High-bandwidth Digital Content Protection - Specification

Specification

HDCP uses three systems:

1. Authentication prevents non-licensed devices from receiving content.
2. Encryption of the data sent over DisplayPort, DVI, HDMI, GVIF, or UDI interfaces prevents eavesdropping of information and man-in-the-middle attacks.
3. 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 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 (unsigned addition modulo 256) its own secret keys together according to a KSV received from another device. Depending on the order of the bits 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 protected content to the revoked device.