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From: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
To: Lars Ingebrigtsen <larsi@gnus.org>
Cc: Robert Pluim <rpluim@gmail.com>, 45631@debbugs.gnu.org
Subject: bug#45631: 27.1; regression: message-forward-ignored-headers no longer applies when forwarding as MIME
Date: Tue, 12 Jan 2021 09:57:48 -0500	[thread overview]
Message-ID: <87ft36jj83.fsf@fifthhorseman.net> (raw)
In-Reply-To: <87czybh5nl.fsf@gnus.org>

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On Mon 2021-01-11 16:09:18 +0100, Lars Ingebrigtsen wrote:
> Well, we'd have to include the relevant headers at a minimum:
>
>   Content-Type: multipart/signed; boundary="=-=-=";
> 	micalg=pgp-sha256; protocol="application/pgp-signature"

I agree, but if the user is stripping the Content-Type header, i think
they're going to break a lot more than digital signatures or encryption
(imagine what that does to a multipart/alternative message).  I think
that stripping Content-Type is more of a case of "don't do that, then".
Maybe we even want to warn if the user tries to strip any of the
Content-* headers more generally.

> But this code is almost a couple of decades old, and I have no idea what
> the thought process behind this was at this date.  Anybody know?

As long as the code doesn't attempt to strip *internal* MIME headers
(that is, headers of subparts of the MIME structure) i think it should
be safe to apply it to the forwarded message.  

Note also that if we care about breakng cryptographic signatures more
generally, DKIM signatures are *more* likely to break if headers are
stripped than PGP/MIME or S/MIME, as DKIM is capable of covering headers
directly.  Even given that concern, i think the most we'd want the
"best" setting to do to constrain header stripping would be to compare
the stripped version of the file to the non-stripped version -- if the
non-stripped version passes DKIM validation, but the stripped does not,
then either produce a warning message about DKIM signature breakage or
(if in an interactive mode) prompt the user about whether they want to
apply the filter or not.

      --dkg

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  reply	other threads:[~2021-01-12 14:57 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-03 17:12 bug#45631: 27.1; regression: message-forward-ignored-headers no longer applies when forwarding as MIME Daniel Kahn Gillmor
2021-01-05 13:06 ` Robert Pluim
2021-01-05 23:25   ` Daniel Kahn Gillmor
2021-01-06 10:31     ` Robert Pluim
2021-01-10 15:18       ` Lars Ingebrigtsen
2021-01-10 17:25         ` Eli Zaretskii
2021-01-11 15:04           ` Lars Ingebrigtsen
2021-01-10 21:41         ` Daniel Kahn Gillmor
2021-01-11 15:09           ` Lars Ingebrigtsen
2021-01-12 14:57             ` Daniel Kahn Gillmor [this message]
2021-01-13  9:25               ` Robert Pluim
2021-01-13 17:35                 ` Daniel Kahn Gillmor
2021-01-18 15:50               ` Lars Ingebrigtsen
2021-01-18 17:43                 ` Daniel Kahn Gillmor
2021-01-19  3:12                   ` Lars Ingebrigtsen
2021-01-19 16:19                     ` Daniel Kahn Gillmor
2021-01-20 16:17                       ` Lars Ingebrigtsen
2021-01-20 19:31                         ` Daniel Kahn Gillmor
2021-01-21 15:46                           ` Lars Ingebrigtsen

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