A scheme for conditional access-based systems using index locations of DCT coefficients

Most of the existing conditional access-based systems (CAS) follow a standard methodology. The service provider shares two copies for single information (specifically image for this work). One copy is a low-resolution version which is shared in the public domain for preview purpose. The other one is a high-resolution version to be provided to the customers through a secure channel on demand (after payment). We propose a novel scheme that can be efficiently adapted in such a scenario. We analyze the images in the discrete cosine transform (DCT) domain and note that polynomials of suitable degree, representing the sorted DCT coefficients together with original index locations, can uniquely represent an image. We show that the arrangement of DCT index locations, after the actual coefficients have been sorted by magnitude, is significantly different for various images and we exploit this to design an efficient CAS-based scheme. We also show that the amount of private data, which a service provider needs to transmit through a secure channel to the customers on demand, is reduced significantly. This reduction in transmitted data makes the system apt for real-time secure applications.