Internet DRAFT - draft-ietf-dcrup-dkim-usage
draft-ietf-dcrup-dkim-usage
Network Working Group S. Kitterman
Internet-Draft Kitterman Technical Services
Updates: 6376 (if approved) November 3, 2017
Intended status: Standards Track
Expires: May 7, 2018
Cryptographic Algorithm and Key Usage Update to DKIM
draft-ietf-dcrup-dkim-usage-06
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 7, 2018.
Copyright Notice
Copyright (c) 2017 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
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Discussion Venue . . . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Conventions Used in This Document . . . . . . . . . . . . . . 3
4. DKIM Signing and Verification Algorithms . . . . . . . . . . 3
4.1. DKIM Signing and Verification Algorithms . . . . . . . . 3
4.2. Key Sizes . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
7.1. Normative References . . . . . . . . . . . . . . . . . . 4
7.2. Informative References . . . . . . . . . . . . . . . . . 4
7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Discussion Venue
RFC EDITOR: Please remove this section before publication.
Discussion about this draft is directed to the dcrup@ietf.org [1]
mailing list.
2. Introduction
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). As discussed in US-CERT 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 sha1 hash algorithm, see
[RFC6194], and the wide availability of the sha256 hash algorithm (it
has been a required part of DKIM [RFC6376] since it was originally
standardized in 2007), the sha1 hash algorithm MUST NOT be used.
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This is being done now to allow the operational community time to
fully shift to sha256 in advance of any sha1 related crisis.
3. 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].
4. DKIM Signing and Verification Algorithms
Section 4.1 updates [RFC6376] Section 3.3.
Section 4.2 updates [RFC6376] Section 3.3.3.
The algorithm described in[RFC6376] Section 3.3.1 is now historic and
no longer used by DKIM.
[RFC6376] Sections 3.3.2 and 3.3.4 are not affected.
4.1. DKIM 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 [RFC6376] Section 3.9.
4.2. 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.
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DKIM signatures with insufficient key sizes (currently rsa-sha256
with less than 1024 bits) have permanently failed evaluation as
discussed in [RFC6376] Section 3.9.
5. 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 [RFC6194] Section 3 are
resolved due to rsa-sha1 no longer being used by DKIM.
6. IANA Considerations
IANA is requested to update the "sha1" registration in the "DKIM Hash
Algorithms" as follows:
+------+-----------+----------+
| TYPE | REFERENCE | STATUS |
+------+-----------+----------+
| sha1 | [RFC6376] | historic |
+------+-----------+----------+
Table 1: DKIM Hash Algorithms Changed Value
7. References
7.1. 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,
<http://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,
<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>.
7.2. Informative References
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[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,
<http://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.
7.3. URIs
[1] mailto:dcrup@ietf.org
Appendix A. Acknowledgements
The author wishes to acknowledge the following for their review and
comment on this proposal: Kurt Andersen, Murray S. Kucherawy, Martin
Thomson, John Levine, Russ Housley, and Jim Fenton.
Thanks to John Levine for his DCRUP work that 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
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