Cryptanalysis of two strongly unforgeable identity-based signatures in the standard model

The strong unforgeability of digital signature means that no attacker can forge a valid signature on a message, even given some previous signatures on the message, which has been widely accepted as a common security requirement. Recently, Tsai et al. and Hung et al. presented an efficient identity-based signature scheme and a revocable identity- based signature scheme, respectively. Meanwhile, they all claimed that their scheme is strongly unforgeable against chosen message attacks. In this paper, we point out that the two schemes cannot meet the requirements of strong unforgeability by giving some concrete attacks and brie y analyze the reasons why the provably-secure schemes are insecure following their security model. © 2018 International Journal of Network Security.