Stealth PGP

However it doesn't do anything special about disguising the encrypted session key.

And as Hal and I have been discussing, that's not at all an obvious problem. A filter for PGP messages cannot make them completely random for all the reasons presented. The session keys must be generated differently if the encrypted form if them is to have a flat distribution. To wit, PGP itself must change in order to make a random PGP output format. Eric