Re: [openpgp] PGP/MIME, RFC 3156, Require empty boundary preamble lines

[Daniel Kahn Gillmor <[e-mail]>]

On Mon 2021-09-20 11:57:53 +0200, Kai Engert wrote:
> Perhaps a future revision of RFC 3156 could state that preamble lines 
> inside the signed payload MUST be empty - in the hope that no transport 
> agent will add their own preamble lines.

RFC 3156 describes (among other things) how to use OpenPGP in the
context RFC 1847 multipart/signed messages.

RFC 1847 §2.1 already says:

>   The entire contents of the multipart/signed container must be treated
>   as opaque while it is in transit from an originator to a recipient.
>   Intermediate message transfer agents must not alter the content of a
>   multipart/signed in any way, including, but not limited to, changing
>   the content transfer encoding of the body part or any of its
>   encapsulated body parts.

If the MTA that mangled the sample message is not treating the signed
message content as opaque, they are already inviolation of RFC 1847.
Making a revision to make those MTAs "more" in violation seems unlikely
to cure the problem.

The best we can do here, i think, is to encourage MUAs to generate
minimally-fancy MIME messages (i.e., empty preamble and postamble seem
the simplest possible, and therefore least likely to be tampered with,
since stripping of preamble and postamble seem like the most likely form
of tampering) and encourage MTAs to comply with the now
quarter-century-old standards that were surely available to them at the
time the current operating version was released.

   --dkg