Well, I see two issues. The first issue of course is the fact that
disk blocks are randomly accessed. A single stream cipher can't be used
to encode the whole disk.
The second issue... actually a requirement in general for both stream
and block ciphers, is that you need a significant amount of random
salt in each randomly accessible unit to protect against various forms
of attack. The salt can be applied as part of the encoding/decoding
stream (it doesn't have to be all up-front), but the question is where
does one store that salt?
-Matt
