In wireless networks, the cooperative relaying that provides a spatial diversity and improves the performance of system is a tendency in the future communications. There are three strategies in the cooperative communications. One is Amplify-and-Forward (AF) mode. Another is Decode-and-Forward (DF) mode and the other is Compress and Forward (CF) mode. Comparing with these three kind modes, AF mode is with the low complexity to implement. Consider the Raleigh fading environment where there are one information source, M relay stations and N destinations. The downlink signal includes two parts. One is from the source and the other comes from the relay stations. In order to achieve the maximum throughput in the system, it should obtain the maximum mutual information between the source and the destination. Hence, how to select an optimal relay station is important in the cooperative communications. This project proposes a relay selection algorithm with Amplify-and-Forward mode employed. The optimal selection scheme is based on the exhaustive search method. It is easy to be realized with a high computational complexity. In order to reduce the computational complexity, the relay station selection scheme with a maximum mutual information finding is proposed. With different relay station selection schemes, the performances are given for comparison.

Many cryptosystems have been developed to solve the problem of information security, and some approaches are based on the self-certified public key proposed by Girault. In Girault’s scheme, the public key is computed cooperatively by both the system authority (SA) and the user. One of the advantages is that the public key is able to implicitly authenticate itself without any additional certificates. Another advantage is that the SA is not able to forge a public key without knowing the user’s secret key. Despite the advantages of Girault’s system, in this paper, the authors demonstrate that the system still suffers from two main weaknesses. As a result, the authors propose a slight improvement on Girault’s system.

Purpose – The purpose of this paper is to discuss the concept of group signature, first introduced by Chaum and van Heyst, which allows individual members in a group to create signatures on behalf of the group.

Abstract A frequently moving mobile user in GSM must re‐register at the HLR frequently, and, therefore, the signaling traffic is overhead and HLR database update cost raised. In this paper, the authors propose a new authentication protocol, based on pointer forwarding to reduce the HLR update cost and traffic load for the MS authentication protocol. The proposed protocol for GSM can achieve some objectives described in Introduction. Compared with other authentication protocols, our protocol is efficient. Copyright © 2007 John Wiley & Sons, Ltd.

In 2003, Hwang et al. proposed an enhanced authentication key exchange protocol. Their protocol can generate multiple shared keys for two entities at a time and takes less computation time than Harn and Lin's protocol. However, we will show that Hwang et al.'s protocol cannot withstand the forgery signature attack. At the same time, we will repair the security flaw in their scheme.

In a (t, n) threshold proxy signature scheme, the original signer delegates the power of signing messages to a designated proxy group of n members. Any t or more proxy signers of the group can cooperatively issue a proxy signature on behalf of the original signer, but (t - 1) or less proxy signers cannot. Previously, all of the proposed threshold proxy signature schemes have been based on the discrete logarithm problem and do not satisfy all proxy requirements. In this paper, we propose a practical, efficient, and secure (t, n) threshold proxy signature scheme based on the RSA cryptosystem. Our scheme satisfies all proxy requirements and uses only a simple Lagrange formula to share the proxy signature key. Furthermore, our scheme requires only 5 percent of the computational overhead and 8 percent of the communicational overhead required in Kim's scheme.

This paper presents a copyright owner detection technique for images and graphics. A digital time-stamped signature for this copyright identification technique is also introduced in this work. The secret key, used to compute the watermark, is generated from the extracted features of the host images in the proposed scheme. The proposed algorithm enables the successful revealing of watermarks under several attacks, such as filtering, lossy compression, cropping, rotating, and so on. In particular, our method is not only suitable for ordinary natural images, but can also be applied to cartoon graphics. Most importantly, the proposed scheme can be applied to the media in which distortion is not allowable, such as medical images, source code of programs, and so on.

In practice, we usually require two cryptosystems, an asymmetric one and a symmetric one, to encrypt a large confidential message. The asymmetric cryptosystem is used to deliver secret key SK, while the symmetric cryptosystem is used to encrypt a large confidential message with the secret key SK. In this article, we propose a simple cryptosystem which allows a large confidential message to be encrypted efficiently. Our scheme is based on the Diffie-Hellman distribution scheme, together with the ElGamal cryptosystem.

Conventional remote password authentication schemes allow a serviceable server to authenticate the legitimacy of a remote login user. However, these schemes are not used for multiserver architecture environments. We present a remote password authentication scheme for multiserver environments. The password authentication system is a pattern classification system based on an artificial neural network. In this scheme, the users only remember user identity and password numbers to log in to various servers. Users can freely choose their password. Furthermore, the system is not required to maintain a verification table and can withstand the replay attack.

The weaknesses of the simple authenticated key agreement (SAKA) protocol were shown by H. Sun (2000). These weaknesses include the following facts: (1) it cannot detect a masquerade, (2) it cannot withstand a dictionary attack, and (3) it cannot provide perfect forward secrecy. In this paper, we improve the SAKA algorithm to overcome these weaknesses. In addition, we show that the improved scheme is more simple and more secure than the original one.

We propose a new image cryptosystem to protect image data. It encrypts the original image into another virtual image. Since both original and virtual images are significant, our new cryptosystem can confuse illegal users. Besides the camouflage, this new cryptosystem has three other benefits. First, our cryptosystem is secure even if the illegal users know that our virtual image is a camouflage. Second, this cryptosystem can compress image data. Finally, our method is more efficient than a method that encrypts the entire image directly.

We propose a new service for digital mobile communication systems. The service enables two or more users to hold a secure conference. Two requirements must be considered: privacy and authentication. Privacy involves ensuring that an eavesdropper cannot intercept the conversations of the parties holding the conference. Authentication involves ensuring that service is not obtained fraudulently in order to avoid usage charges. We present two new conference key distribution schemes for digital mobile communication systems. In these schemes, a group of users can generate a common secret keg over a public channel so that they may hold a secure conference.>