DKIM Crypto Update: Cryptographic Algorithm and Key Usage Update to DomainKeys Identified Mail (DKIM)
DKIM signers must use rsa-sha256 and keys of at least 1024 bits; rsa-sha1 is retired.
Kitterman, S., "Cryptographic Algorithm and Key Usage Update to DomainKeys Identified Mail (DKIM)", RFC 8301, DOI 10.17487/RFC8301, January 2018, <https://www.rfc-editor.org/info/rfc8301>.
@misc{rfc8301,
series = {Request for Comments},
number = 8301,
howpublished = {RFC 8301},
publisher = {RFC Editor},
doi = {10.17487/RFC8301},
url = {https://www.rfc-editor.org/info/rfc8301},
author = {S. Kitterman},
title = {{Cryptographic Algorithm and Key Usage Update to DomainKeys Identified Mail (DKIM)}},
pagetotal = 5,
year = 2018,
month = jan,
} RFC text
Contents
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. 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
AI ELI5
On this page
§1The Problem with Old DKIM Crypto
DKIM signs email by hashing the message headers and content and signing the header hash with a digital signature. Recipients fetch the verification key from the DNS, where it is stored in a TXT record. The defining document, RFC 6376, specified a single signing algorithm, RSA, and recommended key sizes of 1024 to 2048 bits, while still requiring verification of 512-bit keys.
The operational community had recognized that shorter keys compromise the effectiveness of DKIM. Widely used DNS configuration software handles only a single 256-octet string in a TXT record, which puts a practical limit on key sizes because RSA keys significantly longer than 1024 bits do not fit.
Email gets a signature so you can tell it is real. The signing locks were old and some were too small to be safe, so it was time to upgrade them.
§3.1Signing and Verification Algorithms
DKIM defines two algorithms, rsa-sha1 and rsa-sha256. Signers MUST sign using rsa-sha256, and verifiers MUST be able to verify using it. rsa-sha1 MUST NOT be used for signing or verifying.
The reasons given are the recognized weakness of the SHA-1 hash and the wide availability of SHA-256, which has been a required part of DKIM since it was standardized in 2007. The change is made now to give the operational community time to shift before any SHA-1-related crisis. Signatures made with a historic algorithm have permanently failed evaluation.
There were two ways to scramble the message for signing, and one was getting weak. Everyone must now use the stronger one.
§3.2Key Sizes
The RFC calls key size a trade-off between cost, performance and risk. Signers MUST use RSA keys of at least 1024 bits and SHOULD use keys of at least 2048 bits.
Verifiers MUST be able to validate signatures with keys from 1024 bits to 4096 bits, and MAY be able to handle larger ones. Verifiers MUST NOT treat signatures using RSA keys of less than 1024 bits as valid. Such signatures, for example rsa-sha256 with less than 1024 bits, have permanently failed evaluation. Verifier policies can use key length as one measure of whether a signature is acceptable.
A longer key is like a lock with more pins, which is harder to pick. Tiny locks are no longer accepted, and big ones are encouraged.
| Item | Requirement |
|---|---|
| Signing algorithm | MUST be rsa-sha256 |
| rsa-sha1 | MUST NOT be used |
| Signer key size | MUST be 1024 bits or more; SHOULD be 2048 bits or more |
| Verifier key range | MUST handle 1024 to 4096 bits; keys under 1024 are invalid |
§4Security and IANA Notes
The document does not change the Security Considerations of RFC 6376. It reduces the risk of signature compromise from weak cryptography, and the SHA-1 risks in RFC 6194 are resolved because rsa-sha1 is no longer used by DKIM.
IANA updated the sha1 entry in the "DKIM Hash Algorithms" registry so that its references are RFC 6376 and RFC 8301 and its status is historic.
Dropping the weak lock makes fake signatures harder. The official list of locks now marks the old one as retired.