On Tue, Jan 16, 2024 at 2:03 AM Alessandro Vesely <ves...@tana.it> wrote:
> On Mon 15/Jan/2024 20:49:35 +0100 John Levine wrote: > > It appears that Scott Kitterman <skl...@kitterman.com> said: > >>I don't think that's sensible at all. It's not the place of a signing > filter to modify the message. > > A signing filter, as part of an MSA _has to_ modify the message in order > to > enhance the possibility that it is transmitted correctly. Besides usual > changes belonging to the core MSA, such as setting Date:, a signing filter > shall take care of signature breaking cases, such as lines beginning with > "from ". > I also disagree. A signing filter (or an MSA, for that matter) can also decline to sign a message that fails to meet certain criteria like, say, RFC5322. I would contend that this is the right action, since what we're doing here is securing the message, and a filter in that position shouldn't be presuming what the author meant to say. As I recall, with milter in particular, the MTA will add a missing Date field, which the filter never actually sees and thus cannot sign. The filter only sees the message exactly as it was presented to the MTA. As a result, if the message is signed, Date is not, and any verifier that thinks it ought to be will consider the signature invalid. There's another thread of concern here: For a signature to survive transit, the signing filter needs to anticipate any rewrites that will happen to the message. If "Date" is missing and it gets added, that's one thing; if "From" is rewritten outbound by the MTA (which is not uncommon), say from $HOST.$DOMAIN to just $DOMAIN, the signature may be immediately invalidated. There's a lot of complexity here that I think filter authors would just as soon leave to the MTA/MSA. A use case is when submitting co-authored articles or notes. Yes, it is > rare > to type a message four hands, but it can happen, and banning the > possibility to > correctly identify the authors is harsh. > > Thinking twice, saving the original multi-value to Author: is not enough > to > have replies reach every author. Better also use Reply-To:. > This strikes me as something that needs to be codified outside of DMARC, rather than by it. -MSK, participating
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