Recently the research in the watermarking field has concentrated its attention to the security aspects. In a watermarking application one of the most sensitive steps from the point of view of security, is the watermark extraction process: here, a prover has to prove to a verifier that a given watermark is present into the content. In the design of the system, it has to be considered that the prover is not a trusted party: the prover could try to exploit the knowledge acquired during watermark extraction to remove the embedded code and, consequently, to undermine the security of the watermarking system. To tackle this particular issue, it has been proposed to use some cryptographic techniques, defined zero-knowledge protocols, for building a secure layer on top of the watermarking channel, able to protect the watermarking algorithm against a possible information leakage. Up till now, zero-knowledge protocols have been applied to spread-spectrum based detectable watermarking algorithms. In this paper, a novel zero-knowledge protocol designed for a Spread Transform Dither Modulation (ST-DM) watermarking algorithm, belonging to the class of the informed watermarking systems, is proposed.