It's just a string of characters.
I don't think "it's just characters" is a one-simple-trick.
"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.)
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.
Using something leaked always carries some inherent risk.