Security analysis and improvement of threshold proxy signature scheme

The security of the Xue et al.'s threshold proxy signature scheme using self-certified public key is analyzed.Two forgery attacks against this scheme are proposed.A malicious original signer can generate a new valid signature after he gets a valid signature,and a malicious proxy signer can derive a valid signature on message m' in the signature generation procedure for message m.The causes of the attacks existing in this scheme are analyzed and an improvement which can resolve these security deficiencies is further proposed.