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 connections. Types of connections include DisplayPort (DP), Digital Visual Interface (DVI), and High-Definition Multimedia Interface (HDMI), as well as less popular or now deprecated protocols like Gigabit Video InterFace (GVIF) and Unified Display Interface (UDI). The system is meant to stop HDCP-encrypted content from being played on unauthorized devices or devices which have been modified to copy HDCP content. Before sending data, a transmitting device checks that the receiver is authorized to receive it. If so, the transmitter encrypts the data to prevent eavesdropping as it flows to the receiver. In order to make a device that plays HDCP-enabled content, the manufacturer must obtain a license for the patent from Intel subsidiary Digital Content Protection LLC, pay an annual fee, and submit to various conditions. For example, the device cannot be designed to copy; it must "frustrate attempts to defeat the content protection requirements"; it must not transmit high definition protected video to non-HDCP receivers; and DVD-Audio works can be played only at CD-audio quality by non-HDCP digital audio outputs (analog audio outputs have no quality limits). If the device has a feature like Intel Management Engine disabled, HDCP will not work. Cryptanalysis researchers demonstrated flaws in HDCP as early as 2001. In September 2010, an HDCP master key that allows for the generation of valid device keys was released to the public, rendering the key revocation feature of HDCP useless. Intel has confirmed that the crack is real, and believes the master key was reverse engineered rather than leaked. In practical terms, the impact of the crack has been described as "the digital equivalent of pointing a video camera at the TV", and of limited importance for consumers because the encryption of high-definition discs has been attacked directly, with the loss of interactive features like menus. Intel threatened to sue anyone producing an unlicensed device.
Specification HDCP uses three systems:
Authentication prevents non-licensed devices from receiving content. Encryption of the data sent over DisplayPort, DVI, HDMI, GVIF, or UDI interfaces prevents eavesdropping of information and man-in-the-middle attacks. Key revocation prevents devices that have been compromised and cloned from receiving data. Each HDCP-capable device has a unique set of 40 56-bit keys. Failure to keep them secret violates the license agreement. For each set of values, a special private key called a Key selection vector (KSV) is created. Each KSV consists of 40 bits (one bit for each HDCP key), with 20 bits set to 0 and 20 bits set to 1. During authentication, the parties exchange their KSVs under a procedure called Blom's scheme. Each device adds its own secret keys together (using unsigned addition modulo 256) according to a KSV received from another device. Depending on which bits are set to 1 in the KSV, a corresponding secret key is used or ignored in the addition. The generation of keys and KSVs gives both devices the same 56-bit number, which is later used to encrypt data. Encryption is done by a stream cipher. Each decoded pixel is encrypted by applying an XOR operation with a 24-bit number produced by a generator. The HDCP specifications ensure constant updating of keys after each encoded frame. If a particular set of keys is compromised, their corresponding KSV is added to a revocation list burned onto new discs in the DVD and Blu-ray formats. (The lists are signed with a DSA digital signature, which is meant to keep malicious users from revoking legitimate devices.) During authentication, the transmitting device looks for the receiver's KSV on the list, and if it is there, will not send the decrypted work to the revoked device.
Uses
HDCP devices are generally divided into three categories:
Source The source sends the content to be displayed. Examples include set-top boxes, DVD, HD DVD and Blu-ray Disc players, and computer video cards. A source has only an HDCP/HDMI transmitter. Sink The sink renders the content for display so it can be viewed. Examples include TVs and digital projectors. A sink has one or more HDCP/HDMI receivers. Repeater A repeater accepts content, decrypts it, then re-encrypts and retransmits the data. It may perform some signal processing, such as upconverting video into a higher-resolution format, or splitting out the audio portion of the signal. Repeaters have HDMI inputs and outputs. Examples include home theater audio-visual receivers that separate and amplify the audio signal, while re-transmitting the video for display on a TV. A repeater could also simply send the input data stream to multiple outputs for simultaneous display on several screens. Each device may contain one or more HDCP transmitters and/or receivers. (A single transmitter or receiver chip may combine HDCP and HDMI functionality.) In the United States, the Federal Communications Commission (FCC) approved HDCP as a "Digital Output Protection Technology" on 4 August 2004. The FCC's Broadcast flag regulations, which were struck down by the United States Court of Appeals for the District of Columbia Circuit, would have required DRM technologies on all digital outputs from HDTV signal demodulators. Congress is still considering legislation that would implement something similar to the Broadcast Flag. 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. On 19 January 2005, the European Information, Communications, and Consumer Electronics Technology Industry Associations (EICTA) announced that HDCP is a required component of the European "HD ready" label. Microsoft Windows Vista and Windows 7 both use HDCP in computer graphics cards and monitors.
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