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Mailing List Manager (MLM) Transformations
draft-vesely-dmarc-mlm-transform-04

Document Type Active Internet-Draft (individual)
Author Alessandro Vesely
Last updated 2022-01-12 (Latest revision 2022-01-04)
Replaces draft-dmarc-vesely-mlm-transform
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draft-vesely-dmarc-mlm-transform-04
Network Working Group                                          A. Vesely
Internet-Draft                                              January 2022
Intended status: Experimental                                           
Expires: 8 July 2022

               Mailing List Manager (MLM) Transformations
                  draft-vesely-dmarc-mlm-transform-04

Abstract

   The widespread adoption of Domain-based Message Authentication,
   Reporting, and Conformance (DMARC) led Mailing List Managers (MLM) to
   rewrite the From: header field as a workaround.

   This document describes reverting MLM transformations in IETF mailing
   lists.  That way, it is possible to verify DomainKeys Identified Mail
   (DKIM) signatures that were applied at submission time and thereby
   restore original identifiers.

   For reliable results, some compliance is required of all agents
   involved, author domain signers, MLMs, forwarders, and final
   recipients' verifiers.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 5 July 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terms Definitions . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Revertible Transformations  . . . . . . . . . . . . . . . . .   4
     3.1.  Header Transformations  . . . . . . . . . . . . . . . . .   4
       3.1.1.  Subject . . . . . . . . . . . . . . . . . . . . . . .   4
       3.1.2.  From  . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Body Transformations  . . . . . . . . . . . . . . . . . .   5
   4.  Outline of a Reverting Verifier . . . . . . . . . . . . . . .   6
   5.  Actors Roles and Compliance . . . . . . . . . . . . . . . . .   8
     5.1.  Original Signer . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  MLM . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.3.  Verifier  . . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  Provisional Message Header Field Names  . . . . . . . . .  11
   8.  Experimental Goals  . . . . . . . . . . . . . . . . . . . . .  12
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  14
     A.1.  Single-part plain text  . . . . . . . . . . . . . . . . .  15
     A.2.  Multipart added . . . . . . . . . . . . . . . . . . . . .  15
     A.3.  Multipart wrapped . . . . . . . . . . . . . . . . . . . .  17
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   As mailing lists do not adhere to an explicitly standardized
   protocol, their behavior and even their definition vary widely.
   Their social usage is often paired with web fora, for example.  In
   the IETF standardization process and in some software development
   communities, instead, mailing lists are rather terse, and constitute
   a key tool to carry out the core work of a group.  Like in any
   working environment, reciprocal trust is an essential team quality.
   Therefore, participants authentication is not an irrelevant feature.

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   However, Domain-based Message Authentication, Reporting, and
   Conformance (DMARC) ([RFC7489]) hinges on the alignment of the domain
   in the From: header field with a verified DKIM signature.  For that
   reason, MLMs that transform messages have to rewrite From:.  MLM
   transformation reversion reduces the DMARC's effects on indirect mail
   flows.

   In this experiment, we try and restore the end-to-end nature of the
   From: email field after DMARC rewriting, focusing on the behavior of
   mailing lists as used in the IETF.  Several Mailing List Managers
   (MLMs) are configured to add a footer and a subject tag to the
   messages that they redistribute.  Although that behavior slightly
   exceeds the very limited set of modifications and actions described
   by Section 3.9.2 of [RFC5321], it is a welcome, time-honored
   tradition.  According to their configuration, the modifications they
   carry out on messages result in a set of stylized transformations
   that are programmatically revertible.  Reversion allows to verify
   DomainKeys Identified Mail (DKIM) signatures ([RFC6376]) that were
   applied before the transformation.

   Mailbox providers can configure their mail submission agents (MSAs)
   in order to ease MLM transformation reversion.  Or they can make it
   impossible.  Their will is expressed by their DKIM signing policy.

   At the receiver, the DKIM verifier needs a substantial enhancement to
   undo the transformations and verify the original signatures.  That
   undoing is carried out in the same way as canonicalization; that is,
   it does not actually change the message except for adding
   Authentication-Results: header fields.  Details are outlined in
   Section 4.

