System for encrypting broadcast programs in the presence of compromised receiver devices

摘要

A digital broadcast system provides secure transmission of digital programs to in-home digital devices even when some of the devices are unauthorized. A matrix of device keys Sj,i is provided, wherein "i" is a key index variable indicating a position in a key dimension of the matrix and "j" is a sets index variable indicating a position in a sets dimension of the matrix. Each in-home device is assigned plural device keys from the matrix, with one and only one device key for each key index variable "i" being assigned to a device. To generate a session key for a broadcast program, session numbers xi are encrypted with all device keys Sj,i to generate a session key block which is decrypted by the in-home devices and used to generate a session key for decrypting the program. If one of the devices is a compromised device, at least one of the session numbers is a dummy number that is encrypted and decrypted by the corresponding compromised device key, with the resulting session key being useless in decrypting the program.