Using Multiple Signing Algorithms with the ARC (Authenticated Received Chain) Protocol
draft-ietf-dmarc-arc-multi-01

Abstract

The Authenticated Received Chain (ARC) protocol creates a mechanism whereby a series of handlers of an email message can conduct authentication of the email message as it passes among them on the way to its destination.

Initial development of ARC has been done with a single allowed signing algorithm, but parallel work in the DCRUP working group (https://datatracker.ietf.org/wg/dcrup/about/) is expanding the supported algorithms. This specification defines how to extend ARC for multiple signing algorithms.

Status of This Memo

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This Internet-Draft will expire on September 20, 2018.

Copyright Notice

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Table of Contents

• 1. Introduction
• 2. Overview
• 3. Definitions and Terminology
• 4. Supporting Alternate Signing Algorithms
• 5. General Approach
• 5.1. Signers
• 5.2. Validators
• 6. Phases of Algorithm Evolution
• 6.1. Introductory Period
• 6.2. Co-Existence Period
• 6.3. Deprecation Period
• 6.4. Obsolescence Period
• 7. Privacy Considerations
• 8. IANA Considerations
• 9. Security Considerations
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Appendix A. Acknowledgements
• Appendix B. Comments and Feedback
• Authors' Addresses

1. Introduction

The Authenticated Received Chain (ARC) protocol adds a traceable chain of signatures that cover the handling of an email message through a chain of intermediary handlers.

Initial development of ARC has been done with a single allowed signing algorithm, but parallel work in the DCRUP working group (https://datatracker.ietf.org/wg/dcrup/about/) is expanding the supported algorithms. This specification defines how to extend ARC for multiple signing algorithms.

2. Overview

In order to phase in new signing algorithms, this specification identifies how signers and validators process ARC sets found in email messages.

3. Definitions and Terminology

This section defines terms used in the rest of the 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].

Because many of the core concepts and definitions are found in [RFC5598], readers should to be familiar with the contents of [RFC5598], and in particular, the potential roles of intermediaries in the delivery of email and the problems [RFC7960] created by the initial DMARC [RFC7489] .

4. Supporting Alternate Signing Algorithms

During a period where multiple algorithms are allowed, all of the statements in the ARC spec which refer to “exactly one set of ARC headers per instance” need to be understood as “at least one set per instance and no more than one set per instance per algorithm”.

5. General Approach

5.1. Signers

There is a separate independent signing chain for each signing algorithm. Hence, when creating an ARC signature, a signer MUST include only other signatures that use the same algorithm as the signature being created.

Wnen signing a message with no previous ARC signatures, signers MUST sign using all supported algorithms.

A signer MUST continue the longest ARC chain(s) in a message with all algorithms that it supports. That is, if at least one of the longest chains uses an algorithm that a signer supports, the signer continues the chain(s). If none of the longest chains in a message use an algorithm supported by a signer, the signer MUST NOT extend any chains, even if a shorter chain does use a supported algorithm.

5.2. Validators

A validator MUST use the longest ARC chain(s) on the message. If a validator cannot interpret the signing algorithm on any of the longest chains, validation fails, evven if a shorter chain does use a supported algorithm.

If there is more than one longest chain, the overall result reported can be that of of any of the validations. The result used when extending an ARC chain MUST be the result from validating that chain.

6. Phases of Algorithm Evolution

6.1. Introductory Period

Intermediaries MUST be able to validate ARC chains built with either algorithm but MAY create ARC sets with either (or both) algorithm.

The introductory period should be at least six (6) months.

6.2. Co-Existence Period

Intermediaries MUST be able to validate ARC chains build with either algorithm and MUST create ARC sets with both algorithms. Chains ending with either algorithm may be used for the result.

6.3. Deprecation Period

ARC sets built with algorithms that are being deprecated MAY be considered valid within an ARC chain, however, intermediaries MUST NOT create additional sets with the deprecated algorithm.

The deprecation period should be at least two (2) years.

6.4. Obsolescence Period

ARC sets built with algorithms that are obsolete MUST NOT be considered valid within an ARC chain. Intermediaries MUST NOT create any sets with any obsoleted algorithm.

7. Privacy Considerations

No unique privacy considerations are introduced by this specification beyond those of the base [ARC-DRAFT-11] protocol.

8. IANA Considerations

No new IANA considerations are introduced by this specification.

9. Security Considerations

No new security considerations are introduced by this specification beyond those of the base [ARC-DRAFT-11] protocol.

10. References

10.1. Normative References

10.2. Informative References

Appendix A. Acknowledgements

This draft is the work of DMARC Working Group.

Grateful appreciation is extended to the people who provided feedback through the discuss mailing list.

Appendix B. Comments and Feedback

Please address all comments, discussions, and questions to dmarc@ietf.org.

Authors' Addresses