As the information era has arrived, the demanded network services and devices have been rapidly increased. As almost every device (smart phone, tablet PC, smart watch, home appliances, health monitoring sensors) connected to digital network, our primary information will be disclosed to others. Therefore, we should pay considerable attention for the security of information. Although, we have information security based on computational complexity. Various identity crimes and official information leakage incidents have been re-ported, and the number has increased exponentially. Furthermore, information security based on computa-tional complexity can be breakdown by inventing of quantum computer. Therefore, there exist many re-search activities to provide unconditional security against rampant cyber crimes and new technologies like quantum computer.
Quantum Key Distribution (QKD), as one of the candidates, assures the unconditional security by us-ing quantum mechanics. Because the quantum states are very fragile, the states are easily distorted even by a weak perturbation. Then, if an eavesdropper tries to tap information in the quantum states, it is possible to detect the existence of the attacker with checking of the distortions. However, the secure key rate is limited below few Mb/s, since almost transmitted quantum states are lost in the communication channels or covered with noises. Transmission distance is also restricted as below a few tens of kilometers. In addition, compatibil-ity with conventional communication system is too bad to share the communication channels for cost effec-tiveness.
As one of options, Optical Steganography was proposed recently. Steganography is a concealing method by diverting attention of unwanted receivers. To hide the existence of the signal, the optical spectrum is put within the spectrum of Amplified Spontaneous Emission (ASE) noise which is thought as unwanted noise in communication. The method accomplishes the high secure key rate as above Gb/s, because it uses conventional optical communication schemes. However, this method cannot assure unconditional security against an eavesdropper who knows the existence of hidden signal.
To achieve high secure key rate, unconditional security, and compatibility, we propose an uncondi-tional secure optical communication method. Based on the concept of unconditional secrecy, the uncondi-tional secrecy can be obtained by keeping the capacity difference between communicators and eavesdrop-per, we introduce a concept of intensity anti-correlated noises which means that the fluctuations direction of the noises are opposite each other. Then individual noises are transmitted to multi-path separately, then each channel has low capacity by the noises. If an Eavesdropper who does not have all noise information attack the system, she can get a very small portion of transmitted information. However, the targeted receiver who have all the noise information can obtain the whole transmitted information by cancel out the anti-correlated noises.
To show the feasibility, we generated anti-correlated noises by using Reflective Semiconductor Optical Amplifier (RSOA) and single-mode Fabry-Perot Laser Diode (F-P LD). Based on the measured noises, the maximum capacity difference was calculated as 3.01 bits/symbol (= receiver - eavesdropper = 3.98 - 0.97 bits/symbol). For demonstration of the proposed method, a 4-Pulse Amplitude Modulation (4-PAM) modula-tion format (2 bits/symbol) was used, and the data transmission speed was 2 GBaud/s. Finally, we showed the maximum secure key rate as 2.06 Gb/s (= (2 - 0.97) bits/symbol $\ast$ 2 GBaud/s). This can be the highest unconditionally secure data rate ever achieved.
In addition, we analyzed effects of multi-tapping attack by simulation. By introducing chromatic dis-persion intentionally (Eavesdropper cannot get this information), we de-correlate the noises in different path. It is possible to achieve unconditional security, if eavesdroppers unable to compensate dispersion blindly. However, further study is need to prove inability conditions for the blind dispersion compensation.
본 논문은 상보잡음을 다중경로에 주입함으로써 송신자-수신자와 송신자-도청자 사이 채널용량 차이를 보장함으로써 완벽보안을 보장하는 체계를 제안한다. 상보잡음이란 잡음 간의 음의 상관관계(anti-correlation)으로 인해 모든 잡음의 합이 0이 되는 특징을 갖는 잡음의 집합을 의미한다. 이러한 잡음을 다중경로에 주입해주면, 모든 경로 정보를 알고 있는 수신자는 잡음을 상쇄하여 높은 채널 용량을 확보한다. 반면, 경로 정보를 모르는 도청자는 잡음으로 인하여 채널용량이 제한되면서 보안을 보장하는 원리이다. 본 논문에서는 제안하는 방법을 광통신 기반으로 구현 하였다. 상보잡음을 생성하기 위해 Fab-ry-Perot Laser Diode (F-P LD)의 모드분할잡음을 잡음 소스로 사용하였고, 잡음 간의 상관관계(correlation)를 형성하기 위해 Reflective Semiconductor Optical Amplifier (RSOA)의 Gain Saturation을 응용하였다. 그 결과, 최대 3.01 bits/symbol의 Secure Capacity (송신자-수신자와 송신자-도청자 사이 채널 용량차이)를 확보할 수 있음을 확인하였다. 실제로는 4-Pulse Amplitude Modulation (4-PAM) 변조방식을 사용하여 신호를 전송함으로써 최대 2 Gb/s의 Secure Key Generation Rate을 갖는 보안 체계를 구현하였다.
또한, 본 논문에서는 다중경로에 도청을 시도하는 도청자에 대한 대응 체계에 대해 분석하였다. 실제로 도청자가 모든 상보잡음을 가져갈 가능성이 있으며, 신호 간의 correla-tion 정보는 도청자가 높은 채널용량을 확보하는데 중요한 힌트가 된다. 도청자로부터 correlation 정보를 숨기는 방법으로써 Chromatic Dispersion 효과를 적용하였다. 상보잡음에 Dispersion 효과를 주면 잡음의 상관관계가 De-correlation 방향으로 광 펄스의 broadening 이 나타남을 시뮬레이션으로 확인하였다. 그 결과, Fiber 500 km에 해당하는 Dispersion 효과를 적용하였을 때 잡음 간의 경로 차이를 보상해줬음에도 채널 용량이 낮게 유지되는 것을 확인하였다. 반면, 수신자는 Dispersion을 보상해줌으로써 높은 채널 용량을 획득할 수 있었다.
