Cryptographic Algorithm and Key Usage Update to DomainKeys Identified Mail (DKIM)
RFC 8301

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Internet Engineering Task Force (IETF)                      S. Kitterman
Request for Comments: 8301                  Kitterman Technical Services
Updates: 6376                                               January 2018
Category: Standards Track
ISSN: 2070-1721

            Cryptographic Algorithm and Key Usage Update to
                   DomainKeys Identified Mail (DKIM)

Abstract

   The cryptographic algorithm and key size requirements included when
   DomainKeys Identified Mail (DKIM) was designed a decade ago are
   functionally obsolete and in need of immediate revision.  This
   document updates DKIM requirements to those minimally suitable for
   operation with currently specified algorithms.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8301.

Copyright Notice

   Copyright (c) 2018 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
   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.

Kitterman                    Standards Track                    [Page 1]
RFC 8301                DKIM Crypto Usage Update            January 2018

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   2
   3.  Updates to DKIM Signing and Verification Requirements . . . .   3
     3.1.  Signing and Verification Algorithms . . . . . . . . . . .   3
     3.2.  Key Sizes . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   DKIM [RFC6376] signs email 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, RFC 6376 [RFC6376] and its predecessors,
   specify a single signing algorithm, RSA [RFC8017], and recommend key
   sizes of 1024 to 2048 bits (but require verification of 512-bit
   keys).  As discussed in US-CERT Vulnerability Note VU#268267
   [VULNOTE], the operational community has recognized that shorter keys
   compromise the effectiveness of DKIM.  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
   significantly longer than 1024 bits don't fit in 256 octets.

   Due to the recognized weakness of the SHA-1 hash algorithm (see
   [RFC6194]) and the wide availability of the SHA-256 hash algorithm
   (it has been a required part of DKIM [RFC6376] since it was
   originally standardized in 2007), the SHA-1 hash algorithm MUST NOT
   be used.  This is being done now to allow the operational community
   time to fully shift to SHA-256 in advance of any SHA-1-related
   crisis.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

Kitterman                    Standards Track                    [Page 2]
RFC 8301                DKIM Crypto Usage Update            January 2018

3.  Updates to DKIM Signing and Verification Requirements