Internet Engineering Task Force S. Rose
Internet-Draft NIST
Updates: 6376 (if approved) April 6, 2017
Intended status: Standards Track
Expires: October 8, 2017

Defining Elliptic Curve Cryptography Algorithms for use with DKIM


DomainKeys Identified Mail (DKIM) uses digital signature to associate a message with a given sending domain. Currently, there is only one cryptography algorithm defined for use with DKIM (RSA). This document defines four new elliptic curve cryptography algorithms for use with DKIM. This will allow for algorithm agility if a weakness is found in RSA, and allows for smaller key length to provide the same digital signature strength.

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Table of Contents

1. Introduction

DomainKeys Identified Mail (DKIM)[RFC6376] uses digital signatures to associate a sending domain with a given message. Each DKIM signed email message as a digital signature in its header, that can be validated by a receiver by obtaining the appropriate public key stored in the DNS. Currently, DKIM has only one cryptographic algorithm defined for use (RSA) and two digital signature algorithms (RSA/SHA-1 and RSA/SHA-256). In the past, 1024-bit RSA keys were common, equating to (roughly) a security key strength of 80 bits [NIST.800-57.2016]. Today, a minimum of 112 bits is recommended, which equates to 2048 bit RSA keys.

The public portion of 2048 bit RSA keys are still small enough to fit into a DNS TXT RR without issues in performance. The encoded public key is too large to fit into the maximum allowed characters in a string, but a DNS TXT RR allows for multiple strings, so the key can be broken into "chunks" to allow it to be served. Elliptic Curve Cryptography (ECC) has shown to have the same (roughly) equivalent key strength with smaller sizes. A 224 to 255 bit ECDSA key has (roughly) the same key strength as a 2048 bit RSA key (112 bits of strength). This means smaller keys can be used to achieve the same DKIM security strength, as well as being easier to manage in the DNS.

Having additional digital signature algorithms defined for use with DKIM also permits algorithm agility. If a weakness is discovered in one digital signature algorithm, email senders can quickly migrate to another algorithm without waiting for a standards action and subsequent software update.

This document defines two new algorithms for DKIM: ECDSA and Edwards-Curve Digital Signature Algorithm (EdDSA) Ed25519 [RFC8032]. This document also defines a new hash algorithm (sha512) as well. This document updates the IANA registry with new values for the algorithms. This document does not change the DKIM key or signature formats, but only defines new algorithm values using those formats.

1.1. Requirements Language

The 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 RFC 2119 [RFC2119].

2. Defining New ECC algorithms for Use with DKIM

This document defines new digital signature algorithms for use with DKIM. They are ECDSA with P-256 and EdDSA 25519 curve (Ed25519).

          algorithm  |  mnemonic
         ECDSA P-256 |  eccp256
         ECDSA P-348 |  eccp348
         EdDSA 255519|  ed25519
         EdDSA 448   |  ed448

NOTE: Any other algorithms? Or is this too many? Right now, it is two FIPS approved algorithms and two non-NIST curves (for those that want those).

The SHA-512 hash algorithm is also now defined for use with DKIM using the mnemonic 'sha512' for the "h=" DKIM key tag and "a=" sig-a-tag-h DKIM signature tag.

All of these newly defined algorithms MUST be used with either SHA-256 or SHA-512 as the hash algorithm. The SHA-1 hash algorithm MUST NOT be used. The previously defined algorithm (RSA) MAY be used with the SHA-512 hash algorithm.

NOTE: Any other hash algorithms? SHA-3?

3. Sender Considerations

New algorithms for an established protocols take some time to gain wide deployment. There will be a period of time where new algorithms are in operation side by side with older algorithms. There will also be a sizable percentage of DKIM validators that will not understand new algorithms until they are upgraded. This will lead to a period of time where multiple DKIM signature algorithms are in use for a sender. Email administrators MAY want to also sign with RSA/SHA-1 or RSA/SHA-256 for a period of time. This period of time is difficult to measure, but DMARC [RFC7960] aggregate reports could provide a view on DKIM validation rates by receivers.

4. Receiver Considerations

These requirements are for DKIM verifiers (as defined it [RFC6376]). These entities would be the consumers of any end-to-end email security policy and would be the entity responsible for validating DKIM signatures.

DKIM verifiers claiming conformance to this document MUST implement all of the above cryptographic algorithms and SHOULD implement the SHA-512 hash algorithm.

This document does NOT change the behavior of the core DKIM specification in that verifiers MUST ignore unknown algorithms in DKIM signatures.

5. Security Considerations

This document defines the use of new elliptic curve cryptographic algorithms for use with DomainKey Identified Mail (DKIM). This document is not a discussion of the relative strengths or weaknesses of these algorithms, but only defines their use.

There is a risk for mail receivers that do not understand or implement the new algorithms. Attackers could modify or spoof messages from sending zones using one of the newly defined algorithms and it would not be detectable as an attack by ECC-ignorant receivers. Likewise, ECC-ignorant receivers may mark valid DKIM signed email messages as invalid due to unknown algorithms.

6. IANA Considerations

This draft defines the use of new algorithms for DKIM. This draft updates the "DKIM Key Tag" registry to include the following new values:

          algorithm  |  mnemonic  |  Reference
         ECDSA P-256 |  eccp256   | This document
         ECDSA P-348 |  eccp348   | This document
         EdDSA 255519|  ed25519   | This document
         EdDSA 448   |  ed448     | This document

The current DKIM Key Tag registry is located at

This draft also defines a new hash algorithm for use with DKIM. This draft updates the "DKIM Hash Algorithms" registry to include the following new entry:

          algorithm  |  mnemonic  |  Reference
          SHA-512    |  sha512    | This document

The current DKIM Hash Algorithm registry is located at

7. References

7.1. 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.
[RFC6376] Crocker, D., Hansen, T. and M. Kucherawy, "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, September 2011.

7.2. Informative References

[NIST.800-57.2016] National Institute of Standards and Technology, "Recommendations for Key Management Part 1: General", NIST 800-57, January 2016.
[RFC7960] Martin, F., Lear, E., Draegen. Ed., T., Zwicky, E. and K. Andersen, "Interoperability Issues between Domain-based Message Authentication, Reporting, and Conformance (DMARC) and Indirect Email Flows", RFC 7960, DOI 10.17487/RFC7960, September 2016.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital Signature Algorithm (EdDSA)", RFC 8032, DOI 10.17487/RFC8032, January 2017.

Author's Address

Scott Rose NIST 100 Bureau Dr. Gaithersburg, MD 20899 USA Phone: +1 301-975-8439 EMail: