[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Nits]

Versions: 00 draft-ietf-dcrup-dkim-usage

Network Working Group                                       S. Kitterman
Internet-Draft                              Kitterman Technical Services
Updates: 6376 (if approved)                                 May 19, 2017
Intended status: Standards Track
Expires: November 20, 2017


             Cryptographic Algorithm and Key Usage to DKIM
                  draft-kitterman-dcrup-dkim-usage-00

Abstract

   The cryptographic algorithm and key size requirements included when
   DKIM was designed in the last decade are functionally obsolete and in
   need of immediate revision.  This document updates DKIM requirements
   to those minimaly suitable for operation with currently specified
   algorithms.  This document updates RFC 6376.

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 November 20, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.












Kitterman               Expires November 20, 2017               [Page 1]


Internet-Draft          DKIM Crypto Usage Update                May 2017


   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
   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
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   3.  DKIM Signing and Verification Algorithms  . . . . . . . . . .   3
     3.1.  The rsa-sha1 Signing Algorithm  . . . . . . . . . . . . .   3
     3.2.  The rsa-sha256 Signing Algorithm  . . . . . . . . . . . .   3
     3.3.  Key Sizes . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.4.  Other Algorithms  . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  DKIM Hash Algorithms  . . . . . . . . . . . . . . . . . .   4
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

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
   [RFC8017], and recommends key sizes of 1024 to 2048 bits (but require
   verification of 512 bit keys).  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 1024 bits don't fit in 256 octets.









Kitterman               Expires November 20, 2017               [Page 2]


Internet-Draft          DKIM Crypto Usage Update                May 2017


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].

3.  DKIM Signing and Verification Algorithms

   This section replaces [RFC6376] Section 3.3 in its entirety.

   DKIM supports multiple digital signature algorithms.  Two algorithms
   were defined by [RFC6376]: rsa-sha1 and rsa-sha256.  Signers MUST
   implement and sign using rsa-sha256.  Verifiers MUST implement rsa-
   sha256.  The rsa-sha1 signing algorithm is obsolete and MUST NOT be
   used.

3.1.  The rsa-sha1 Signing Algorithm

   This algorithm is obsolete and MUST NOT be used.

3.2.  The rsa-sha256 Signing Algorithm

   The rsa-sha256 Signing Algorithm computes a message hash as described
   in [RFC6376], Section 3.7 using SHA-256 [FIPS-180-3-2008] as the
   hash-alg.  That hash is then signed by the Signer using the RSA
   algorithm (defined in PKCS#1 version 1.5 [RFC8017]) as the crypt-alg
   and the Signer's private key.  The hash MUST NOT be truncated or
   converted into any form other than the native binary form before
   being signed.  The signing algorithm SHOULD use a public exponent of
   65537.

3.3.  Key Sizes

   Selecting appropriate key sizes is a trade-off between cost,
   performance, and risk.  Since short RSA keys more easily succumb to
   off-line attacks, Signers MUST use RSA keys of at least 1024 bits for
   all keys.  Verifiers MUST be able to validate signatures with keys
   ranging from 1024 bits to 4096 bits, and they MAY be able to validate
   signatures with larger keys.  Verifier policies can use the length of
   the signing key as one metric for determining whether a signature is
   acceptable.

   Factors that should influence the key size choice include the
   following:

   o  The practical constraint that large (e.g., 4096-bit) keys might
      not fit within a 512-byte DNS UDP response packet



Kitterman               Expires November 20, 2017               [Page 3]


Internet-Draft          DKIM Crypto Usage Update                May 2017


   o  The security constraint that keys smaller than 2048 bits may be
      subject to off-line attacks

   o  Larger keys impose higher CPU costs to verify and sign email

   o  Keys can be replaced on a regular basis; thus, their lifetime can
      be relatively short

   o  The security goals of DKIM,[RFC6376], are modest compared to
      typical goals of other systems that employ digital signatures

   See [RFC3766] for further discussion on selecting key sizes.

3.4.  Other Algorithms

   Other algorithms will be defined in the future.  Verifiers MUST
   ignore any signatures using algorithms that they do not implement.

4.  Security Considerations

   This document does not change the Security Considerations of
   [RFC6376].  It reduces the risk of signature compromise due to weak
   cryptography.

5.  IANA Considerations

   IANA is requested to update registries as follows.

5.1.  DKIM Hash Algorithms

   The following value is changed in the DKIM Hash Algorithms

                   +------+-----------------+----------+
                   | TYPE | REFERENCE       | STATUS   |
                   +------+-----------------+----------+
                   | sha1 | (this document) | obsolete |
                   +------+-----------------+----------+

                Table 1: DKIM Hash Algorithms Changed Value

6.  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>.





Kitterman               Expires November 20, 2017               [Page 4]


Internet-Draft          DKIM Crypto Usage Update                May 2017


   [RFC3766]  Orman, H. and P. Hoffman, "Determining Strengths For
              Public Keys Used For Exchanging Symmetric Keys", BCP 86,
              RFC 3766, DOI 10.17487/RFC3766, April 2004,
              <http://www.rfc-editor.org/info/rfc3766>.

   [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>.

   [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,
              <http://www.rfc-editor.org/info/rfc8017>.

Appendix A.  Acknowledgements

   The author wishes to acknowledge the following for their review and
   constructive criticism of this proposal: TBD (surely there will be
   someone).

   Thanks to John Levine for draft-ietf-dcrup-dkim-crypto-00, which was
   the source for much of the introductory material in this draft.

Author's Address

   Scott Kitterman
   Kitterman Technical Services
   3611 Scheel Dr
   Ellicott City, MD  21042

   Phone: +1 301 325-5475
   Email: scott@kitterman.com


















Kitterman               Expires November 20, 2017               [Page 5]


Html markup produced by rfcmarkup 1.123, available from https://tools.ietf.org/tools/rfcmarkup/