The problem of secure and efficient revocation of membership without incurring big costs has been considered, but no satisfactory solution was reported. In this paper, we propose a new revocation method of membership based on the ACJT group scheme. Our solution is efficient in that it only needs one multiplication and one exponentiation, which the length of exponent is fixed, to update the public key for the group manager to exclude a group member, and the signing and verifying procedure are independent of the number of current group members and excluded group members. To the best of our knowledge, the signing and verifying procedures in prior revocation schemes are dependent of the number either current group members or excluded group members, and thus the group manager needs a heavy computation load to update the public key.

A new discrete wavelet transform-based image fragile watermarking scheme exploiting the features of the human visual system is presented. Fusion of multi-resolution tamper detections, and mathe-matical morphology operations are developed for accurate tamper detection. As a result, malicious tamper and the tampered regions can be deteeted, while toleration towards high quality Iossy image compression is achieved.

The authors propose a segmentation-based hybrid image coding algorithm taking the features of the human visual system into account. A novel segmentation criterion based on luminance masking is presented. The image is segmented into blocks of different sizes and different properties. Block mean, polynomial approximation and discrete cosine transform coding are then applied to the blocks accordingly.

An adaptive filtering algorithm for artefact removal in block coded images is presented. Based on an analysis of block artefacts in the frequency domain, the edge image is notch filtered and then used for a neighbourhood analysis. A space-variant filter is applied to the coded image adaptively, dependent on a frequency analysis.

An image watermarking scheme in a DCT domain is proposed, it is adaptive in that a different strength of watermark is applied to different blocks according to their block classification. For the classification, visual masking (both luminance and texture masking) is taken into account. Consequently, the watermarks thus generated are robust and invisible.