Design of time-stamped proxy signatures with traceable receivers

A proxy signature scheme is a method which allows an original signer to delegate his signing power to a proxy signer. Most proxy signature schemes use a warrant appearing in the signature verification equation to declare the valid delegation period. However, the declaration in the warrant is useless because no-one can know the exact time when the proxy signer signed a message. To avoid the proxy signer abusing the signing capability, the original signer may hope to know the identity of who received the proxy signature from the proxy signer. Recently Sun and Chen proposed the concept of time-stamped proxy signatures with traceable receivers to solve these two problems. A time-stamped proxy signature scheme with traceable receivers is a proxy signature scheme which can ascertain whether a proxy signature is created during the delegation period, and can trace who actually received the proxy signatures from the proxy signer. The author shows that Sun and Chen´s scheme suffers from weaknesses and consequently proposes a new time-stamped proxy signature scheme which doesn´t suffer from the same weaknesses