But the "decompressor" here is also more or less fixed - it's the environment in which the challenger program needs to run. So, the organizer of the challenge w

But the "decompressor" here is also more or less fixed - it's the environment in which the challenger program needs to run. So, the organizer of the challenge would need to be unlucky to pick a file that is compressible for this particular decompressor. Mathematically this has non-zero (albeit very small) probability but I'd wager that even having such file it would be unfeasible to find its compressed form. Maybe the file is the third million of bytes of pi squared, but how do you find this fact?