   Besides IETF mailing lists, that setting also works for any MLM
   operators which abide by the same rules.  The outcome is twofold:

   1.  Author domains receive feedback about DKIM verification of
       mailing list traffic.  That might eventually lead them to harden
       their DMARC policy.

   2.  Final recipient's mail delivery agents (MDAs), which know by the
       Authentication-Results: field whether a rewritten From: header
       was verified, can safely undo From: rewriting (after any external
       forwarding).  That way, the annoyance causes by munging to end
       recipients is avoided.

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2.  Terms Definitions

   *Signers* and *verifiers* are defined in [RFC6376].  The use of the
   term *Mailing List Manager*, almost always abbreviated *MLM* follows
   [RFC6377].  A MLM is a kind of *Mediator* in [RFC5598] parlance.

   *Message* is defined in [RFC5322].  It consists of a *header* made up
   of one or more *fields* and a *body*, possibly composed of various
   MIME *entities*, the latter being defined in [RFC2045] and
   companions.

   The term *original* is used here to refer to the Author or parts of
   the Author's message as it was sent out by the Author's domain, where
   *Author* is defined in [RFC5598] and [RFC9057].

3.  Revertible Transformations

   Message modifications can affect the header and/or the body of a
   message.  This document only considers a very limited set of
   transformations, described in the following subsections.  They turn
   out to be revertible.

3.1.  Header Transformations

3.1.1.  Subject

   MLM often modify the Subject: field by inserting a tag at the
   beginning of its value.  A tag consists of a short text delimited by
   square brackets.  For example:

     Subject: [added tag] Original value of subject

   This transformation is easily reverted by removing the tag.  For
   security reasons, subject tags must not exceed 20 characters.

   Note that some MLMs carry out further changes to this field.  For
   example:

     Subject: AW: [MLM-tag] German reply subject

   can be transformed to:

     Subject: Re: [MLM-tag] German reply subject

   Therefore, if the field is signed, it is clever to save a copy of it
   as Original-Subject:.

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3.1.2.  From

   From: rewriting is necessary for DMARC.  That way, the MLM domain
   becomes the primary identifier of a message, in the DMARC sense.  It
   is often achieved by transforming a field like this:

     From: Original User <user@example.com>

   into one like the following:

     From: Original User via MLM <MLM.post@list.example>

   MLMs can save the original value of From: in a variety of places,
   including Reply-To:, Cc:, X-Original-From:.  When the original value
   is known, the transformation is revertible.

   Author's domain submission agents can also provide a copy of From: in
   one of the fields Author: [RFC9057] and Original-From: [RFC5703].
   Besides providing revertibility, signing that field signals
   participation and implicitly asks for feedback reports.

3.2.  Body Transformations

   We only consider footer addition.  It is often performed in one of
   three ways, according to the format of the original message.

   Single-part plain text
      When the original message is not structured, a footer can be
      appended at the end of the original text.  See example in
      Appendix A.1

   Multipart added
      The footer stands in its own MIME entity, which is appended as the
      last part of an original multipart/mixed structure.  See example
      in Appendix A.2

   Multipart wrapped
      The footer stands in the second entity of a new multipart/mixed
      MIME structure whose first entity consists of the original body.
      See example in Appendix A.3

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   The footer begins with a line consisting exclusively of underscore
   ("_", ASCII 95) characters, at least four of them.  Alternatively, a
   footer can consist of the three characters "-- " (dash, dash, space),
   the Usenet signature convention (see for example Section 4.3 of
   [RFC3676]).  For security reasons, the footer must belong to an
   entity of Content-Type: text/plain in all cases.  In addition,
   footers cannot exceed 10 lines of text, each shorter than 80
   characters.  If these restrictions are not met, the transformation
   cannot be reverted safely.

4.  Outline of a Reverting Verifier

   The algorithm described here is implemented in a mail filter
   [zdkimfilter].  The filter usually reads the input message twice
   -first pass, verify; last pass, write Authentication-Results and the
   rest of the message to follow.  When enabling MLM transformation
   reversion, there can be a retry pass in between those two.  The
   result is yielded during the SMTP dialogue with no noticeable delay.
   Implementing reversion changed the software from 22730 lines of C
   code to 26762.  The bulk of such ~18% increase is due to the addition
   of encoding conversion functions.  Changes involve both verifying and
   signing functions (see Section 5.1 for the latter).

