Abstract-Histogram-based watermarking schemes are invariant against pixel permutations and can be combined with permutation-based ciphers. However, typical histogram-based watermarking schemes based on comparison of histogram bins are prone to de-synchronization attacks, where the whole histogram is shifted by a certain amount. In this paper we investigate the possibility of avoiding this kind of attacks by synchronizing the embedding and detection processes, using the mean of the histogram as a calibration point. The resulting watermarking scheme is resistant to three common types of shifts of the histogram, while the advantages of previous histogrambased schemes, especially commutativity of watermarking and permutation-based encryption, are preserved.
I. INTRODUCTIONIt is well known that histogram-based watermarking schemes are resistant to permutations of image pixels. In particular, this implies robustness against rotation, scaling and translation (RST) of images. Recently, this fact has been utilized to devise a commutative watermarking-encryption (CWE) scheme by choosing a permutation cipher for encryption and a histogram-based scheme for watermarking [1]. However, typical histogram-based watermarking schemes like [1], [2] work by comparing selected histogram bins, where the selection process is controlled by a watermarking key. If the whole histogram is shifted by a small amount, i.e. by adding a small number to each pixel's value, the detector will use different bin pairs for extracting the embedded watermark and will produce wrong results. To overcome this problem, in the present paper, we extend the scheme described in [1] by deploying a synchronization process between embedder and detector that is based on the global mean of the histogram. A similar approach is used in the scheme [3], but this watermarking scheme does not use a sufficiently long secret watermarking key and has a limited capacity.The rest of the paper is organized as follows: In Section II we briefly summarize previous histogram-based watermarking algorithms. Section III describes the three types