Random padding

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.

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 length.