Internet DRAFT - draft-levine-dcrup-dkim-crypto
draft-levine-dcrup-dkim-crypto
Network Working Group J. Levine
Internet-Draft Taughannock Networks
Intended status: Standards Track March 29, 2017
Expires: September 30, 2017
Cryptographic Update to DKIM
draft-levine-dcrup-dkim-crypto-00
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.
Status of This Memo
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This Internet-Draft will expire on September 30, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 2
3. ECDH-SHA256 Signing Algorithm . . . . . . . . . . . . . . . . 3
4. Public key fingerprints . . . . . . . . . . . . . . . . . . . 3
5. Transition Considerations . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 4
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
7.1. DKIM Signature Tag Registry . . . . . . . . . . . . . . . 4
7.2. DKIM Key Type registry . . . . . . . . . . . . . . . . . 4
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 5
8.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 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 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 1024 bits don't fit in 256 octets.
This document adds a new signing algorithm, Elliptic Curve Diffie-
Hellman (ECDH), 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].
Syntax descriptions use Augmented BNF (ABNF) [RFC5234]. The ABNF
tokens sig-a-tag-k, key-k-tag-type, and base64tring are imported from
[RFC6376].
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3. ECDH-SHA256 Signing Algorithm
The ecdh-sha256 signing algorithm computes a message hash as defined
in section 3 of [RFC6376], and signs it using the ECDH algorithm
defined in [RFC7748]. 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.
ECDH signatures MUST NOT use SHA-1 hashes.
The syntax of DKIM signature tags is updated as follows:
ABNF:
sig-a-tag-k = "rsa" / "rsafp" / "ecdh" / "ecdhfp" x-sig-a-tag-k
The syntax of DKIM key tags is updated as follows:
ABNF:
key-k-tag-type = "rsa" / "rsafp" / "ecdh" / "ecdhfp"
/ 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 ECDH signature the Signing Algorithm is ecdhfp-sha256. The
corresponding public key is included in the signature as a k= tag.
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. 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.
6. Security Considerations
ECDH 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.
7. IANA Considerations
IANA is requested to update registries as follows.
7.1. DKIM Signature Tag Registry
The following value is added to the DKIM Signature Tag Registry
+------+-----------------+--------+
| TYPE | REFERENCE | STATUS |
+------+-----------------+--------+
| k | (this document) | active |
+------+-----------------+--------+
Table 1: DKIM Signature Tag Registry Added Value
7.2. DKIM Key Type registry
The following values is added to the DKIM Key Type Registry
+--------+-----------+--------+
| TYPE | REFERENCE | STATUS |
+--------+-----------+--------+
| rsafp | [RFC3447] | active |
| ecdh | [RFC7748] | active |
| ecdhfp | [RFC7748] | active |
+--------+-----------+--------+
Table 2: DKIM Key Type Registry Added Values
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8. References
8.1. 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>.
[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>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <http://www.rfc-editor.org/info/rfc7748>.
8.2. URIs
[1] mailto:we-need-a-list
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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