On the security of proxy blind multi-signature scheme without a secure channel

The proxy blind signature scheme allows the designated proxy signer using the proxy secret key to generate a blind signature on behalf of the original signer, and most of proxy blind signature schemes should rely on secure channels to transmit proxy secret key. Recently, Lu et al. presented a proxy blind multi-signature scheme which did not need a secure channel. However, in the paper, we show that Lu et al.´s scheme does not satisfy the unforgeability.