Attacks on Two Buyer-Seller Watermarking Protocols and an Improvement for Revocable Anonymity

Buyer-seller watermarking protocols incorporate digital watermarking with cryptography, in order to protect digital copyrights and privacy rights for the seller and the buyer before, during, and after trading activities in e-commerce. In this paper, we present attacks on two recently proposed buyer-seller watermarking protocols, and prove that these protocols are not able to provide security for both the buyer and the seller simultaneously. Further, we point out that both protocols don´t function properly when employing homomorphic probabilistic cryptosystems. We also show that the buyer´s anonymity and/or the transaction unlinkability is not achieved in these protocols. We propose an improved secure and anonymous buyer-seller protocol, which is secure and fair for both the seller and the buyer. In contrast to early work, our scheme is able to provide all the security properties that a secure buyer-seller watermarking protocol is expected to hold.