Can anyone see a better way of reliably determining which is the 'most
recent sender'? One option would be for the signing party to _remove_
the Sender: header if signing a Resent-Sender: address. I'm not sure I
like that much though. Better options?
Well, the signature can say which address it signed.
On the theory of not trying too hard, I think a signature system
should have a really simple algorithm to verify the signature, one
that doesn't go anywhere near heuristics about guessing which header
means what or was applied by whom when. If a message is too mutated
to pass, then it doesn't pass, and we tell whoever's running the
mutator to sign if they want to play.
There are a lot of forwarders that forward without mutating at all,
the pobox.com and ieee.org and .forward style ones that are typically
set up at the request of the recipient and forward to a single place.
It's also common for mail to hop from host to host within a mail
system, again without any mutation. Those are the only ones that I
think are worth trying to survive.
I agree that if our goal were purely to do per-hop validation,
something like starttls would suffice, but I also think there is
a useful middle ground between per-hop and the end-to-end goal
that s/mime and gpg have.
Regards,
John Levine, johnl(_at_)taugh(_dot_)com, Taughannock Networks, Trumansburg NY
http://www.taugh.com