External attack. In the proposed protocol, the decoy state technique is used to prevent external eavesdroppers form attacking the protocol. To achieve that, a sequence of single decoy qubits is randomly selected from the states {|+⟩, |−⟩, |0⟩, |1⟩} and then inserted in random positions into the secret message. The eavesdropper (Eve) cannot distinguish between the decoy-states and secret message states. Eve may try to entangle a secret message state with an auxiliary quantum state (|⟩) by applying a unitary operation () as follows: |0⟩|⟩ = 1|0⟩|00⟩ + 2|1⟩|01⟩, (5) |0⟩|⟩ = 1|0⟩|00⟩ + 2|1⟩|01⟩, (6) |+⟩|⟩ = 1 [|+⟩( | ⟩ + | ⟩ + |0⟩, |0⟩, |0⟩, |1⟩ |0⟩, |0⟩, |1⟩ |0⟩, |1⟩, |−⟩, |+⟩) and
External attack. There is an eavesdropper called Eve who wants to exact the secret key. She can try to use three common method to test the quantum system and exact the secret key, which are “intercept-and-resend”, “control-not” and “fake-participant”. According to the idea before, Eve can get each bit of final key if she gets one operation of any participants. Therefore, this section only discusses a interaction between Eve and one of any participants.
External attack
