ADSK: The Additional Decryption Subkey

refactor of "big brother key", but now as a subkey, so technically it's easier to manage in valid use cases. previously it was too heavy handed i guess.

lots of trickery to reduce size. wonder how it impact attack times...

if anyone have any valid reason for this, simply encrypting twice once for target then for archive/group/etc would probably be better for everyone. but what do i know?