"Your honor, I wasn't copying that movie. You see, I applied a mathematical formula to the .zip file, and it just happened to produce the movie as output. Coincidence!"
(That's not to say the key is copyrightable, it's not. I think the relevant law would be the DMCA anti-circumvention provision.)
"I didn't distribute the movie, just a file that XOR'd every byte with 255!"
Technical people tend to see the law as a technical thing, where technical arguments will win. Courts are generally unamused, since every judge has years of experience with defendants who think that they've discovered one simple trick.