   While reading the header in the first pass, the verifier looks for
   specific fields:

   *  From:

   *  Author:

   *  Original-From:

   *  X-Original-From:

   *  Reply-To:

   *  Cc:

   These are candidates to the original mailbox.  Note that Reply-To:
   and Cc: may contain multiple mailboxes.

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   The verifier also collects the Subject: and any field named
   Original-* that the original signer might have set to ease the
   reversion.  On reaching the end of the header, during the first pass,
   the verifier sorts the candidate original mailboxes according to the
   display name, which MLMs try and keep unaltered.  The best candidate
   is then added to the collected set of Original-* fields.  If the
   Subject: begins with a tag, its version without tag is added to that
   set as well, unless one is there already.

   Next, before reading the body, the verifier looks for prospect
   signatures; that is, signatures whose "d=" domain is not aligned with
   SPF credentials ([RFC7208]), List-Post: ([RFC4201]), Sender:, or the
   rewritten From: (if deemed to have been rewritten).  If any such
   signature exist, along with MLM or other signatures, then the
   verifier enables parsing the body to look for a footer.

   Reversing verifiers also have to watch out for idiosyncrasies used to
   mask DKIM signatures.  For example, a MLM introduced a header field
   named X-Mailman-Original-DKIM-Signature, because some receivers took
   the habit to downgrade messages with failed signatures, despite
   [RFC6376] recommendation to consider an unauthenticated message
   regardless of whether or not it looks like it was signed.  For
   authentication purposes, the first 19 characters of that field can be
   discarded.

   Body parsing is done in parallel with body canonicalization during
   the first pass.  For multipart, track top level entities.  Set
   transformation type to "wrapped" if there are exactly two entities,
   "added" otherwise.  However, some lists, perhaps out of
   misconfiguration, insert an empty attachment before the one
   containing the footer.  As it is unlikely that a mail client sends an
   empty attachment, heuristically it may be preferable to just not
   count it.  For single-part, body parsing must avail of encoding
   conversions as needed.  Assume identity encoding, 7bit or 8bit,
   unless otherwise directed by an Original-Content-Transfer-Encoding:
   field.

   At the end of the first pass, the verifier knows how prospect
   signatures did.  Let's recall that DKIM signature verification
   results from two independent operations, steps 3 and 4 in
   Section 6.1.3 of [RFC6376].  The signature in the "b=" tag depends on
   the header, while the body hash in the "bh=" tag depends on the body:

   *  If the signature "b=" did not verify and the set of Original-*
      fields is not empty, then it is worth to try and re-canonicalize
      the header using the values in the set of Original-* fields.

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   *  If the body hash "bh=" did not match and a footer was found, then
      it is worth to try and re-canonicalize the body excluding the
      footer.

   None, one, or both of the above operations are performed in the retry
   pass.

   On writing Authentication-Results, if a prospect signature verifies
   after replacing the From: field, the verifier writes a prominent,
   well documented "reason" in the relevant resinfo stanza (Section 2.2
   of [RFC7601]).  For example:

   Authentication-Results: example.com;
     spf=pass smtp.mailfrom=list.example;
     dkim=pass reason="transformed" header.d=example.org;
     dkim=pass (whitelisted) header.d=list.example;
     dmarc=pass header.from=example.org;

   That way, reversion elements can be easily recognized and parsed by
   downstream agents.

5.  Actors Roles and Compliance

5.1.  Original Signer

   Signers who wish their users to be able to participate to mailing
   lists can adopt rules apt to ease MLM transformations reversion.  A
   sender might abide by the following rules for all outgoing mail, or,
   if it had some idea which recipients are MLMs, could apply the rules
   only to mail to those recipients.

   A first rule is the addition of an Author: or Original-From: header
   field with a value identical to the one signed in From:.  Author: is
   defined for exactly this purpose by [RFC9057].  Original-From: is
   defined by [RFC5703] in the context of Sieve Email Filtering.  As
   Sieve operates at time of final delivery, DKIM verifiers which act at
   the time of message transit can reliably use it.

   If Author: is also signed by the author's domain, a reverting
   verifier can send DMARC feedback reports also to the original signer,
   even though From: was rewritten.

