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

Abstract

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 dev ice 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.

Dependencies are inferred from the claim text.