Cryptographic Algorithm and Key Usage Update to DKIM
draft-ietf-dcrup-dkim-usage-03
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (dcrup WG) | |
|---|---|---|---|
| Author | Scott Kitterman | ||
| Last updated | 2017-08-20 (Latest revision 2017-07-31) | ||
| Replaces | draft-kitterman-dcrup-dkim-usage | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text xml htmlized pdfized bibtex | ||
| Reviews |
SECDIR Last Call review
(of
-04)
Has Issues
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| Stream | WG state | WG Consensus: Waiting for Write-Up | |
| Document shepherd | Seth Blank | ||
| Shepherd write-up | Show Last changed 2017-08-18 | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | Seth Blank <seth@sethblank.com> |
draft-ietf-dcrup-dkim-usage-03
Network Working Group S. Kitterman
Internet-Draft Kitterman Technical Services
Updates: 6376 (if approved) July 31, 2017
Intended status: Standards Track
Expires: February 1, 2018
Cryptographic Algorithm and Key Usage Update to DKIM
draft-ietf-dcrup-dkim-usage-03
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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 1, 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
Provisions Relating to IETF Documents
(http://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
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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. The rsa-sha256 Signing Algorithm . . . . . . . . . . . . 3
4.2. Key Sizes . . . . . . . . . . . . . . . . . . . . . . . . 3
4.3. Other Algorithms . . . . . . . . . . . . . . . . . . . . 4
5. The DKIM-Signature Header Field . . . . . . . . . . . . . . . 4
6. Key Management and Representation . . . . . . . . . . . . . . 4
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
9.1. Normative References . . . . . . . . . . . . . . . . . . 5
9.2. Informative References . . . . . . . . . . . . . . . . . 6
9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
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.
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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 is removed from the
protocol. 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
This section replaces [RFC6376] Section 3.3 in its entirety.
Generally, DKIM supports multiple digital signature algorithms. One
algorithm, rsa-sha256, is currenlty defined. Signers MUST implement
and sign using rsa-sha256. Verifiers MUST implement and verify using
rsa-sha256.
4.1. The rsa-sha256 Signing Algorithm
The rsa-sha256 Signing Algorithm computes a message hash as described
in [RFC6376], Section 3.7 using SHA-256 [FIPS-180-3-2008] as the
hash-alg. That hash is then signed by the Signer using the RSA
algorithm (defined in PKCS#1 version 1.5 [RFC8017]) as the crypt-alg
and the Signer's private key. The hash MUST NOT be truncated or
converted into any form other than the native binary form before
being signed. The signing algorithm SHOULD use a public exponent of
65537.
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. 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.
Factors that should influence the key size choice include the
following:
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o The practical constraint that large (e.g., 4096-bit) keys might
not fit within a 512-byte DNS UDP response packet
o The security constraint that keys smaller than 2048 bits may be
subject to off-line attacks
o Larger keys impose higher CPU costs to verify and sign email
o Keys can be replaced on a regular basis; thus, their lifetime can
be relatively short
o The security goals of DKIM [RFC6376] are modest compared to
typical goals of other systems that employ digital signatures
See [RFC3766] for further discussion on selecting key sizes.
4.3. Other Algorithms
The rsa-sha1 was formerly used by DKIM [RFC6376]. Signers MUST NOT
sign with rsa-sha1 and verifiers MUST NOT verify using rsa-sha1.
Other algorithms will be defined in the future. Verifiers MUST
ignore any signatures using algorithms that they do not implement.
5. The DKIM-Signature Header Field
This section updates the a= tag in [RFC6376] Section 3.5.
The text description of the tag is now:
a= The algorithm used to generate the signature (plain-text;
REQUIRED). Verifiers MUST support "rsa-sha256"; Signers MUST
sign using "rsa-sha256". See [RFC6376] Section 3.3 (as updated
by this document) for a description of the algorithms.
The following ABNF element is updated:
ABNF:
sig-a-tag-h = "sha256" / x-sig-a-tag-h
6. Key Management and Representation
This section updates the h= tag in [RFC6376] Section 3.6.1.
The following ABNF element is updated:
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ABNF:
key-h-tag-alg = "sha256" / x-key-h-tag-alg
7. 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 the removal of rsa-sha1 from DKIM.
8. IANA Considerations
IANA is requested to update the "sha1" registration in the "DKIM Hash
Algorithms" as follows:
+------+-----------------+----------+
| TYPE | REFERENCE | STATUS |
+------+-----------------+----------+
| sha1 | (this document) | obsolete |
+------+-----------------+----------+
Table 1: DKIM Hash Algorithms Changed Value
9. References
9.1. 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>.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys", BCP 86,
RFC 3766, DOI 10.17487/RFC3766, April 2004,
<http://www.rfc-editor.org/info/rfc3766>.
[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>.
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9.2. 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,
<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.
9.3. 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 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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