Cryptographic Algorithm and Key Usage Update to DKIM
draft-ietf-dcrup-dkim-usage-01

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

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 December 7, 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. Conventions Used in This Document
• 3. DKIM Signing and Verification Algorithms
• 3.1. The rsa-sha1 Signing Algorithm
• 3.2. The rsa-sha256 Signing Algorithm
• 3.3. Key Sizes
• 3.4. Other Algorithms
• 4. The DKIM-Signature Header Field
• 5. Key Management and Representation
• 6. Security Considerations
• 7. IANA Considerations
• 7.1. DKIM Hash Algorithms
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Appendix A. Acknowledgements
• Author's Address

1. Introduction

Discussion Venue:

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

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 longer than 1024 bits don't fit in 256 octets.

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:

• The practical constraint that large (e.g., 4096-bit) keys might not fit within a 512-byte DNS UDP response packet
• The security constraint that keys smaller than 2048 bits may be subject to off-line attacks
• Larger keys impose higher CPU costs to verify and sign email
• Keys can be replaced on a regular basis; thus, their lifetime can be relatively short
• 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. 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

5. Key Management and Representation

This section updates the h= tag in [RFC6376] Section 3.6.1.

The following ABNF element is updated:

      ABNF:

      key-h-tag-alg   = "sha256" / x-key-h-tag-alg

6. 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.

7. IANA Considerations

IANA is requested to update registries as follows.

7.1. DKIM Hash Algorithms

The following value is changed in the DKIM Hash Algorithms

8. References

8.1. Normative References

8.2. Informative References

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