With the rapid development and rich applications of mobile Internet, security issues become urgent. For example, piracy of media content (image, audio, or video) becomes easier because mobile Internet provides more ways to re-distribute media contents. Watermarking has been regarded as a potential solution for identifying pirate copies. However, the watermarking algorithms' security and robustness are still not ideal enough for practical applications, especially in mobile scenarios with more attacks. In this paper, a novel hybrid multiplicative watermarking algorithm is proposed to obtain higher security and good robustness. In this algorithm, two embedding rules are changed according to the secret key, which embed the watermark into Discrete Wavelet Transform (DWT) coefficients. The secret key improves the watermark system's security. Additionally, the optimum and locally optimum hybrid multiplicative watermarking decoders are proposed, respectively, to detect the watermark. Because it works with two rules, it obtains a tradeoff in robustness. Various comparative experiments are carried out, and results are given to prove the valid of theoretical analysis. Copyright © 2013 John Wiley & Sons, Ltd.