"Master HDCP key released: http://bit.ly/aM84GD (please mirror and RT!) #drm #hdcp #defectivebydesign," the tweet exclaims with a link to a Pastebin page that describes the key as "a forty times forty element matrix of fifty-six bit hexadecimal numbers."
Most legally purchased HD content currently requires HDCP DRM technology to play. HDCP DRM works by embedding keys to encrypt and decrypt protected data in the devices and ports; both the input (say a Blu-ray player) and the output (say an HDTV) need to be compliant. The technology allows for banning individual HDCP keys in the event that they're compromised, but this master key would let anyone to create their own source and sink key, thereby permanently bypassing the DRM protocol. In other words, users could play high-quality pirated content and the authorities would not be able to throw the kill-switch via another firmware update.
It's still not clear whether the key is indeed legitimate, and if so, how it was found. It has been proposed, however, that as few as 50 keys to different devices could let one find the master key via reverse engineering (but a straight out leak is also possible). Still, for now it means that only those technically competent enough to flash the firmware of HDCP compliant devices could render their protection ineffective.