Abstract
Recently, Tsai et al. constructed an efficient identity-based signature (IBS) scheme and claimed that it was strongly unforgeable in the standard model. Unfortunately, we find that their scheme is insecure. By giving concrete attack, we show that their scheme does not meet the requirement of strong unforgeability. Meanwhile, we demonstrate that there are serious flaws in their security proof. The simulator cannot correctly answer the signing query in the security model. Furthermore, we propose an improved strongly unforgeable IBS scheme without random oracles. Compared with other strongly unforgeable IBS schemes in the standard model, our scheme is more efficient in terms of computation cost and signature size.
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