[bulletsvshumans]

The subtle thing that I think the notion of 'illegal prime' misses is that it's not actually the prime number that is illegal.

To complete the 'illegality' you need three pieces of information used in tandem:

1) the prime number (which is innocent in its own right)

2) the decompression algorithm (also innocent in its own right)

3) the knowledge that some specific prime number, when viewed through the lens of the decompression algorithm, yields an 'illegal' interpretation. (not illegal in its own right?)

The issue only comes from all three being present. If you have #2 and #3, #1 can be the last puzzle piece needed to create the required set, but no one piece is in-and-of-itself the source of the issue.

[Zak]

I think you need a fourth element: intent to produce the illegal representation.

Of course, if the compression algorithm is one that's very commonly used like gzip or jpeg, a jury will infer intent from the totality of the circumstances and your lawyer will have a really hard time talking them out of it.