Quantum public-key cryptosystem without exchanging qubits between any two users

A quantum public-key cryptosystem using entangled states is present in this paper. Every user shares a set of entangled quantum systems with the key management center (KMC) as the (private key, public key) pair. When a user wants to send encrypted message to another user, he or she asks KMC for the latter’s public key and produces a string to encrypt his or her pain text. Then the latter can get the same string using the correlations between the subsystems of the entangled quantum systems and recover the plain text while no third one can make it. So users can achieve secret communication and digital signature by the help of KMC. The principles of quantum physics guarantee the unconditional security of the cryptosystem. Any two users needn’t performing exchanging quantum systems with each other. So it is easier to carry out and more robust in practice.