Right, that’s what I was getting at with my parenthetical. Obviously the work has to have an owned copyright in order to be protected by copyright law.
If you mean that you might be able to decrypt a copyrighted work because you used that same encryption method on a non-copyrighted work, then definitely not. The work under protection will be considered. (Otherwise, I am unsure what you meant.)
From what I recall, it was the actual protection method that was protected by DMCA - when DVD protection was cracked it was forbidden to distribute a particular section of code so they just printed it on a Tee-shirt to troll the powers that be.
