Man-in-the-middle attack on BB84 protocol and its defence

Man-in-the-middle attack on the BB84 protocol is presented. The man in the middle can intercept the message from the sender and send his message to the receiver in every step of the BB84 protocol. The attack can be analogously used on other quantum key distribution schemes. Some methods are suggested to defense against the attack by discovering the attack or handicapping the attack, and the security of BB84 protocol is analyzed.