We provide an asymmetrical quantum key distribution protocol based on the correlations of EPR pairs. It is a variant of the modified Lo–Chau protocol where the EPR pair is not in the state Φ+ but is in one of the four states {Φ±, Ψ±}. The two sides communicating with each other are not equal in the process of establishing the key. A public key system can be built on the protocol. It differs from classical public key system in that there are three keys: the public key, the secret key and the real key.
Security of quantum-key-distribution protocol by using the post-selection technique