DCRUP                                                            S. Rose
Internet-Draft                                                      NIST
Updates: 6376 (if approved)                                June 21, 2017
Intended status: Standards Track
Expires: December 23, 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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 23, 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must

Rose                    Expires December 23, 2017               [Page 1]

Internet-Draft                  dkim-ecc                       June 2017

   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.  Changes to ABNF Definitions of DKIM Keys and Signatures . . .   3
     3.1.  Changes to DKIM Key Definition  . . . . . . . . . . . . .   3
     3.2.  Changes to DKIM Signature Definition  . . . . . . . . . .   4
   4.  Sender Considerations . . . . . . . . . . . . . . . . . . . .   5
   5.  Receiver Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

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
   single string, but a DNS TXT RR allows for multiple strings, so the
   key can be broken into "chunks" to allow it to be served.  However,
   some code components may not correctly handle TXT RRs with multiple
   strings which will result in errors in validation.

   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.

Rose                    Expires December 23, 2017               [Page 2]

Internet-Draft                  dkim-ecc                       June 2017

   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 a ECDSA as a new algorithms for DKIM.  This
   document also defines a new hash algorithm for use with DKIM
   signatures.  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

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Defining New ECC algorithms for Use with DKIM

   This document defines a new digital signature algorithm for use with

             algorithm  |  mnemonic
            ECDSA P-256 |  ecdsa256

   For ECDSA, the SHA-1 hash algorithm MUST NOT be used.

3.  Changes to ABNF Definitions of DKIM Keys and Signatures

   The original definition of DKIM signatues and keys are defined in
   [RFC6376].  The following are changes to the definition to include
   the new digital signature algorithm and secure hash algorithm.

3.1.  Changes to DKIM Key Definition

   The original definition of the textual representation of DKIM keys is
   found in section 3.6.1 of [RFC6376].  The only changes to the
   definition is below.  The entire key:tag definition is included for
   clarity.  All other tags:value pairs are unchanged.  References to
   the definitions below have also been updated to reflect the current
   state of the art.

   h=   Acceptable hash algorithms (plain-text; OPTIONAL, defaults to
      "sha256").  A colon-separated list of hash algorithms that might

Rose                    Expires December 23, 2017               [Page 3]

Internet-Draft                  dkim-ecc                       June 2017

      be used.  Unrecognized algorithms MUST be ignored.  Refer to
      [RFC6376]Section 3.3 for a discussion of the hash algorithms
      implemented by Signers and Verifiers.  The set of algorithms
      listed in this tag in each record is an operational choice made by
      the Signer.


           key-h-tag       = %x68 [FWS] "=" [FWS] key-h-tag-alg
           *( [FWS] ":" [FWS] key-h-tag-alg )
           key-h-tag-alg   = "sha1" / "sha256" / x-key-h-tag-alg
           x-key-h-tag-alg = hyphenated-word  ; for future extension

   k=   Key type (plain-text; OPTIONAL, default is "rsa").  Signers and
      Verifiers MUST support the "rsa" key type.  The "rsa" key type
      indicates that an ASN.1 DER-encoded [UTI.X680.2002] RSAPublicKey
      (see [RFC8017], Sections 3.1 and A.1.1) is being used in the "p="
      tag.  The "ecdsa256" key type indicates an ASN.1 DER-encoded
      [UTI.X680.2002] PublicKey (see [RFC5480], Section 2.2) is being
      used in the "p=" tag.  (Note: the "p=" tag further encodes the
      value using the base64 algorithm.)  Unrecognized key types MUST be


           key-k-tag        = %x76 [FWS] "=" [FWS] key-k-tag-type
           key-k-tag-type   = "rsa" / "ecdsa256" / x-key-k-tag-type
           x-key-k-tag-type = hyphenated-word   ; for future extension

3.2.  Changes to DKIM Signature Definition

   The original definition of the textual representation of DKIM
   signatures is found in section 3.5 of [RFC6376].  The only changes to
   the definition is below.  The entire key:tag definition is included
   for clarity.  All other tags:value pairs are unchanged.  References
   to the definitions below have also been updated to reflect the
   current state of the art.

   a= The algorithm used to generate the signature (plain-text;
      REQUIRED).  Verifiers MUST support "rsa-sha1" and "rsa-sha256" and
      SHOULD support "ecdsa256-sha256"; Signers MUST NOT use "sha1" with
      "ecdsa256".  See [RFC6376] Section 3.3 for a description of RSA
      and [FIPS.186-4.2013] Section 6 for a brief description of ECDSA.

Rose                    Expires December 23, 2017               [Page 4]

Internet-Draft                  dkim-ecc                       June 2017


           sig-a-tag     = %x61 [FWS] "=" [FWS] sig-a-tag-alg
           sig-a-tag-alg = sig-a-tag-k "-" sig-a-tag-h
           sig-a-tag-k   = "rsa" / "ecdsa256" / x-sig-a-tag-k
           sig-a-tag-h   = "sha1" / "sha256" / x-sig-a-tag-h
           x-sig-a-tag-k = ALPHA *(ALPHA / DIGIT)
           ; for later extension
           x-sig-a-tag-h = ALPHA *(ALPHA / DIGIT)
           ; for later extension

4.  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.

5.  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.

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

6.  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

Rose                    Expires December 23, 2017               [Page 5]

Internet-Draft                  dkim-ecc                       June 2017

   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.

7.  IANA Considerations

   This draft defines the use of a new algorithm for DKIM.  This draft
   updates the "DKIM Key Tag" registry to include the following new

             algorithm  |  mnemonic  |  Reference
            ECDSA P-256 |  ecdsa256  | This document

   The current DKIM Key Tag registry is located at

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,

   [RFC5480]  Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
              "Elliptic Curve Cryptography Subject Public Key
              Information", RFC 5480, DOI 10.17487/RFC5480, March 2009,

   [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,

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,

Rose                    Expires December 23, 2017               [Page 6]

Internet-Draft                  dkim-ecc                       June 2017

              "ITU-T Recommendation X.680 (2002) | ISO/IEC 8825-1:2002,
              Information Technology - ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ITU X680, 2002.

8.2.  Informative References

              National Institute of Standards and Technology, "Digital
              Signature Standard", FIPS PUB 186-4, July 2013,

              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,

Author's Address

   Scott Rose
   100 Bureau Dr.
   Gaithersburg, MD  20899

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

Rose                    Expires December 23, 2017               [Page 7]