On the security of an identity-based signature scheme

An identity-based signature is an important cryptographic primitive which can realize the functionality of signing in identity-based cryptography and can also simplify the key management procedures including the generation, management and revocation of digital certificates in the traditional digital signatures. The first identity-based signature that can be proven secure in the standard model was proposed by Paterson and Schuldt, unfortunately, the scheme suffers from low efficiency in computation and heavy cost in communication. An improvement due to Li and Jiang was given, and the scheme was claimed as being provably secure in the standard model, enjoying shorter signature length and less computation operation. However, in this paper, we demonstrate that the new scheme is insecure against the forgery attack by providing a concrete security analysis in their security model. This attack is due to the universal forgery of the key extraction in their scheme. We also show the flaws in their security proof: the view of the adversary is not independent of the event that the simulation succeeds.