Internet DRAFT - draft-kitterman-dcrup-dkim-usage
draft-kitterman-dcrup-dkim-usage
Network Working Group S. Kitterman
Internet-Draft Kitterman Technical Services
Updates: 6376 (if approved) May 19, 2017
Intended status: Standards Track
Expires: November 20, 2017
Cryptographic Algorithm and Key Usage to DKIM
draft-kitterman-dcrup-dkim-usage-00
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. This document updates RFC 6376.
Status of This Memo
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This Internet-Draft will expire on November 20, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. DKIM Signing and Verification Algorithms . . . . . . . . . . 3
3.1. The rsa-sha1 Signing Algorithm . . . . . . . . . . . . . 3
3.2. The rsa-sha256 Signing Algorithm . . . . . . . . . . . . 3
3.3. Key Sizes . . . . . . . . . . . . . . . . . . . . . . . . 3
3.4. Other Algorithms . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
5.1. DKIM Hash Algorithms . . . . . . . . . . . . . . . . . . 4
6. Normative References . . . . . . . . . . . . . . . . . . . . 4
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
Discussion Venue: Discussion about this draft is directed to the
dcrup@ietf.org [1] mailing list.
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). 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
longer than 1024 bits don't fit in 256 octets.
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2. 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].
3. DKIM Signing and Verification Algorithms
This section replaces [RFC6376] Section 3.3 in its entirety.
DKIM supports multiple digital signature algorithms. Two algorithms
were defined by [RFC6376]: rsa-sha1 and rsa-sha256. Signers MUST
implement and sign using rsa-sha256. Verifiers MUST implement rsa-
sha256. The rsa-sha1 signing algorithm is obsolete and MUST NOT be
used.
3.1. The rsa-sha1 Signing Algorithm
This algorithm is obsolete and MUST NOT be used.
3.2. 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.
3.3. 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:
o The practical constraint that large (e.g., 4096-bit) keys might
not fit within a 512-byte DNS UDP response packet
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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.
3.4. Other Algorithms
Other algorithms will be defined in the future. Verifiers MUST
ignore any signatures using algorithms that they do not implement.
4. Security Considerations
This document does not change the Security Considerations of
[RFC6376]. It reduces the risk of signature compromise due to weak
cryptography.
5. IANA Considerations
IANA is requested to update registries as follows.
5.1. DKIM Hash Algorithms
The following value is changed in the DKIM Hash Algorithms
+------+-----------------+----------+
| TYPE | REFERENCE | STATUS |
+------+-----------------+----------+
| sha1 | (this document) | obsolete |
+------+-----------------+----------+
Table 1: DKIM Hash Algorithms Changed Value
6. 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>.
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[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>.
Appendix A. Acknowledgements
The author wishes to acknowledge the following for their review and
constructive criticism of this proposal: TBD (surely there will be
someone).
Thanks to John Levine for draft-ietf-dcrup-dkim-crypto-00, which 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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