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In the existing watermarking protocols, a trusted third party (TTP) is introduced to guarantee that a protocol is fair to both the seller and buyer in a digital content transaction. However, the TTP decreases the security and affects the protocol implementation. To address this issue, in this article a secure buyer-seller watermarking protocol without the assistance of a TTP is proposed in which there are only two participants, a seller and a buyer. Based on the idea of sharing a secret, a watermark embedded in digital content to trace piracy is composed of two pieces of secret information, one produced by the seller and one by the buyer. Since neither knows the exact watermark, the buyer cannot remove the watermark from watermarked digital content, and at the same time the seller cannot fabricate piracy to frame an innocent buyer. In other words, the proposed protocol can trace piracy and protect the customer's rights. In addition, because no third party is introduced into the proposed protocol, the problem of a seller (or a buyer) colluding with a third party to cheat the buyer (or the seller), namely, the conspiracy problem, can be avoided.

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Abstract. Certificateless Public Key Cryptography (CL-PKC) has very appealing features, namely it does not require any public key certification (cf. traditional Public Key Cryptography) nor having key escrow problem (cf. Identity-Based Cryptography). However, it does suffer to the Denialof-Decryption (DoD) Attack called by Liu and Au [1], as its nature is similar to the well known Denial-of-Service (DoS) Attack. Based on CL-PKC, they introduced a new paradigm called Self-Generated-Certificate Public Key Cryptography (SGC-PKC) that captured the DoD Attack and proposed a first scheme derived from a novel application of Water's Identity-Based Encryption scheme. In this paper, we propose a new SGC-PKE scheme that does not depend on the bilinear pairings, which make it be more efficient and more short public keys than Liu and Au's scheme. More importantly, our scheme reaches Girault's trusted level 3 (cf. Girault's trusted level 2 of Liu and Au's scheme), the same level as is enjoyed in a traditional PKI.

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