Recognized Transformations of Messages Bearing DomainKeys Identified Mail (DKIM) Signatures
draft-kucherawy-dkim-transform-02

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Network Working Group                                       M. Kucherawy
Internet-Draft                                              July 5, 2020
Intended status: Experimental
Expires: January 6, 2021

  Recognized Transformations of Messages Bearing DomainKeys Identified
                         Mail (DKIM) Signatures
                   draft-kucherawy-dkim-transform-02

Abstract

   DomainKeys Identified Mail (DKIM) introduced a mechanism whereby a
   mail operator can affix a signature to a message that validates at
   the level of the signer's domain name.  It specified two possible
   ways of converting the message body to a canonical form, one
   intolerant of changes and the other tolerant of simple changes to
   whitespace within the message body.

   The provided canonicalization schemes do not tolerate changes in a
   message such as conversion between transfer encodings or addition of
   new message content.  It is useful to have these capabilities to
   allow for transport through gateways, and also for transport through
   handlers (such as mailing list services) that might add content that
   would invalidate a signature generated using the existing
   canonicalization schemes.

   This document presents a mechanism for declaring that a message
   underwent any of a handful of well-defined transformations prior to
   being re-signed by a mediator, so that a verifier might rewind such
   modification(s) and thereby confirm that the original signature still
   verifies against the original content.

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Kucherawy                Expires January 6, 2021                [Page 1]
Internet-Draft            DKIM Transformations                 July 2020

   This Internet-Draft will expire on January 6, 2021.

Copyright Notice

   Copyright (c) 2020 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
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Table of Contents

   1.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  The 'tf' DKIM Signature Tag  . . . . . . . . . . . . . . . . .  4
   4.  DKIM Operational Flow  . . . . . . . . . . . . . . . . . . . .  4
     4.1.  Detail . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  The 'subject' Transformation . . . . . . . . . . . . . . . . .  6
   6.  The 'footer' Transformation  . . . . . . . . . . . . . . . . .  6
   7.  The 'mimeify' Transformation . . . . . . . . . . . . . . . . .  7
   8.  The 'add-part' Transformation  . . . . . . . . . . . . . . . .  8
   9.  The 'mime-wrap' Transformation . . . . . . . . . . . . . . . .  9
   10. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 11
     11.1. Imported from DKIM . . . . . . . . . . . . . . . . . . . . 11
     11.2. False Transformation Claims  . . . . . . . . . . . . . . . 12
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
     12.1. DKIM Transformations Registry  . . . . . . . . . . . . . . 12
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 13
     13.2. Informative References . . . . . . . . . . . . . . . . . . 14
   Appendix A.  Example . . . . . . . . . . . . . . . . . . . . . . . 14
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 15
   Appendix C.  To-Do List  . . . . . . . . . . . . . . . . . . . . . 15
   Appendix D.  Change Log  . . . . . . . . . . . . . . . . . . . . . 16

Kucherawy                Expires January 6, 2021                [Page 2]
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1.  Background

   DomainKeys Identified Mail (DKIM) [RFC6376] defines a mechanism
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