Cryptographic Algorithm and Key Usage Update to DomainKeys Identified Mail (DKIM)
RFC 8301
| Document | Type |
RFC - Proposed Standard
(January 2018; No errata)
Updates RFC 6376
|
|
|---|---|---|---|
| Author | Scott Kitterman | ||
| Last updated | 2018-01-19 | ||
| Replaces | draft-kitterman-dcrup-dkim-usage | ||
| Stream | IETF | ||
| Formats | plain text html pdf htmlized bibtex | ||
| Reviews | |||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Seth Blank | ||
| Shepherd write-up | Show (last changed 2017-08-22) | ||
| IESG | IESG state | RFC 8301 (Proposed Standard) | |
| Action Holders |
(None)
|
||
| Consensus Boilerplate | Yes | ||
| Telechat date | |||
| Responsible AD | Alexey Melnikov | ||
| Send notices to | Seth Blank <seth@sethblank.com> | ||
| IANA | IANA review state | Version Changed - Review Needed | |
| IANA action state | RFC-Ed-Ack | ||
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
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