Does it need trusted third party? Design of buyer-seller watermarking protocol without trusted third party

Buyer-seller watermarking protocol is a combination of traditional watermarking and fingerprinting techniques. For example, in applications where multimedia content is electronically distributed over a network, the content owner can embed a distinct watermark (a fingerprint), in each copy of the data that is distributed. If unauthorized copies of the data are found, then the origin of the copy can be determined by retrieving the unique watermark corresponding to each buyer. Recently, Ju and Kim proposed an anonymous buyer-seller watermarking protocol, where a buyer can purchase contents anonymously, but the anonymity can be controlled. They used two trusted parties: the watermark certification authority and the judge. The significance of this protocol is that it offered anonymity to watermarking protocol. But this protocol has the problem that honest buyers can be found as guilty, because sellers can recreate the same contents as the buyer's one if he/she colludes with the watermark certification authority and the judge. Thus this scheme must assume existence of the trusted third parties for its security. In this paper, we show shortcomings of this protocol and suggest a buyer-seller watermarking protocol that provides security of buyers and sellers without trusted third party.