   Note that [RFC7960] suggests that ReSenders can add an Original-From:
   too.  Likewise, [RFC9057] suggests that Author: can be added by
   Mediators.

   Other generic rules to ease reversion are as follows:

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   *  DKIM signatures must deploy the "relaxed" canonicalization, at
      least for the header, since MLMs may reflow header fields.

   *  The quoted-printable encoding must not be used for the body of
      single-part text/plain messages, as it is impossible to guess
      original soft line breaks after re-encoding.  Base64 is much more
      robust.

   *  Single-part text/plain messages encoded as base64 must follow a
      constant column width of 76 characters.  The encoding must be
      advertised by adding a new header field as follows:

     Original-Content-Transfer-Encoding: base64

   *  If the original Subject: begins with a tag (not Re: followed by a
      tag), its value must be copied to an Original-Subject: header
      field.  The latter field is also defined by [RFC5703], and the
      same usage considerations hold.

   *  Content-Type: and Content-Transfer-Encoding: are fields related to
      the data form.  Section 5.4.1 of [RFC6376] does not recommend to
      sign them.  Mailers often rewrite them, so they must not be signed
      if signature robustness is a concern.  If signed, their Original-
      counterpart should be set too.

   *  When signing Cc: or Reply-To:, add their Original- counterparts to
      the header, as MLMs are likely to change them, especially if they
      have multiple mailboxes.

   *  Original-*: fields with an empty value stand for non-existing
      counterparts.

   *  Except as noted above for Author:, Original-* fields need not be
      signed.  If original signatures can be recovered, that suffices;
      otherwise, the unverified signature is irrelevant.

5.2.  MLM

   Participating MLMs must not operate transformations other than those
   listed in Section 3.  Since DKIM is MIME-agnostic, attention must be
   paid to preserve the exact preamble and epilogue of the original MIME
   structure.

   MLMs must apply their own DKIM signature.  The presence of signatures
   by multiple domains can be used by verifiers to infer that a message
   underwent MLM transformations.

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   It is recommended that MLMs add a mailbox entry to Reply-To: or Cc:
   in order to ease off-list replies as well as to allow transformation
   reversion.

   MLMs which collect posts from other MLMs must avoid to add their own
   footer and subject tag.  Transformation reversion cannot be stacked.
   A second-level MLM can modify or replace the content of previous
   transformations.  Attention must be paid to not exceed tag and footer
   length limits.

5.3.  Verifier

   Attempts to verify original signatures can be done as outlined in
   Section 4.  The reversion must not alter the messages signed and
   distributed by MLMs, except for adding an Authentication-Results:
   header field, and possibly an Author: or an Original-From: field.

   If an original signature with rewritten From: is recovered, the
   verifier must make sure that the original value of From: is written
   out in a field agreed upon by downstream agents, typically Original-
   From:.  An MDA downstream may combine the Authentication-Results:
   with that field to restore the original value of From:.  This is the
   only recommended modification to the distributed message.  It must be
   done after any dot-forward processing, so that external verifiers
   receive the message as distributed by the MLM, and can revert
   transformations by themselves.

   If the Author: field is found and if it was included in the h= tag of
   the original signature, the corresponding DMARC record may be looked
   up and its "rua=" and "ruf=" tags considered for feedback reports,
   whatever the result.  However, if applying DMARC policies is
   considered, it is the From: field which rules, not the Author:,
   Original-From:, Sender:, nor any other mailbox domain.

6.  Security Considerations

   Rewriting the From: header field is a treacherous modification to
   messages.  It fosters the belief that the display name of a mailbox
   is more true than the angle address.  A belief further consented by
   the tendency to not even display the latter.  Bad actors take
   advantage of this belief by displaying the names of trusted
   institutions paired with trash email addresses hidden between angle
   brackets.  That trick defeats DMARC's purpose.

   It is out of this document's scope to suggest how mail user agents
   (MUAs) could counter phishing by highlighting security indicators
   (for the extent that indicators can actually help preventing phishing
   attacks).  Let's just note that MUAs have to cope with MLM and

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   phishing alike, which makes it hard to devise a pattern to tell apart
   one from the other without getting involved with the reputation of
   the specific domains.

