Bijective padding is optimal for certain file distributions. In particular if
the probability of transmission of a file of length N is 256 times
the probability of a file of length N - 1, it would be hard to improve on this
In practice files rarely follow this distribution. Usually the probability
of file transmission does not depend very strongly on file length.
Under such conditions, no deterministic padding algorithm will work terribly well.
If a genuine random source is freely available, then what is needed under those
circumstances is a form of random padding.