Identity-Based Concurrent Signature Scheme with Improved Accountability

In Eurocrypt´04, the concept of the concurrent signature was introduced by Chen, Kulda and Paterson, in which they proposed a somewhat weaker concept to solve the traditional fair exchange problem. Concurrent signatures provide a new idea for fair exchange without the help of the trusted third party. Only two parties interact to produce two signatures. However two signatures are still ambiguous from any third party´s point of view, unless an extra piece of information (called the keystone) is released. Upon the keystone is released by initial signer, both signatures are binding to their true signer and effective concurrently. In this paper, we present an identity-based concurrent signatures scheme from bilinear pairings with improved accountability. Each user uses his/ her ID (IP address, e-mail address) as public key for simplification of the key management and does not need to maintain the certificates of the users. The proposed scheme can resist the message substitute attack and achieve the property of real accountability.