This provision goes after the provision of hacking tools, making the tool itself illegal if its predominant purpose is to help crack copyright encryption, whether or not that’s what any particular person was using it for.

This provision targets hackers. Jon Johansen, a 15-year-old Norwegian student who cracked the CSS scrambling system for DVDs with seven lines of code, was sued under this Act and lost.

The Copyright Office gets to carve out exceptions, allowing hacking for some legitimate purposes. A recent copyright office rulemaking allowed “jailbreaking” of smartphones, for instance.

The DMCA was a 1998 compromise between copyright owners, who wanted the right to sue those who cracked encryption or copy-protection systems, and the growing Internet industry, which wanted some protection from copyright suits for material posted by others on their systems.

At the time it was passed, most observers concluded that copyright owners got the best end of the bargain, because the limitations on liability had lots of loopholes. But the limitations have turned out to be extremely important in protecting sites like YouTube and Google from infringement claims for material they link to or that other people post. In fact, RapGenius itself can exist only because of the protection of the DMCA.