HDCP (High-bandwidth Digital Content Protection)is a specification developed by Intel Corporation to protect digital entertainment content across the DVI interface. The HDCP specification provides a method for transmitting and receiving digital entertainment content to DVI-compliant digital displays. The HDCP specification is proprietary and implementation of HDCP requires a license.
HDCP was first broken in 2001, prior to its adoption in any commercial product. Scott Crosby of CMU authored a paper with Ian Goldberg, Robert Johnson, Dawn Song, and David Wagner entitled A Cryptanalysis of the High-bandwidth Digital Content Protection System. This paper was presented at ACM-CCS8 DRM WORKSHOP on November 5th, 2001.
The paper's conclusion is that "HDCP's linear key exchange is a fundamental weaknesses. We can:
Eavesdrop on any data
Clone any device with only their public key
Avoid any blacklist on devices
Create new device keyvectors.
In aggregate, we can usurp the authority completely."
Around the same time that Scott Crosby and co-authors were writting this paper noted cryptographer Niels Ferguson independently claims to have broken the HDCP scheme, but he was unable to publish his research due to legal concerns arising from the Digital Millenium Copyright Act.
The Federal Communications Commission approved HDCP as a "Digital Output Protection Technology" on August 4th, 2004 http://hraunfoss.fcc.gov/edocs_public/attachmatch/DOC-250532A1.pdf despite its known flaws. By FCC regulation digital output protection technologies are mandatory on all digital outputs from HDTV signal demodulators as of July 1st, 2005. Analog outputs from digital receievers do not require output protections, despite the fact that converting signals between analog and digital is trivial and can result in impreceptibly degraded signals when using a Digital to analog converter.