Security Analysis of two Key Based Watermarking Schemes Based on QR Decomposition

Watermarking refers to a way of protecting copyright. From one perspective, watermarking algorithms are categorized as key-based and non-key-based. In the key-based watermarking, targeted blocks are selected by using a key to embed the watermark bits; in non-key-based watermarking on the other hand, watermark bits are embedded in a predefined position of the host image. In general, key-based schemes are expected to be more robust against targeted removal attacks. In this paper we propose two attacks against two key-based watermarking algorithm, which are based on the QR decomposition. Although the key based watermarking algorithms are supposed to be more robust, we show that they are vulnerable to our proposed attacks. Our experimental results indicate that the existence of a key is also not a guarantee for the safety of watermarking methods.