Kwok-Tung Lo

On the Design of Perceptual MPEG-Video Encryption Algorithms

In this paper, some existing perceptual encryption algorithms of MPEG videos are reviewed and some problems, especially security defects of two recently proposed MPEG-video perceptual encryption schemes, are pointed out. Then, a simpler and more effective design is suggested, which selectively encrypts fixed-length codewords in MPEG-video bit streams under the control of three perceptibility factors. The proposed design is actually an encryption configuration that can work with any stream cipher or block cipher. Compared with the previously-proposed schemes, the new design provides more useful features, such as strict size-preservation, on-the-fly encryption and multiple perceptibility, which make it possible to support more applications with different requirements. In addition, four different measures are suggested to provide better security against known/chosen-plaintext attacks

Cryptanalysis of the RCES/RSES image encryption scheme

Recently, a chaos-based image encryption scheme called RCES (also called RSES) was proposed. This paper analyses the security of RCES, and points out that it is insecure against the known/chosen-plaintext attacks: the number of required known/chosen plain-images is only one or two to succeed an attack. In addition, the security of RCES against the brute-force attack was overestimated. Both theoretical and experimental analyses are given to show the performance of the suggested known/chosen-plaintext attacks. The insecurity of RCES is due to its special design, which makes it a typical example of insecure image encryption schemes. A number of lessons are drawn from the reported cryptanalysis of RCES, consequently suggesting some common principles for ensuring a high level of security of an image encryption scheme.

Adaptive block matching motion estimation algorithm using bit-plane matching

Motion estimation/compensation techniques are extensively used in video compression because of their abilities to exploit the high temporal correlation between successive frames of an image sequence. We first propose a new matching criterion called bit-plane matching (BPM) for distance measurement in the block matching algorithm. This criterion not only considers the magnitude but also the directive information of the pixel elements in the block. A simple adaptive block matching algorithm is also developed to adjust the search origin and search range at a block-by-block basis. Simulation results show that the proposed algorithm achieves similar MSE performance for motion-predicted images but requires about 8 times less the number of computations than the full-search method.

Shot reconstruction degree: a novel criterion for key frame selection

Abstract In this paper, the authors treat the key frame extraction problem from a viewpoint of shot reconstruction. As a result, a novel criterion called shot reconstruction degree (SRD) is proposed for key frame selection based on the degree of retaining motion dynamics of a video shot. Compared with the widely used fidelity criterion, the key frame set produced by SRD can better capture the detailed dynamics of the shot. Using the new SRD criterion, a novel inflexion-based key frame selection algorithm is developed. Simulation results show that the new algorithm results in good performance in terms of both fidelity and shot reconstruction degree.

Dynamic selection and effective compression of key frames for video abstraction

This paper reports on a new key frame based video abstraction method. With our method, a video sequence is first segmented into a number of video shots. Several key frames are selected in each shot using a dynamic selection technique. For these key frames, a motion-based clustering algorithm is applied so that key frames in the same cluster are alike in sense of motion compensation error, while those from different clusters are quit dissimilar. Then a novel cluster-based coding scheme is developed for efficient representation of the key frames. Simulations show that the proposed method can select key frames according to the dynamics of a video sequence and abstract the video with different levels of scalability.

Error concealment for MPEG video transmissions

Packet loss is a critical problem for video transmission on packet-switched networks. An error compensation method called the modified boundary matching algorithm is developed for recovering lost video due to packet loss at the video decoder. Simulation results demonstrate that the proposed method preserves good image quality even when the cell loss rate is very high.

Cryptanalysis of a chaotic block cipher with external key and its improved version

Recently, Pareek et al. proposed a symmetric key block cipher using multiple one-dimensional chaotic maps. This paper reports some new findings on the security problems of this kind of chaotic cipher: (1) a number of weak keys exist; (2) some important intermediate data of the cipher are not sufficiently random; (3) the whole secret key can be broken by a known-plaintext attack with only 120 consecutive known plain-bytes in one known plaintext. In addition, it is pointed out that an improved version of the chaotic cipher proposed by Wei et al. still suffers from all the same security defects.

Cryptanalysis of an Image Scrambling Scheme Without Bandwidth Expansion

Recently, a new image scrambling (i.e., encryption) scheme without bandwidth expansion was proposed based on two-dimensional discrete prolate spheroidal sequences. A comprehensive cryptanalysis is given here on this image scrambling scheme, showing that it is not sufficiently secure against various cryptographical attacks including ciphertext-only attack, known/chosen-plaintext attack, and chosen-ciphertext attack. Detailed cryptanalytic results suggest that the image scrambling scheme can only be used to realize perceptual encryption but not to provide content protection for digital images.

Cryptanalyzing an Encryption Scheme Based on Blind Source Separation

Recently, there was a proposal of using the underdetermined blind source separation (BSS) principle to design image and speech encryption. In this paper, we report a cryptanalysis of this BSS-based encryption scheme and point out that it is not secure against known-/chosen-plaintext attack and chosen-ciphertext attack. In addition, we discuss some other security defects of the schemes: (1) it has a low sensitivity to part of the key and to the plaintext; (2) it is weak against a ciphertext-only differential attack; and (3) a divide-and-conquer (DAC) attack can be used to break part of the key. We finally analyze the role of BSS in this approach towards cryptographically secure ciphers.

Image watermarking using tree-based spatial-frequency feature of wavelet transform

Abstract In this paper, an image watermarking scheme is developed using the tree-based spatial-frequency feature of wavelet transform. With our approach, the watermark sequence is inserted in those high activity texture regions of an image having the maximum strength of just noticeable distortion (JND) tolerance of the human visual system (HVS). Simulation results show that the proposed method achieves a good compromise between the robustness and transparency.