An alterable-capacity authentication watermarking algorithm with superior localization

To meet different security demands, this work proposes an alterable-capacity algorithm for JPEG images. First, the original image is divided into 8×8 blocks. The authentication watermarks of each block are computed by its quantized DCT coefficients and the secret-matrix generated by user key, where the size of secret-matrix depends on the watermark capacity. The authentication watermarks of each block are embedded into the middle quantized DCT coefficients in other blocks. The validity of the test block is determined by comparing the number of inconsistent blocks in eight neighbor blocks of test block with the average of that of its mapping blocks. To further enhance the performance of tamper detection, the invalid block will be set valid if there are three and less invalid blocks in its eight neighbor blocks. We also discuss that the interaction among watermark capacity, invisibility and tamper detection performance. Experimental results show that the proposed scheme outperforms the literature [8] in tamper detection with the same watermark capacity.