k:
ABNF:
sig-a-tag-k = "rsa" / "rsafp" / "eddsa"
/ x-sig-a-tag-k
The following tag is added to the list of tags on the DKIM-Signature
header field in section 3.5 of [RFC6376].
k= The public key (base64; REQUIRED). White space 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
5.2. Key syntax
The syntax of DKIM key tags in section 3.6.1 of [RFC6376] is updated
as follows, where this rule replaces the existing rule for key-k-tag-
type:
ABNF:
key-k-tag-type = "rsa" / "rsafp" / "eddsa"
/ x-key-k-tag-type
Levine Expires January 2, 2018 [Page 4]
Internet-Draft DKIM Crypto Update July 2017
6. Key and algorithm choice and strength
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-sha256 and eddsa-sha256 algorithms.
Signers that use rsa-sha256 or rsafp-sha256 signatures MUST use keys
at least 1024 bits long and SHOULD use keys 2048 bits long.
Verifiers SHOULD NOT accept rsa-sha256 or rsafp-sha256 signatures
with keys less than 1024 bits long.
7. 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.
8. 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.
DKIM signatures that use SHA-1 hashes have been deprecated since
[RFC4871] in 2007, and this document finally removes them from DKIM.
Since SHA-1 is known to be significantly weaker than SHA-256 and
there is at least one known practical SHA-1 hash collision, switching
all DKIM signatures to SHA-256 should improve DKIM's security
9. IANA Considerations
IANA is requested to update registries as follows.
9.1. DKIM Signature Tag Registry
The following value is added to the DKIM Signature Tag Registry
Levine Expires January 2, 2018 [Page 5]
Internet-Draft DKIM Crypto Update July 2017
+------+-----------------+--------+
| TYPE | REFERENCE | STATUS |
+------+-----------------+--------+
| k | (this document) | active |
+------+-----------------+--------+
Table 1: DKIM Signature Tag Registry Added Value
9.2. 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
9.3. DKIM Key Type registry
The following values are added to the DKIM Key Type Registry
+-------+-----------+--------+
| TYPE | REFERENCE | STATUS |
+-------+-----------+--------+
| rsafp | [RFC3447] | active |
| eddsa | [RFC8032] | active |
+-------+-----------+--------+
Table 3: DKIM Key Type Registry Added Values
10. References
10.1. 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>.
Levine Expires January 2, 2018 [Page 6]
Internet-Draft DKIM Crypto Update July 2017
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://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,
<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>.
10.2. Informative References
[RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, DOI 10.17487/RFC4871, May 2007,
<http://www.rfc-editor.org/info/rfc4871>.
10.3. URIs
[1] mailto:dcrup@ietf.org
Appendix A. Change log
02 to 03: Remove hashed eddsa keys. Fix typos and clarify text.
Move syntax updates to separate section. Say something insecure
about SHA-1.
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
Levine Expires January 2, 2018 [Page 7]