   By safely restoring munged From: to the original value, that contrast
   is eliminated.  Then, perhaps, deceptive mailboxes might become
   amenable to some kind of efficient indication.

   Of course, MLM role can be played by miscreants as well.  However,
   replaying a signed message, even with revertible transformations, has
   more limits than forging scam messages anew.  Therefore, the risk
   introduced by easing transformation reversion is considerably lower
   than that of not signing, or of keeping DMARC policy at "none".

   An unlikely risk is that of a fake MLM sending messages with Author:
   signed by a broken signature in order to trick a reverting verifier
   into sending false feedback reports.

   Compared with the use of "l=" tag (Section 8.2 of [RFC6376]), the
   fact that footers are written in plain text removes the main security
   objection about footer additions.  Namely, footers cannot completely
   replace the original content in the end recipient's eyes by
   exploiting lax HTML parsing in the MUA.

   Still, a footer can contain dangerous URLs and deceiving text.  That
   possibility has to be countered by usual mail filtering and savvy
   behavior.

7.  IANA Considerations

   IANA maintains the "Message Header" registry with several
   subregistries.  IANA is asked to make the assignments set out in the
   following section.

7.1.  Provisional Message Header Field Names

   IANA is asked to create new entries in the "Provisional Message
   Header Field Names" registry as follows.

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    +===================+==========+==========+==========+===========+
    | Header Field Name | Template | Protocol | Status   | Reference |
    +===================+==========+==========+==========+===========+
    | Original-Content- |          | mail     | standard | this I-D  |
    | Transfer-Encoding |          |          |          |           |
    +-------------------+----------+----------+----------+-----------+
    | Original-Reply-To |          | mail     | standard | this I-D  |
    +-------------------+----------+----------+----------+-----------+
    | Original-Cc       |          | mail     | standard | this I-D  |
    +-------------------+----------+----------+----------+-----------+

                                 Table 1

8.  Experimental Goals

   Mailing lists are a tool of the trade in a number of communities,
   including IETF.  In order to preserve the confidence in it as is
   provided by the end-to-end nature of mail identifiers also in the
   face of DMARC disruption, the feedback this experiment seeks can be
   sketched in the following goals:

   *  Are signers or MUA willing to add the Author: field?

   *  How much change is needed to adapt more DKIM verifiers to revert
      the restricted set of transformations described here so as to
      verify original signatures and restore the original value of
      From:?

   *  Alternatively, a receiver could trust an Authenticated Received
      Chain (ARC) [RFC8617] and use the value of Author: to restore the
      original value of From:.  There are some outstanding loose points
      in doing so, but how do these two possibilities compare?  What is
      the security vs. ease of setup tradeoff?

   *  What are the incentives to make the changes?

9.  References

9.1.  Normative References

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/info/rfc2045>.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <https://www.rfc-editor.org/info/rfc5321>.

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   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [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,
              <https://www.rfc-editor.org/info/rfc6376>.

   [RFC7489]  Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
              Message Authentication, Reporting, and Conformance
              (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
              <https://www.rfc-editor.org/info/rfc7489>.

   [RFC9057]  Crocker, D., "Email Author Header Field", RFC 9057,
              DOI 10.17487/RFC9057, June 2021,
              <https://www.rfc-editor.org/info/rfc9057>.

9.2.  Informative References

   [RFC3676]  Gellens, R., "The Text/Plain Format and DelSp Parameters",
              RFC 3676, DOI 10.17487/RFC3676, February 2004,
              <https://www.rfc-editor.org/info/rfc3676>.

   [RFC4201]  Kompella, K., Rekhter, Y., and L. Berger, "Link Bundling
              in MPLS Traffic Engineering (TE)", RFC 4201,
              DOI 10.17487/RFC4201, October 2005,
              <https://www.rfc-editor.org/info/rfc4201>.

   [RFC5703]  Hansen, T. and C. Daboo, "Sieve Email Filtering: MIME Part
              Tests, Iteration, Extraction, Replacement, and Enclosure",
              RFC 5703, DOI 10.17487/RFC5703, October 2009,
              <https://www.rfc-editor.org/info/rfc5703>.

   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,
              DOI 10.17487/RFC5598, July 2009,
              <https://www.rfc-editor.org/info/rfc5598>.

