Cryptoanlaysis of Two Proxy Blind Signature Schemes with Proxy Revocation

As we know blind signature and proxy signature have their respective advantages. Proxy signature allows an original signer to delegate his signing capability to a proxy signer who can produce a signature on behalf of the original signer. Blind signature is the concept with an outstanding feature that the signer can not make a linkage between the blind signature and the signed message, thus, it realize the anonymity of the signed message. Because of these properties, proxy signature and blind signature are used widely in electronic commerce. To adapt practical requirements, recently, by combining proxy signature with blind signature, Li et.al gave a fair blind proxy blind signature scheme with the property of proxy revocation and Liu et.al gave a new proxy blind signature scheme with proxy revocation. They claimed that their scheme were secure in their security model, respectively. Unfortunately, by analyzing the security of the two blind signature schemes, we show that the two blind signature schemes are not secure. They are universally forgeable in that anyone can forge a blind signature on an arbitrary message. Finally, we give the corresponding attack and the improved discussion, respectively.