The AACS (Advanced Access Content System) copy-protection body has threatened legal action against bloggers who posted to the web a software key that unlocks the DRM (digital rights management) encryption on some commercial HD-DVD discs.

The row erupted after Digg removed from its site stories featuring the key, which first leaked six to eight weeks ago. Many of the bloggers responsible for publishing the 32-digit code believe their free speech was infringed by the story being spiked.

The bloggers responded to Digg’s action by posting the 'HD-DVD DRM-cracking key' on other sites. It has since appeared on YouTube and even on the front of T-shirts. At the time of writing more than 800,000 separate websites feature the code.

But the AACS group has said it will take “necessary action”, both legal and technical, to prevent further breaches of copyright on HD-DVD optical discs. The code itself is no longer a threat to copyright holders, however, as it's been revoked and is now useless, according to Michael Ayers, chair of the AACS business group, the body that built the encryption software.

Ayers also explained that the copyright-protection on the HD-DVDs was not, in fact, broken. The key was leaked, he said, resulting in some HD-DVD discs being able to have their copy protection removed so they could be watched on two different software players.

The key did not affect hardware players and now it’s been revoked it will be useless, Ayers said.

Ayers added that the HD-DVD copy protection system was designed to cope with such breaches.