   [RFC6377]  Kucherawy, M., "DomainKeys Identified Mail (DKIM) and
              Mailing Lists", BCP 167, RFC 6377, DOI 10.17487/RFC6377,
              September 2011, <https://www.rfc-editor.org/info/rfc6377>.

   [RFC7208]  Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in Email, Version 1", RFC 7208,
              DOI 10.17487/RFC7208, April 2014,
              <https://www.rfc-editor.org/info/rfc7208>.

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   [RFC7601]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 7601,
              DOI 10.17487/RFC7601, August 2015,
              <https://www.rfc-editor.org/info/rfc7601>.

   [RFC7960]  Martin, F., Ed., Lear, E., Ed., Draegen, T., Ed., Zwicky,
              E., Ed., and K. Andersen, Ed., "Interoperability Issues
              between Domain-based Message Authentication, Reporting,
              and Conformance (DMARC) and Indirect Email Flows",
              RFC 7960, DOI 10.17487/RFC7960, September 2016,
              <https://www.rfc-editor.org/info/rfc7960>.

   [RFC8617]  Andersen, K., Long, B., Ed., Blank, S., Ed., and M.
              Kucherawy, Ed., "The Authenticated Received Chain (ARC)
              Protocol", RFC 8617, DOI 10.17487/RFC8617, July 2019,
              <https://www.rfc-editor.org/info/rfc8617>.

   [zdkimfilter]
              "zdkimfilter", <https://www.tana.it/sw/zdkimfilter/>.

Appendix A.  Examples

   In the examples that follow, the first character of each wrapped line
   of DKIM-Signature: fields should be a TAB.  For editorial reasons, it
   is rendered as four spaces.  While visually there is little
   difference, those signatures won't verify unless replacing them with
   a TAB.

   To verify the examples, public keys can be set as follows:

   s._domainkey.example.com IN TXT ( "v=DKIM1; g=*; k=rsa; "
   "p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCqlye7m5zLLXoIpBp2OO05LNMqK"
   "u0zKowoHOpyRpviOVqOaNCk5uZ+wY00JwrKbt5u1G1ghuXsFkFkl0h00LBurz7ivyZH"
   "3LohSWOZ8okgR+8kuGu9GHtQ+MqgRd16tlCF8PlWS2kGaBQKua1zk+ZCDwFy82Uo5G2"
   "1nu/+Nn2sUwIDAQAB" )

   s._domainkey.lists.example IN TXT ( "v=DKIM1; k=rsa; "
   "p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDgnLb2TZ6KECBMBo9ZLqDFt4ZBz"
   "NHFrgBj/LVJVFU8IQP8uH4G8Pj0mEHRo1qpf0vuFI2HVpe/3NhzkT4Ay/1ZIIsxY754"
   "f2thlhBvKh4AAgZFmzRvA3aZs6Tb/ERmD+a51liEMFaTOmY4mWeLi9wOM51usQ9Q65i"
   "8IP/vjHM3rQIDAQAB" )

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A.1.  Single-part plain text

   Base64 encoding has to be decoded in order to locate the footer.  The
   original encoding was text/plain, this can be inferred by the
   verifier from the absence of an Original-Content-Transfer-Encoding:
   field.  The original body hash will match after decoding and removing
   the footer.  Note that an "l=" tag couldn't have done the trick in
   this case.

   Received: from lists.example by subscriber.example.org with ESMTP
   DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=lists.example; s=s;
       t=1603901305; bh=MjC5ikx26j8beyDJiz7Rk/4W+ppdGOmqh6koz0gLa8o=;
       h=Date:From:To:Subject;
       b=PNIYHGd7aytHEvew44WRpSfl4Py3c/9mKjovvQ1ps/xdpkl1/z+gWeu8e8ZmR7gdE
        iT2TsJ7ni3Lfp5oUpGCko5MvCoqcKX7Zmq3CmXTxRTwwvVZrAp/ir8UTvG+rJFnyEZ
        Yi3dSTX4rKe2LotyLkqcs+/uXaWEADbqcBp/9iHo=
   Received: from mail.example.com by lists.example with ESMTP
   DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=example.com; s=s;
       t=1603889142; bh=hrDXocZNPy1+eUFYIk1PVRKa6mUMb8+ql9CFNABacww=;
       h=Date:From:To:Subject;
       b=YFLwvvW5bGbE5HpJwBM1JoL1F9b8AxdVFlwE/vOkL0p/pPpr7g9KnPXqwoEXZgFI0
        /kkTHK/Afy4gaWZQfwDZ77LuxYSMFjwpNorSc0YEGzHYzLCN7rL1e+xE7B7kOCThiq
        ebaMdcaHeZF6QUmWcUkEj8LVkxrvWi+bTzd3RnaA=
   Original-From: Author <user@example.com>
   Received: from mua.example.com by mail.example.com with ESMTPA
   Message-ID: <123456@author.example>
   Date: Mon, 28 Oct 2020 13:12:55 +0100
   From: Author <user@example.com>
   MIME-Version: 1.0
   To: MLM@lists.example
   Subject: [example] Check simple MLM message
   Content-Type: text/plain; charset=us-ascii
   Content-Transfer-Encoding: base64

