Security analysis of transformed-key asymmetric watermarking system

Asymmetric watermarking schemes have been widely discussed recently. Transformed-key asymmetric watermarking (TKAW) system is one of the schemes, which was proposed by Choi et al. This letter discusses the weaknesses of TKAW. In TKAW, the inner product of the received signal and public key almost equals to zero. As a result, it cannot resist projection attack. To prove this point, we will demonstrate how to find the most relevant unwatermarked signal y˜ for a given watermark embedded signal y and prove that ||y-y||≤|α|, where α is a constant adjusted to a value that makes the watermark imperceptible. Moreover, we also find that the feature y-y˜, a predicted watermark of a stego image, can be copied from a stego image to another image, which means that the TKAW system cannot resist copy attack either.