An act of Congress enacted in 1998 which made some key changes to U.S. Copyright law, including: limitations on liability for online service providers, rules for publication and protection of copyright management information, and prohibitions against circumvention of measures to protect copyrighted material.
One of the most controversial aspects of the DMCA was the anti-circumvention provision. The DMCA made it a violation of copyright law to circumvent any technical measure that controls access to copyright protected works. For example, disabling encryption software used to prevent unauthorized access to a movie in digital form. Furthermore, it prohibits the sale, manufacture, distribution, or use of any device or technology used to circumvent protective measures.
One high profile illustration of the DMCA was the case against Napster. Napster was a file sharing platform that enabled users to share and access digital files stored across all computers running the software. Inevitably, users ended up sharing digital music files and the copyright owners sued, claiming the platform circumvented the copyrights in violation of the DMCA. The court decided in favor of the copyright owners, even though Napster had useful applications and even though much of the content was legitimately owned by users of the network.