encryptwithmdc( <plaintext packet>, <mdc packet> )
(Note that what I showed here as "<plaintext packet>" could actually
be multiple packets long. It would be better described as an "<OpenPGP
Message>" in the syntax of section 10.2 of RFC2440.)
If we did go with something like this, how would we handle the hashing
of the packet headers on the mdc packet? They should probably be hashed
too, don't you think?
The simplest case from my point of view would be that we hash all the
encrypted material except for the last 20 bytes (assuming we settle on
a SHA-1 hash).
Hal