The basic idea
If a source of genuinely random numbers is available - and one doesn't mind
inflating the size of the file somewhat - padding to the nearest block boundary
can be done using random padding.
This takes the form of adding between 1 and 16 bytes of random data (assuming
we are padding out a 8-bit granular file out to a 128-bit block boundary).
The exact number of bytes added has one subtracted from it, and the result is
stored where the last four bits of the padding were.
This type of random padding has been described before.
The section entitled "Dealing with the Incomplete Byte" on John Savard's
"Tying Up loose Ends" page
covers a closely related scheme.
This type of padding is optimal in the case where files are distributed in such
a manner that probability of file transmission does not depend on file