The proxy signature allows a proxy signer to sign on behalf of an original signer and can be verified by anyone with access to the original signers public key. Recently, Dai et al. proposed a privacy-protecting proxy signature scheme. In this scheme, the messages the original signer entrust to the proxy signer to sign on behalf of him are kept secret from the proxy signer during the generation of the proxy signature except the receiver designated by the original signer. Therefore, the privacy of the original signer is protected. Unfortunately, Dai et al.s scheme is insecure and inefficient. Particularly, the receiver can cheat the proxy signer and obtain a proxy signature on any message. To eliminate these weaknesses, we propose an improved scheme based on Nyberg-Rueppel signature.
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