bug#45631: 27.1; regression: message-forward-ignored-headers no longer applies when forwarding as MIME

[Daniel Kahn Gillmor <dkg@fifthhorseman.net>]

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Hi Lars--

On Sun 2021-01-10 16:18:23 +0100, Lars Ingebrigtsen wrote:
> Looking over that mess again, I think my analysis of when that variable
> is used, and when it's supposed to be used, was wrong.  I've now make it
> respect that variable even if message-forward-show-mml is nil.  The only
> instance it won't be used is if message-forward-show-mml is `best', and
> we're forwarding an encrypted/signed message.

Thanks for taking a look at this.  I'm trying to understand the
rationale for *not* trimming headers when message-forward-show-mml is
`best' and we're forwarding an encrypted/signed message.

If the headers being trimmed are strictly in the header section of the
forwarded message, then they aren't in the cryptographic envelope [0],
which means that they aren't implicated in either a standard PGP/MIME or
S/MIME signature or encryption.

Is the expectation that headers of internal parts of the message are
being trimmed (in which case, they might be implicated in the signature
or encryption)?  or, is there some situation i'm missing where they
might have an impact on the cryptographic structure?

      --dkg

[0]
    https://www.ietf.org/archive/id/draft-dkg-lamps-e2e-mail-guidance-00.html
    has definitions of "cryptographic envelope" and other
    hopefully-useful concepts and terminology.

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