(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.HDCP devices are generally divided into three categories: A repeater accepts content, decrypts it, then re-encrypts and retransmits the data.
For each set of values, a special public key called a KSV (Key Selection Vector) 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 2) according to a KSV received from another device.
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.
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.In November 2011 Professor Tim Güneysu of Ruhr-Universität Bochum revealed he had broken the HDCP 1.3 encryption standard.On September 14, 2010, Engadget reported the release of a possible genuine HDCP master key which can create device keys that can authenticate with other HDCP compliant devices without obtaining valid keys from The Digital Content Protection LLC.That makes the encryptor (receiver) unaware of whether it encrypts or decrypts the key.Further, the input parameters for the XOR and the AES above it are fixed from the receiver side, meaning the transmitter can enforce repeating the same operation.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.