Cryptographic Algorithm and Key Usage Update to DKIM
draft-ietf-dcrup-dkim-usage-05

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 April 30, 2018.

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. Discussion Venue
• 2. Introduction
• 3. Conventions Used in This Document
• 4. DKIM Signing and Verification Algorithms
• 4.1. Key Sizes
• 5. Security Considerations
• 6. IANA Considerations
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Appendix A. Acknowledgements
• Author's Address

1. Discussion Venue

RFC EDITOR: Please remove this section before publication.

Discussion about this draft is directed to the dcrup@ietf.org mailing list.

2. Introduction

DKIM 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, and recommends key sizes of 1024 to 2048 bits (but require verification of 512 bit keys). As discussed in US-CERT VU#268267, 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 since it was originally standardized in 2007), the sha1 hash algorithm MUST NOT be used. 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 updates [RFC6376] Section 3.3.

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 signed with historic algorithms (currently rsa-sha1) or with insufficient key sizes (currently rsa-sha256 with less than 1024 bits) have permanently failed evaluation as discussed in [RFC6376] Section 3.9.

4.1. 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.

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:

7. References

7.1. Normative References

7.2. Informative References

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