HDCP

High-bandwidth Digital Content Protection (HDCP) is a form of digital copy protection developed by Intel Corporation to prevent copying of digital audio and video content as it travels across Display Port, Digital Visual Interface (DVI), High-Definition Multimedia Interface (HDMI), Gigabit Video Interface (GVIF), or Unified Display Interface (UDI) connections. The specification is proprietary, and implementing HDCP requires a license. For DVI interfaces, HDCP is optional. HDCP is licensed by Digital Content Protection, LLC, a subsidiary of Intel. For example, high-definition digital video sources must not transmit protected content to non-HDCP-compliant receivers. Additionally, DVD-Audio content is restricted to CD-audio quality or less on non-HDCP-digital audio outputs (analog audio outputs have no quality limits). Licensed adopters cannot allow their devices to make copies of content, and must design their products in ways that "effectively frustrate attempts to defeat the content protection requirements." HDCP's stated purpose is to protect high definition content during transmission from a source device to a display device. Three systems were developed to achieve that goal, as the authentication process prevents non-licensed devices from receiving content, encryption of the actual data sent over Display Port, DVI, HDMI, GVIF, or UDI interfaces prevents eavesdropping of information and man-in-the-middle attacks and key revocation procedures ensure that devices that have been compromised and cloned can be blocked from receiving data. Each HDCP-capable device has a unique set of keys; there are 40 keys, each 56 bits long. Each KSV consists of 40 bits, i.e. one bit for each HDCP key, with exactly 20 bits set to 0 and 20 bits set to 1. Then each device adds,  its own secret keys together according to a KSV received from another device. This key exchanging procedure is known as Blom's scheme. The HDCP specifications ensure constant updating of keys, after each encoded frame. As far as the types of HDCP devices are concerned, these include three types of devices: sources, sinks and repeaters. HDCP protects data as it is transmitted between each of these devices when they are connected via HDMI or other HDCP-protected digital interfaces. Each device contains one or more HDCP transmitters or receivers, or it may contain both receivers and transmitters. Sometimes HDCP and HDMI functionality are combined into a single transmitter or receiver chip. A source has only an HDCP/HDMI transmitter. A sink has one or more HDCP/HDMI receivers. HDCP protects content using authentication and encryption. Before sending HDCP-protected data, the transmitting device initiates an authentication process to confirm that the receiver is authorized to receive the data. In the United States, the Federal Communications Commission approved HDCP as a "Digital Output Protection Technology" on August 4, 2004. The HDCP standard is more restrictive than the FCC's Digital Output Protection Technology requirement. HDCP bans compliant products from converting HDCP-restricted content to full-resolution analog form, presumably in an attempt to reduce the size of the analog hole. Prior to HDCP, digital interfaces provided the same digital content without content protection between the player and display. As HDCP was introduced as a content protection standard, many non-HDCP-compliant devices were rendered unable to display HDCP-protected content unless fitted with a device to circumvent HDCP content protection, often referred to as "HDCP strippers." These are devices that remove the HDCP information from the video signal, leaving the video playable on non-HDCP-compliant displays.

In the case of HDCP, we need a minimum of 39 device keys in order to reconstruct the secret symmetrical master matrix that has been used to compute all device keys. The most well-known attack on HDCP is the conspiracy attack, where a number of devices are compromised and the information gathered is used to reproduce the private key of the central authority.