There was a legal battle in the early 2000s about circumventing DVD copy protections. The DVD CSS (content scramble system). The court ruling declared that the program was a 'copyright circumvention device'. It was illegal to distribute the source code. This lead to people creating illegal t-shirts with the source printed on it.

http://en.wikipedia.org/wiki/DeCSS

Later the same thing happened for Blu-ray. Only it was an encryption key that was discovered and subsequently illegal to distribute under the DMCA.

http://en.wikipedia.org/wiki/AACS_encryption_key_controversy