Breaking a proxy ring signature with revocable anonymity in MINE2009

Proxy signature is an important delegation technique. Since it was introduced in 1996, many variants of proxy signature schemes have been proposed. Among of them, the requirement of proxy signer´s privacy protection and the revocation of proxy signer´s anonymous identity are needful in some practical applications. Recently, Wang et.al gave a new signature type: proxy ring signature with revocable anonymity. It allows that proxy signer anonymously produces a proxy signature. However, it is anonymous for the others except that the original signer can know exactly who is the actual proxy signer. Unfortunately, in this paper, by analyzing the scheme, we show that the scheme does not satisfy anonymity, that is to say, given an anonymous proxy signature, the others also can know the actual identity of proxy signer beside original signer. At the same time, it cannot also revoke anonymity of proxy signer´s identity. Finally, we give the corresponding attacks on the scheme and the reasons to produce suck attacks.