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.
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.
3. Updates to DKIM Signing and Verification Requirements
This document updates [RFC6376] as follows:
o Section 3.1 of this document updates Section 3.3 of [RFC6376].
o Section 3.2 of this document updates Section 3.3.3 of [RFC6376].
o The algorithm described in Section 3.3.1 of [RFC6376] is now
historic and no longer used by DKIM.
Sections 3.3.2 and 3.3.4 of [RFC6376] are not affected.
3.1. Signing and Verification Algorithms
DKIM supports multiple digital signature algorithms. Two algorithms
are defined by this specification at this time: rsa-sha1 and rsa-
sha256. Signers MUST sign using rsa-sha256. Verifiers MUST be able
to verify using rsa-sha256. rsa-sha1 MUST NOT be used for signing or
verifying.
DKIM signatures identified as having been signed with historic
algorithms (currently, rsa-sha1) have permanently failed evaluation
as discussed in Section 3.9 of [RFC6376].
3.2. 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. Signers SHOULD use RSA keys of at least 2048 bits.
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. Verifiers MUST NOT consider signatures using RSA keys of
less than 1024 bits as valid signatures.
DKIM signatures with insufficient key sizes (currently, rsa-sha256
with less than 1024 bits) have permanently failed evaluation as
discussed in Section 3.9 of [RFC6376].
4. Security Considerations
This document does not change the Security Considerations of
[RFC6376]. It reduces the risk of signature compromise due to weak
cryptography. The SHA-1 risks discussed in Section 3 of [RFC6194]
are resolved due to rsa-sha1 no longer being used by DKIM.
5. IANA Considerations
IANA has updated the Reference and Status fields of the "sha1"
registration in the "DKIM Hash Algorithms" registry registry. The
registration now appears as follows:
+------+---------------------+----------+
| Type | Reference | Status |
+------+---------------------+----------+
| sha1 | [RFC6376] [RFC8301] | historic |
+------+---------------------+----------+
6. References
6.1. 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>.
[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>.
[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,
<https://www.rfc-editor.org/info/rfc8017>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
6.2. Informative References
[RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
Considerations for the SHA-0 and SHA-1 Message-Digest
Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
<https://www.rfc-editor.org/info/rfc6194>.
[VULNOTE] US-CERT, "Vulnerability Note VU#268267: DomainKeys
Identified Mail (DKIM) Verifiers may inappropriately
convey message trust", October 2012,
<http://www.kb.cert.org/vuls/id/268267>.
Acknowledgements
The author wishes to acknowledge the following individuals for their
review and comments on this proposal: Kurt Andersen, Murray
S. Kucherawy, Martin Thomson, John Levine, Russ Housley, and Jim
Fenton.
Thanks to John Levine for his DKIM Crypto Update (DCRUP) work that
was the source for much of the introductory material in this
document.
Author's Address
Scott Kitterman
Kitterman Technical Services
3611 Scheel Dr
Ellicott City, MD 21042
United States of America
Phone: +1 301 325-5475
Email: scott@kitterman.com