[ietf-dkim] A few SSP axioms

[John Levine]

>> In my book it's the same as A signed by A.  The only concern I would have
>> is if B added content, what to do about that, I'm not sure.

I'd appreciate a concrete example where B adds and signs content without
breaking A's signature.

There's a few scenarios that have come up: 

* The first signature has l= and B adds stuff at the end.

* The first signature didn't have MIME headers and B adds them,
perhaps making a lot of the original message invisible in a newly
defined MIME part.

Note that these two are easy to defend against: always sign MIME headers,
even if there aren't any, and don't use l=.

If people think there are other scenarios where a second signer can
make signficant changes to a message without breaking an existing
signature, we have worse problems than SSP.

R's,
John