   VGhpcyBpcyBhIHBsYWluIHRleHQgbWVzc2FnZSBzdWJtaXR0ZWQgdG8gYSBtYWlsaW5nIGxpc3Qu
   ClRoZSBtYWlsaW5nIGxpc3QgaXMgZXhwZWN0ZWQgdG8gYWRkIGEgZm9vdGVyIGFuZCBhIHN1Ympl
   Y3QgdGFnLgoKQmVzdApBdXRob3IKCl9fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19f
   X19fX18KdGhpcyBtZXNzYWdlIHdhcyBtb2RpZmllZCBieSBNTE0gZXhhbXBsZQphZGRpbmcgdGhp
   cyBmb290ZXIgYW5kIHRoZSBzdWJqZWN0IHRhZwoobm90ZSB0aGF0IGw9IGlzIG5vdCBzZXQpCg==

A.2.  Multipart added

   When the original message has a MIME structure, MLMs can append an
   entity.

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   Received: from lists.example by subscriber.example.org with ESMTP
   DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=lists.example; s=s;
       t=1603974193; bh=sEPYSlJlh90leqy5+63oPn1iU+9P684R92cZHXa9ENw=;
       h=Date:From:To:Subject;
       b=fTSAMcaEatofQCuAeUhlTXmVl5j9bPbwWgc84NWtoSt5zT+SSNp37DTzhYIGHozEk
        bpldArGQ+GygJE1b2witi6NctBd1O/xsUwDcJQxDXkF63QlCcalbKWypHZOhRqncUQ
        zgUzdcuYgqTYMJ0NoTP8fqu0HdgmjD2LJXjV3pVI=
   Old-Authentication-Results: lists.example;
     dkim=pass header.d=example.com
   Received: from mail.example.com by lists.example with ESMTP
   DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=example.com; s=s;
       t=1603973996; bh=eWqyE53pjRVCFGyHY1zGQTkCEvucN1vNN4cTcWk90WU=;
       h=Date:From:To:Subject;
       b=LGP1M3IX6XORfLs8HRLCFOcymzsPn+8+ljgQlmeNlCC/2Cl1+aBDCIEnzWI0pceCb
        zg32vFfEeryvRDHB1L1K4rrKCEznvO0J3p1xkUPEWpSpzxUGw+PK9KA9ePZ5qdz7cI
        /hXf7zjebznNdDQJnxajf7QHnx1tXmxijsJ1jiGQ=
   Old-Authentication-Results: example.com; auth=pass (details omitted)
   Original-From: Author <user@example.com>
   Received: from mua.example.com by mail.example.com with ESMTPA
   Message-ID: <123456@author.example>
   Date: Mon, 28 Oct 2020 13:12:55 +0100
   From: Author via MLM <MLM@lists.example>
   MIME-Version: 1.0
   To: MLM@lists.example
   Subject: [example] Check simple MLM message
   Content-Type: multipart/mixed; boundary=original-boundary

   Original preamble must be preserved!

   --original-boundary
   Content-Type: text/plain; charset=us-ascii
   Content-Transfer-Encoding: 7bit

   This is a plain text message submitted to a mailing list.
   The mailing list is expected to add a footer and a subject tag.

