Fingerprinting protocol for on-line trade using information gap between buyer and merchant

The homomorphic property of the public key cryptosystem has been exploited in order to achieve asymmetric fingerprinting such that only a buyer can obtain fingerprinted content. However, this requires many computations and a wide-band network channel because the entire uncompressed content must be encrypted based on the public key cryptosystem. In this paper, instead of the homomorphic property, we introduce the management of the enciphering keys for the symmetric cryptosystem. Based on a buyer's identity, a trusted center issues the buyer a partial sequence which is one of the two elements in the entire sequence. Although a merchant shares the entire sequence with the center, he cannot extract the buyer's key sequence from it. Such an information gap enables our protocol to be asymmetric and efficient. For each packet of content, the merchant produces two marked packets that contains a "0" or "1" information bit, and they are enciphered using the two elements from the entire sequence. Subsequently, the buyer obtains the two ciphertexts (the encrypted marked packets) containing the information bits of his identity. Since the merchant does not know the ciphertext decrypted by the buyer, an asymmetric property is achieved. In our protocol, before trade between a buyer and a merchant, the merchant can produce and compress the marked packets: this enables the reduction of both the computational costs for the encryption and the amount of data for transmission. Since only the enciphering operation is performed by a merchant in the on-line protocol, real-time operation may be possible.