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A new method to encrypt a compressed multimedia bit stream, which is the output of any compression system using an entropy coder in the last stage, is proposed in this research. The basic idea is to partition a coded bit stream into random-sized blocks and perform a random rotation in each block. The proposed encryption scheme demands low computational overhead and does not increase the size of compressed bit stream. Security analysis to both the ciphertext-only attack and the known/chosen-plaintext attack shows that the proposed algorithm achieves high security.

Efficient multimedia encryption algorithms play a key role in multimedia security protection. One algorithm known as the MHT (multiple Huffman tables) method was recently developed by Wu and Kuo (2001; 2000). Although MHT has many desirable advantages, it is vulnerable under the chosen-plaintext attack (CPA). In this work, an enhanced MHT encryption scheme is proposed to remedy this drawback using the idea of key hopping. It is demonstrated by mathematical analysis that our proposed scheme is secure against chosen plaintext attack.

The robust stabilization problem (RSP) of an interval plant family is considered. By representing the nominal plant with the normalized coprime factorization description, the RSP of interval plant family is converted to a particular H/sub /spl infin// control problem. An analytical formula for calculating the parametric stability radius /spl delta//sub max/ of the closed-loop system consisting of the interval plant family and the H/sub /spl infin// controller is given. Using /spl delta//sub max/, the stability margin /spl epsi//sub IP/, of the closed-loop system, described in terms of the H/sub /spl infin//-norm of the uncertain factors in the NCF description, is calculated. Numerical example shows that /spl epsi//sub IP/ is larger than the stability margin of the general unstructured uncertainties case.