Generalized traitor tracing for nested codes

Nested or concatenated codes are often used for traitor tracing schemes as they require a small symbol size to accommodate a given number of users. Typically, tracing is performed on each layer of the code separately - tracing is first performed on the inner code and the information obtained is subsequently used to perform tracing on the outer code. In such situations, the collusion resistance is determined by the minimum of the coalition sizes that can be tolerated by the individual codes. Due to the small symbol size, the inner codes can tolerate a smaller number of colluders resulting in a small overall collusion resistance. Further, recovering more attacked versions does not enable identification of larger coalitions. To improve the collusion resistance, in this paper we propose to pass soft information from the inner code tracing to the outer code tracing. We demonstrate through simulations and formal analysis that the proposed technique improves the collusion resistance of tracing systems employing nested codes.