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.

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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.

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Table of Contents

• 1. Introduction
• 2. Conventions Used in This Document
• 3. EdDSA-SHA256 Signing Algorithm
• 4. Public key fingerprints
• 5. Signature and key syntax
• 5.1. Signature syntax
• 5.2. Key syntax
• 6. Key and algorithm choice and strength
• 7. Transition Considerations
• 8. Security Considerations
• 9. IANA Considerations
• 9.1. DKIM Signature Tag Registry
• 9.2. DKIM Hash Algorithms Registry
• 9.3. DKIM Key Type registry
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Appendix A. Change log
• Author's Address

1. Introduction

Discussion Venue:

Discussion about this draft is directed to the dcrup@ietf.org mailing list.

DKIM 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, 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.

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). 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. 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=.

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

    ABNF:

    sig-a-tag-k = "rsa" / "rsafp" / "eddsa"
          / x-sig-a-tag-k

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:

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

    ABNF:

    key-k-tag-type  = "rsa" / "rsafp" / "eddsa"
          / x-key-k-tag-type

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:

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

9.2. DKIM Hash Algorithms Registry

The following value is updated in the DKIM Hash Algorithms Registry

9.3. DKIM Key Type registry

The following values are added to the DKIM Key Type Registry

10. References

10.1. Normative References

10.2. Informative References

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