Internet DRAFT - draft-srose-dkim-ecc

draft-srose-dkim-ecc







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
                        draft-srose-dkim-ecc-00

Abstract

   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.

Status of This Memo

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   This Internet-Draft will expire on October 8, 2017.

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
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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Defining New ECC algorithms for Use with DKIM . . . . . . . .   3
   3.  Sender Considerations . . . . . . . . . . . . . . . . . . . .   3
   4.  Receiver Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

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.




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   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



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   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















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   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
   https://www.iana.org/assignments/dkim-parameters/dkim-
   parameters.xhtml#dkim-parameters-6

   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
   https://www.iana.org/assignments/dkim-parameters/dkim-
   parameters.xhtml#dkim-parameters-7

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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [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>.

7.2.  Informative References







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   [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., Ed., Lear, E., Ed., Draegen. Ed., T., Zwicky,
              E., Ed., and K. Andersen, Ed., "Interoperability Issues
              between Domain-based Message Authentication, Reporting,
              and Conformance (DMARC) and Indirect Email Flows",
              RFC 7960, DOI 10.17487/RFC7960, September 2016,
              <http://www.rfc-editor.org/info/rfc7960>.

   [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>.

Author's Address

   Scott Rose
   NIST
   100 Bureau Dr.
   Gaithersburg, MD  20899
   USA

   Phone: +1 301-975-8439
   Email: scott.rose@nist.gov
























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