Can't you just add some random length data at the end. You are defeating compression a little bit, but are also making the length non deterministic. I thought pgp did that.
So long as you do not compress the random data you add it will work. Compressing it will not help the situation at all since the compression ratio will be constant for the random data. The problem is you have to then seriously negate the effects of compression.
Consider compressing "silence" in a telephone call.. You can't compress it well if you also need to have the non silence elements be indistinguishable from it by adding random noise. You must add enough random noise to cover up any compression differential, otherwise statistical artefacts still persist. That amount of random noise will be up to the maximum compression ratio you can achieve.
