Cryptanalysis of and improvement on the Hwang-Chen multi-proxy multi-signature schemes

Hwang and Chen recently proposed new multi-proxy multi-signature schemes that allow a group of authorized proxy signers to sign messages on behalf of a group of original signers. This paper shows that their schemes are insecure because a malicious proxy signer can forge a signature for a message secretly while participating in the message signing process with the other proxy signers. This paper then proposes a method to remove this weakness with only small computational overheads and without impairing the security of the original schemes.