Abstract: A proxy signature with proxy signer privacy protection is proposed by Fu-Kou-Xiao,which has two characteristics listed as follows:the proxy signer’s identity is hidden behind an alias;the anonymity can be revoked when it is need to identify the proxy signer without a trusted third party.In this paper,the authors show that Fu-Kou-Xiao’s scheme is insecure.The original signer may use the capability of proxy signature freely.Also the proxy signer can repudiate a valid proxy signature generated by himself.Therefore,the scheme does not satisfy the properties of undeniable and misuse of the proxy signature.

Key words: proxy signature, proxy signature with proxy signer privacy protection, anonymity revocation, misuse of the proxy signature, strong undeniability