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Versions: (draft-levine-dcrup-dkim-crypto) 00 01 02 03 04 05 06

Network Working Group                                          J. Levine
Internet-Draft                                      Taughannock Networks
Updates: 6376 (if approved)                           September 12, 2017
Intended status: Standards Track
Expires: March 16, 2018


              New cryptographic signature methods for DKIM
                    draft-ietf-dcrup-dkim-crypto-06

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.

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 https://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 March 16, 2018.

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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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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.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   3.  Public key fingerprints . . . . . . . . . . . . . . . . . . .   3
   4.  Ed25519-SHA256 Signing Algorithm  . . . . . . . . . . . . . .   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
     8.1.  Duplicate Signature Key Selection . . . . . . . . . . . .   5
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  DKIM Signature Tag Registry . . . . . . . . . . . . . . .   6
     9.2.  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 body 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
   recommend 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 stronger signing algorithm, Edwards-Curve
   Digital Signature Algorithm using the Curve25519 curve (ed25519),
   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 fixed length public key fingerprint
   that fits in a 256 octet string 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.  Public key fingerprints

   Rather than using a public key stored in the DNS, an RSA signature
   can 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.

   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 in the p=
   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 p= tag is known in advance, it is
   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 p= 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.

4.  Ed25519-SHA256 Signing Algorithm

   The ed25519-sha256 signing algorithm computes a message hash as
   defined in section 3 of [RFC6376], and signs it with the Pure variant
   of Ed25519, as defined in in RFC 8032 section 5.1 [RFC8032].  The



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   signing algorithm is PureEdDSA, since the input to the signing
   algorithm has already been hashed.  The DNS record for the
   verification public key MUST have a "k=ed25519" tag to indicate that
   the key is an Ed25519 rather than RSA key.

   The signature MAY contain a base64 copy of the public key in the p=
   tag, to enable checks for a duplicate signature key selection
   cryptographic attack described in the Security section below.

   Note: since Ed25519 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 ed25519 key fingerprints in the key record rather than the
   keys themselves.

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] is
   updated by adding this rule to the existing rule for sig-a-tag-k:

       ABNF:

       sig-a-tag-k =/ "rsafp" / "ed25519"

   The following tag is added to the list of tags on the DKIM-Signature
   header field in section 3.5 of [RFC6376].

   p=    The public key (base64; REQUIRED for rsafp signatures, OPTIONAL
         otherwise).  White space is ignored in this value and MUST be
         ignored when reassembling the original key.

         ABNF:

         sig-p-tag       = %x70 [FWS] "=" [FWS] sig-p-tag-data
         sig-p-tag-data  = base64string

5.2.  Key syntax

   The syntax of DKIM key tags in section 3.6.1 of [RFC6376] is updated
   by adding this rule to the existing rule for key-k-tag-type:

       ABNF:

       key-k-tag-type  =/ "rsafp" / "ed25519"




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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 SHOULD implement and verifiers MUST implement the rsafp-
   sha256 and ed25519-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 MUST 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

   Ed25519 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.

8.1.  Duplicate Signature Key Selection

   The rsafp signature scheme describes a method where the public key is
   hashed.  The primary motivation for this design is allowing for a
   small representation of large public keys.  Hashing has a secondary
   effect: the public key is included in messages and is signed.
   Including and signing the public key renders duplicate signature key
   selection attacks [Koblitz13] infeasible.  An attacker cannot claim a
   message by constructing a key that would be valid for a signed
   message because the public key is covered by the signature.

   There is currently no known way that an attacker might use a
   duplicate signature key selection attack to their advantage and so
   defending against the attack is not mandated by this specification.
   In the event that a potential attack becomes known, a signer could
   include the public key in messages using the `p=` parameter.  If the
   `p=` parameter is present, a verifier MUST ensure that the parameter
   contains the public key that it uses to verify the message signature.






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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

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

             Table 1: DKIM Signature Tag Registry Added Value

9.2.  DKIM Key Type registry

   The following values are added to the DKIM Key Type Registry

                     +---------+-----------+--------+
                     |   TYPE  | REFERENCE | STATUS |
                     +---------+-----------+--------+
                     |  rsafp  | [RFC3447] | active |
                     | ed25519 | [RFC8032] | active |
                     +---------+-----------+--------+

               Table 2: 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,
              <https://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, <https://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,
              <https://www.rfc-editor.org/info/rfc5234>.




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   [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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc8032>.

10.2.  Informative References

   [Koblitz13]
              Koblitz, N. and A. Menezes, "Another look at security
              definitions", DOI 10.3934/amc.2013.7.1, 2013,
              <http://aimsciences.org/journals/
              displayArticlesnew.jsp?paperID=8249>.

              Advances in Mathematics of Communications, Vol 7, Num 1,
              pp. 1-38.

10.3.  URIs

   [1] mailto:dcrup@ietf.org

Appendix A.  Change log

   05 to 06:  Editorial changes only.

   04 to 05:  Remove deprecation cruft and inconsistent key advice.  Fix
      p= and k= text.

   03 to 04:  Change eddsa to ed25519.  Add Martin's key regeneration
      issue.  Remove hashed ed25519 keys.  Fix typos and clarify text.
      Move syntax updates to separate section.  Take out SHA-1 stuff.

   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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