Improved Identity Based Online/Offline Signature Scheme

Online/offline signatures are used in many applications where the signer must respond quickly once the message to be signed is presented. The idea is to split the signing process into two phases: The first phase is performed offline before the message to be signed is available and the second phase is performed online after the message to be signed is provided. Recently, an identity-based online/offline signature scheme was proposed for authentication for routing in Mobile Ad Hoc Network. Unfortunately, we show that this scheme is insecure in the paper, it is universally forgeable. Anyone can produce a forged signature on arbitrary message without the private key of a signer and a valid signature of a message. To overcome the drawback, an improved scheme is proposed. A security analysis shows that the proposed scheme is proved secure against existential forgery on an adaptively chosen message attack under the assumption of Computational Diffie Hellman in the random oracle model.