A hacker claims to have discovered a cryptographic key that can be used to circumvent copy restrictions on HD DVD and Blu-ray movies.

The key, which was published Sunday on the Doom9.org discussion forum, is a further step toward undermining the next-generation AACS (Advanced Access Content System) encryption system used to copy-protect high-definition media.

The hacker, going by the name of Arnezami, said he discovered the key by examining what was happening in his computer's memory while it processed an HD DVD video.

A spokeswoman for the group that sets the AACS specification, called the AACS Licensing Administrator, said Arnezami's claims were being investigated but declined to provide further comment.

In late December, a different hacker, named Muslix64, posted a software program that could decrypt high-definition movies. Users needed to first enter another type of encryption key, called the "volume key," for the software to work. More than 100 of these volume keys have since popped up, allowing users to freely copy such films as King Kong, Mission: Impossible and Jarhead.

The publication of this latest key, called a processing key, gives users a much easier way to figure out the volume keys they need in order to make movie copies with the HDDVDBackup software, according to Arnezami.

Introduced in April 2005, AACS is supported by media and technology companies such as Microsoft, Matsushita Electric Industrial (Panasonic), Sony, Toshiba, Walt Disney and Warner Bros.

The encryption system is designed to be more robust than the CSS (content scrambling system) encryption scheme used by DVDs, which was completely cracked in late 1999.