Network Working Group J. Levine
Internet-Draft Taughannock Networks
Updates: 6376 (if approved) April 28, 2017
Intended status: Standards Track
Expires: October 30, 2017

Cryptographic Update to DKIM


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.

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Table of Contents

1. Introduction

Discussion Venue:
Discussion about this draft is directed to the 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].

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.


    sig-a-tag-k = "rsa" / "rsafp" / "ecdh" / "ecdhfp" x-sig-a-tag-k

The syntax of DKIM signature tags is updated as follows:


    key-k-tag-type  = "rsa" / "rsafp" / "ecdh" / "ecdhfp"
          / x-key-k-tag-type

The syntax of DKIM key tags is updated as follows:

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.

The public key (base64; REQUIRED). Whitespace is ignored in this value and MUST be ignored when reassembling the original key.


      sig-k-tag       = %x6b [FWS] "=" [FWS] sig-k-tag-data
      sig-k-tag-data  = base64string

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 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

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.

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

DKIM Signature Tag Registry Added Value
k (this document) active

8.2. DKIM Key Type registry

The following values is added to the DKIM Key Type Registry

DKIM Key Type Registry Added Values
rsafp [RFC3447] active
ecdh [RFC7748] active
ecdhfp [RFC7748] active

9. 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.
[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.
[RFC6376] Crocker, D., Hansen, T. and M. Kucherawy, "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, September 2011.
[RFC7748] Langley, A., Hamburg, M. and S. Turner, "Elliptic Curves for Security", RFC 7748, DOI 10.17487/RFC7748, January 2016.

Author's Address

John Levine Taughannock Networks PO Box 727 Trumansburg, NY 14886 Phone: +1 831 480 2300 EMail: