Re: [dmarc-ietf] Apropos of the de-munging draft

[Alessandro Vesely <[e-mail]>]

On 2020-07-29 1:41 a.m., John R Levine (quoting Jesse) wrote:
> 
>> I think that draft-kucherawy-dkim-transform-02 is getting at what
>> I was originally thinking. In my opinion, MLMs will *always* need
>> to munge, because they will never know if an arbitrary receiver
>> will trust their non-munged mail.  Giving the receivers a way to
>> un-munge (if they can and/or want and/or trust) would be a
>> productive path forward out of this situation.> 
> We already have a couple of ways to do reversible message munging,
> starting with MIME message wrapping. In principle it works fine, in
> practice it's awful because MUAs don't show wrapped messages
> consistently and often in ways that are painful, e.g., you can see the
> original author address but there's no button you can push to respond
> to it.
> 
> Unwrapping a MIME attachment is a lot easier than the proposed DKIM
> unmunging but I doubt either is going to show up in MUAs any time
> soon. Perhaps you could do it in a mail gateway.


Looking at the steps required to carry out the proposed unmunging, "a lot easier" doesn't seem to be an accurate measurement.  Actually, reversing the tf=footer is simpler than unwrapping a message/rfc822 attachment.[*]  The major difficulty is for MLMs to produce modifications that consist of the allowed transformations _only_.  Perhaps, it is a lot easier to create a message/rfc822 without breaking its signatures...?

From: rewriting in particular should be added to the set of allowed transforms.  The MLM should make sure the Author: field mirrors the original From: exactly.  Then it rewrites From:.  It may seem redundant to set a tf= tag on the one hand and undo transforms on the other, since a munged From: is enough to pass.  However, if receivers send aggregate reports back to the MLM, we can hope that one day they'll all succeed and the MLM can stop rewriting From:.

In the interim, an MX which verified the original signature up to allowed transforms can replace the value of From: with that of Author:.  This action is legitimate if all agree that the only reason why MLMs rewrite From: is to pass DMARC checking.  I'd leave footers and subject tags in place.

Best
Ale
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[*] https://mailarchive.ietf.org/arch/msg/dmarc/2ZN7DS5NktoyEPItZ5vzr-xd0Mc