   Best
   Author

   --original-boundary
   Content-Type: image/png
   Content-Transfer-Encoding: base64

   iVBORw0KGgoAAAANSUhEUgAAAAYAAAAGCAYAAADgzO9IAAAABHNCSVQICAgIfAhkiAAAAAlwSFlz
   AAAHKgAAByoB49HU1wAAABl0RVh0U29mdHdhcmUAd3d3Lmlua3NjYXBlLm9yZ5vuPBoAAAB+SURB
   VAiZNcGxDYUgAEXRhxTMYWLFVlDTOAUjOIEzWDqEC1igCQ0LSLi/+ueotUZKieu6uO+bdV2ptaLz
   PDHGsG0b+74jieM40Pd91Fr5K6UAMC3LImutxhgaY8g5p3meNcUYFULQ+756nkchBMUYpd47OWe8
   93jvyTnTe+cHXqRZbKSV4EoAAAAASUVORK5CYII=

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   --original-boundary
   Content-Tyep: text/plain

   ________________________________________
   this message was modified by MLM example
   adding this footer and the subject tag
   (note that l= cannot work in this case)

   --original-boundary--

A.3.  Multipart wrapped

   When the original body is multipart/alternative, MLMs have to wrap
   the whole body into the first entity of a multipart/mixed structure.
   Indeed, appending an entity to a multipart/alternative would result
   in it either hiding or being hidden by the existing ones.

   Received: from lists.example by subscriber.example.org with ESMTP
   DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=lists.example; s=s;
       t=1603962061; bh=n4/RahgnfVg7htgJtCr7TwEW4eKA1O5oiNaQFA5HU+A=;
       h=Date:From:To:Subject;
       b=RJlq/Fu40AC1hdJfljd+KPU69Vq2M7capbGQyEMhDWvaN7xDPJdXotwnTwiz91iZY
        5W3ITY7YXKHsWweLxu1Rph3ST3bbYQ1cifztpmtu4VPifBkm9MAe7OMDLHhk5ua9YL
        VzJOsXieiIw5a8JhOsr6F/05/K05kNiEXvuLgKd8=
   Old-Authentication-Results: lists.example;
     dkim=pass header.d=example.com
   Received: from mail.example.com by lists.example with ESMTP
   DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=example.com; s=s;
       t=1603961679; bh=XiCPbOV1vcu2Q2TyEUOuT4SMun2AjYj/Va6KRPa1lv0=;
       h=Date:From:To:Subject;
       b=gvM5grV2dbtinFMLcExv+gMATILzY+c8RY7QPVBJSFohH5HMgOLwrgSH8uwOcZxq0
        FoXtBcHnukonqo97l8nY0faHi0Dp0LAmqn9e4ijwXw9IWwhFuUiCwICRaLEzrNUVBN
        TWtzkQKnHpEXnPGBD7Q9f924mBe+eZsDyRc41ZvQ=
   Old-Authentication-Results: example.com; auth=pass (details omitted)
   Original-From: Author <user@example.com>
   Received: from mua.example.com by mail.example.com with ESMTPA
   Message-ID: <123456@author.example>
   Date: Mon, 28 Oct 2020 13:12:55 +0100
   From: Author via MLM <MLM@lists.example>
   MIME-Version: 1.0
   To: MLM@lists.example
   Subject: [example] Check simple MLM message
   Content-Type: multipart/mixed; boundary=MLM-boundary

   This is the MLM preamble, not signed by Author.

   --MLM-boundary
   Content-Type: multipart/alternative; boundary=original-boundary

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   Original preamble must be preserved!

   --original-boundary
   Content-Type: text/plain;

   This is a plain text message submitted to a mailing list.
   The mailing list is expected to add a footer and a subject tag.

   Best
   Author

   --original-boundary
   Content-Type: text/html;

   <p>This is a plain text message submitted to a mailing list.
   The mailing list is expected to add a footer and a subject tag.

   <p>Best<br>
   Author<br>

   --original-boundary--

   Original epilogue

   --MLM-boundary
   Content-Type: text/plain

   ________________________________________
   this message was modified by MLM example
   adding this footer and the subject tag
   (note that l= is not set)

   --MLM-boundary--

   MLM epilogue

Author's Address

   Alessandro Vesely
   v. L. Anelli 13
   20122 Milano MI
   Italy

   Email: vesely@tana.it

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