Cryptanalysis and improvement of a self-certified proxy signature scheme

In this paper, the self-certified proxy signature scheme, which is proposed by Youan Xiao independently, is analyzed and the scheme turns out to be insecure. Concretely, there are two flaws in their scheme: Firstly, in the digital signing power delegation phase, the proxy secret key is generated without the use of the secret key of the proxy signer. Then, the proxy signer can disavow the signature which is really done by him, because the proxy secret key and proxy public key can be generated by the original signer only. Furthermore, we also point out that such flaw also exist in some other papers. Secondly, the self-certified proxy signature scheme can suffer from the public key replacement attack so that any one can forge a valid signature on any message. Finally, to overcome the above two flaws, an improved scheme is proposed.