Improvement of an ID-based proxy signature scheme without trusted PKG

By analyzing the ID-based proxy signature scheme without trusted PKG proposed by Xu, the paper proves that the scheme is not security because it cannot withstand the attack which the original signer can tamper with the proxy warrant allayed with PKG. So the scheme is improved. In the paper, the improved scheme is proved that it is a security and effectual scheme. And the improved scheme overcomes the security vulnerability of Xu´s scheme without increasing the computing cost. The improved scheme is more safe to be suit for being applied into e-cash and e-voting.