An asymmetric traceability scheme for copyright protection without trust assumptions

Traceability schemes have been proposed as a method to establish copyright protection of broadcast information. With asymmetric traceability, the merchant cannot frame an innocent user, while no user can abuse the system without being detected. We propose an asymmetric solution for traceability, based on the very efficient symmetric scheme of Kurosawa-Desmedt [13]. We do not make any trust assumptions about the broadcasting center or other authorities. Furthermore, we establish anonymity protection for all honest users: the identity of a user is protected, until a "fingerprint" of that user is found on a pirate decoder. We make use of well-known cryptographic techniques, such as oblivious transfer, time-lock puzzles and blind signatures. Finally, we propose a cut-and-choose technique to assure the correctness of the decryption keys.