Cryptographic Update to DKIM
draft-ietf-dcrup-dkim-crypto-02

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Document Type Active Internet-Draft (dcrup WG)
Author John Levine 
Last updated 2017-06-20
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Network Working Group                                          J. Levine
Internet-Draft                                      Taughannock Networks
Updates: 6376 (if approved)                                June 20, 2017
Intended status: Standards Track
Expires: December 22, 2017

                      Cryptographic Update to DKIM
                    draft-ietf-dcrup-dkim-crypto-02

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.

   Internet-Drafts are working documents of the Internet Engineering
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 22, 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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   described in the Simplified BSD License.

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Table of Contents

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

1.  Introduction

   Discussion Venue:    Discussion about this draft is directed to the
      dcrup@ietf.org [1] 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 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,
   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].

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   Syntax descriptions use Augmented BNF (ABNF) [RFC5234].  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 [RFC8032].  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" or "k=eddsafp" tag to indicate that the key is
   an EdDSA rather than RSA key.

   The syntax of DKIM signature tags is updated as follows, where this
   rule replaces the existing rule for sig-a-tag-k:

       ABNF:

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

   The syntax of DKIM key tags is updated as follows, where this rule
   replaces the existing rule for key-k-tag-type:

       ABNF:

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

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 EdDSA signature the Signing Algorithm is eddsafp-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 or
   k=eddsafp.

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

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         ABNF:

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

   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.  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, eddsa-sha256, and eddsafp-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.

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.

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7.  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.

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

                    +------+-----------------+--------+
                    | TYPE | REFERENCE       | STATUS |
                    +------+-----------------+--------+
                    |  k   | (this document) | active |
                    +------+-----------------+--------+

             Table 1: DKIM Signature Tag Registry Added Value

8.2.  DKIM Hash Algorithms Registry

   The following value is updated in the DKIM Hash Algorithms Registry

                  +------+-------------------+----------+
                  | TYPE | REFERENCE         | STATUS   |
                  +------+-------------------+----------+
                  | sha1 | [FIPS-180-3-2008] | historic |
                  +------+-------------------+----------+

           Table 2: DKIM Hash Algorithms Registry Updated Value

8.3.  DKIM Key Type registry

   The following values are added to the DKIM Key Type Registry

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                     +---------+-----------+--------+
                     |   TYPE  | REFERENCE | STATUS |
                     +---------+-----------+--------+
                     |  rsafp  | [RFC3447] | active |
                     |  eddsa  | [RFC8032] | active |
                     | eddsafp | [RFC8032] | active |
                     +---------+-----------+--------+

               Table 3: DKIM Key Type Registry Added Values

9.  References

9.1.  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>.

   [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, <http://www.rfc-editor.org/info/rfc3447>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [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>.

   [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>.

9.2.  URIs

   [1] mailto:dcrup@ietf.org

Appendix A.  Change log

   01 to 02:  Clarify EdDSA algorithm is ed25519 with Pure version of
      the signing.  Make references to tags and fields consistent.

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Author's Address

   John Levine
   Taughannock Networks
   PO Box 727
   Trumansburg, NY  14886

   Phone: +1 831 480 2300
   Email: standards@taugh.com

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