The notion of concurrent signatures was introduced by Chen, Kudla and Paterson in their seminal paper in Eurocrypt 2004. In concurrent signature schemes, two entities can produce two signatures that are not binding, until an extra piece of information (namely the keystone) is released by one of the parties. Upon release of the keystone, both signatures become binding to their true signers concurrently. In ICICS 2005, two identity-based perfect concurrent signature schemes were proposed by Chow and Susilo. In this paper, we show that these two schemes are unfair. In which the initial signer can cheat the matching signer. We present a formal definition of ID-based concurrent signatures which redress the flaw of Chow et al.'s definition and then propose two simple but significant improvements to fix our attacks.

For a large website adopting web server cluster, how to organize and distribute web documents is a challenging problem. In this paper, we propose a strategy to distribute web documents in web server cluster, whose aim is to reduce system's average response time. The strategy uses queuing model to analyze cluster system, and translates the document distribution problem into a 0-1 integer programming problem. Aimed at such kind of 0-1 integer programming problem, we propose a chaotic searching algorithm to solve it. The chaotic searching algorithm lets many isolated chaotic variables search in their tracks, so the corresponding 0-1 distribution matrix built by these variables can experience every possible distribution, thereby it can find the global optimal solution in enough long time. Simulation tests show that the chaotic searching algorithm can find the global optimal solution.

Certificateless public key cryptography was introduced to remove the use of certificate to ensure the authentication of the user's public key in the traditional certificate-based public key cryptography and overcome the key escrow problem in the identity-based public key cryptography. Concurrent signatures were introduced as an alternative approach to solving the problem of fair exchange of signatures. Combining the concept of certificateless cryptography with the concept of concurrent signature, in this paper, we present a notion of certificateless concurrent signature with a formal security model and propose a provably secure scheme assuming the hardness of Computational Diffie-Hellman Problem.

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