Network Working Group                                          A. Vesely
Internet-Draft                                             November 2020
Intended status: Informational
Expires: 20 May 2021


               Mailing List Manager (MLM) Transformations
                  draft-dmarc-vesely-mlm-transform-00

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 cases where it is possible to revert MLM
   transformations and hence verify DomainKeys Identified Mail (DKIM)
   signatures that were applied at submission time.  For reliable
   results, some compliance is required of all agents involved, author
   domain signers, MLMs, forwarders, and final recipients.

   MLM transformation reversion reduces the DMARC's effects on indirect
   mail flows.

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 May 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.






Vesely                     Expires 20 May 2021                  [Page 1]


Internet-Draft             MLM Transformations             November 2020


   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 . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Revertible Transformations  . . . . . . . . . . . . . . . . .   3
     3.1.  Header Transformations  . . . . . . . . . . . . . . . . .   3
     3.2.  Body Transformations  . . . . . . . . . . . . . . . . . .   4
   4.  Outline of a Reverting Verifier . . . . . . . . . . . . . . .   4
   5.  Actors Roles and Compliance . . . . . . . . . . . . . . . . .   6
     5.1.  Original Signer . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  MLM . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.3.  Verifier  . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Permanent Message Header Field Names  . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  11
     A.1.  Single-part plain text  . . . . . . . . . . . . . . . . .  11
     A.2.  Multipart added . . . . . . . . . . . . . . . . . . . . .  12
     A.3.  Multipart wrapped . . . . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   Mailing List Managers (MLMs) can be 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 may 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.

   Domain-based Message Authentication, Reporting, and Conformance
   (DMARC) ([I-D.ietf-dmarc-dmarcbis]) 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:.  A
   deed which can be mitigated in some cases.




Vesely                     Expires 20 May 2021                  [Page 2]


Internet-Draft             MLM Transformations             November 2020


   Mailbox providers can configure their mail submission agents (MSAs)
   in order to ease MLM transformation reversion.  Or they can make it
   impossible.  It is their policy and their will.  MLM operators make a
   similar decision.  When they both agree on revertibility, a well
   equipped receiver can verify the original signatures.  The outcome is
   twofold:

   1.  Author domains receive positive 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: munging (after any external
       forwarding).

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].

3.  Revertible Transformations

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

3.1.  Header Transformations

   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.



Vesely                     Expires 20 May 2021                  [Page 3]


Internet-Draft             MLM Transformations             November 2020


   A more recent modification carried out by MLMs is From: rewriting.
   It alters the value of From: in order to pass DMARC filters.  MLMs
   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.

3.2.  Body Transformations

   Footer addition 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

   The footer begins with a line consisting exclusively of underscore
   ("_", ASCII 95) characters, at least four of them.  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.

4.  Outline of a Reverting Verifier

   The algorithm described here is implemented in a mail filter.  It
   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:




Vesely                     Expires 20 May 2021                  [Page 4]


Internet-Draft             MLM Transformations             November 2020


   *  From:

   *  Original-From:

   *  X-Original-From:

   *  Reply-To:

   *  Cc:

   These are candidates to the original mailbox.

   The verifier also collects the Subject: and any field named
   Original-* that the original signer might have set to ease the
   reversion.  At the end of the header, candidate original mailboxes
   are sorted 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.

   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.  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.






Vesely                     Expires 20 May 2021                  [Page 5]


Internet-Draft             MLM Transformations             November 2020


   *  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]).  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.
   Doing so can slightly weaken DKIM'S stiffness, and expose to the risk
   of malicious MLMs.  A sender that doesn't know which of its mail
   recipients are likely to be MLMs might abide by the following rules
   for all outgoing mail, in the expectation that few of its users
   correspondents are likely to be malicious.  A sender that had some
   idea which recipients are MLMs could apply the rules only to mail to
   those recipients.  Or a sender might apply the rules to all mail
   except that sent to recipients with poor reputations.

   A special rule is the addition of an Original-From: header field with
   a value identical to the one signed in From:.  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.

   Original-From: is special because verifiers may infer that the field
   was added by the original signer rather than by MLMs.  In that case,
   they can send DMARC feedback reports to the original signer even if
   From: was rewritten.

   Other generic rules to ease reversion are as follows:

   *  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.



Vesely                     Expires 20 May 2021                  [Page 6]


Internet-Draft             MLM Transformations             November 2020


   *  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, 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.  Mailers often rewrite them, so they should not be
      signed.  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.

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

   *  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.

   MLMs must not set the Original-From: field, which is reserved to
   original signers.  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, but only in case the Original-From:
   is missing.

   MLMs may set Original-* fields other than Original-From:, but only if
   the original message contains no Original-* field at all.  That is,
   when the author's domain is not aware of the possibility to ease MLM
   transformation reversion.






Vesely                     Expires 20 May 2021                  [Page 7]


Internet-Draft             MLM Transformations             November 2020


   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 replace the messages signed and
   distributed by MLMs, with one exception detailed in the next
   paragraph.  Only the result of the verification is written out.

   If an original signature with rewritten From: is recovered, the
   verifier must make sure that an Original-From: field with the
   verified mailbox is written out.  An MDA downstream may combine the
   Authentication-Results: and Original-From: fields 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 Original-From: is set, the corresponding DMARC record may be
   looked up and its "rua=" and "ruf=" tags considered for feedback
   reports.  If DMARC policies are considered, it is the the From: field
   which rules, not the Original-From: nor any other mailbox value,
   unless verified.

6.  Security Considerations

   Rewriting the From: header field is an unwelcome modification to
   messages.  It fosters the belief that the display name of a mailbox
   is more trustworthy 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
   institution 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
   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.






Vesely                     Expires 20 May 2021                  [Page 8]


Internet-Draft             MLM Transformations             November 2020


   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".

   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.  Permanent Message Header Field Names

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

    +===================+==========+==========+==========+===========+
    | 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.  References

8.1.  Normative References




Vesely                     Expires 20 May 2021                  [Page 9]


Internet-Draft             MLM Transformations             November 2020


   [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>.

   [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>.

   [I-D.ietf-dmarc-dmarcbis]
              Gustafsson, E., Herr, T., and J. Levine, "Domain-based
              Message Authentication, Reporting, and Conformance
              (DMARC)", Work in Progress, Internet-Draft, draft-ietf-
              dmarc-dmarcbis-00, 11 November 2020,
              <https://tools.ietf.org/html/draft-ietf-dmarc-dmarcbis-
              00>.

8.2.  Informative References

   [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>.






Vesely                     Expires 20 May 2021                 [Page 10]


Internet-Draft             MLM Transformations             November 2020


   [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>.

   [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>.

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" )

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.











Vesely                     Expires 20 May 2021                 [Page 11]


Internet-Draft             MLM Transformations             November 2020


   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.

   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=;



Vesely                     Expires 20 May 2021                 [Page 12]


Internet-Draft             MLM Transformations             November 2020


       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=

   --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--





Vesely                     Expires 20 May 2021                 [Page 13]


Internet-Draft             MLM Transformations             November 2020


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

   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



Vesely                     Expires 20 May 2021                 [Page 14]


Internet-Draft             MLM Transformations             November 2020


   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
















Vesely                     Expires 20 May 2021                 [Page 15]