Cryptographic Update to DKIM
draft-ietf-dcrup-dkim-crypto-03

Network Working Group                                          J. Levine
Internet-Draft                                      Taughannock Networks
Updates: 6376 (if approved)                                 July 1, 2017
Intended status: Standards Track
Expires: January 2, 2018

                      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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   3.  EdDSA-SHA256 Signing Algorithm  . . . . . . . . . . . . . . .   3
   4.  Public key fingerprints . . . . . . . . . . . . . . . . . . .   3
   5.  Signature and key syntax  . . . . . . . . . . . . . . . . . .   4
     5.1.  Signature syntax  . . . . . . . . . . . . . . . . . . . .   4
     5.2.  Key syntax  . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Key and algorithm choice and strength . . . . . . . . . . . .   5
   7.  Transition Considerations . . . . . . . . . . . . . . . . . .   5
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     9.1.  DKIM Signature Tag Registry . . . . . . . . . . . . . . .   5
     9.2.  DKIM Hash Algorithms Registry . . . . . . . . . . . . . .   6
     9.3.  DKIM Key Type registry  . . . . . . . . . . . . . . . . .   6
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   6
     10.2.  Informative References . . . . . . . . . . . . . . . . .   7
     10.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Appendix A.  Change log . . . . . . . . . . . . . . . . . . . . .   7
   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
   for RSA keys, with the key itself in the signature and a shorter key
   fingerprint that fits in 256 octets in the DNS.

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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) [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" 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=.

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   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

   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:

       ABNF:

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

   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:

       ABNF:

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

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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

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

             Table 1: DKIM Signature Tag Registry Added Value

9.2.  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

9.3.  DKIM Key Type registry

   The following values are added to the DKIM Key Type Registry

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

               Table 3: DKIM Key Type Registry Added Values

10.  References

10.1.  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>.

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   [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>.

10.2.  Informative References

   [RFC4871]  Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
              J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
              Signatures", RFC 4871, DOI 10.17487/RFC4871, May 2007,
              <http://www.rfc-editor.org/info/rfc4871>.

10.3.  URIs

   [1] mailto:dcrup@ietf.org

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

   John Levine
   Taughannock Networks
   PO Box 727
   Trumansburg, NY  14886

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

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