A related idea: It should be possible to write a zip (rar?) file that encodes a PRNG, such that it contains one zip: itself with the PRNG stepped forward one.

A related idea: It should be possible to write a zip (rar?) file that encodes a PRNG, such that it contains one zip: itself with the PRNG stepped forward one. (Even a "PRNG" that just outputs 1, then 11, then 111, etc, would be sufficient in most cases) The hard part is making sure the checksum is consistent. This would wreak havoc with any file checker that checks for recursive archive files.