# Key Distribution Scheme for Optical Fiber Channel Based on SNR Feature Measurement
## 1 Introduction  
With the sharp increase in the speed and distance of optical communications, optical networks have become more accessible; accordingly, more threats and higher risks may be posed. A reliable key distribution system is required to solve this problem and ensure communication security. Traditional public key-based key distribution security primarily depends on the complexity of an algorithm, such as the RSA algorithm [1, 2]. However, traditional cryptography is susceptible to the rapid progress of hardware and algorithms. The robustness of these algorithms faces severe challenges with the development of computer systems, especially quantum computers [3, 4]. Quantum key distribution (QKD) is theoretically considered as the only solution to guarantee absolute security at the physical level [5, 6, 7, 8]. However, QKD requires highly sensitive optical detection equipment instead of optical amplifiers. Therefore, it is still challenging to realize QKD at longer distances and higher key rates.  
A promising and cost-effective approach is to take advantage of the unpredictable and random characteristics of the transmission channel to convert the random characteristics of the environment into a secure key. In this approach, the key is only highly correlated to the legitimate users ([PERSON] and [PERSON]), but not to the signal being eavesdropped on by [PERSON]. This idea has already been put into practice in wireless and fiber-optic communications systems. In a wireless communication system, for key generation, the random fading effects of the wireless channel are utilized [9, 10], such as received signal strength, channel impulse response, and frequency phase in key distribution schemes [11, 12].  
Unlike wireless communication, the optic fiber links have stronger resistance to environmental disturbance. The proposed method achieves the implementation of a classical physical layer secure key distribution (SKGD) with unique fiber characteristics, which extracts the key from the fiber channel characteristics. For the key distribution scheme based on chaotic synchronization, the security has been improved [13; 14], but it is incompatible with the existing optic fiber link configuration, and the key distribution distance is short. The SKGD scheme based on the unique characteristics of the physical layer has the advantages of high security, low cost, and a simple structure. The polarization mode dispersion (PMD) depends on the birefringence distribution in instantaneous space [15; 16; 17; 18] and fluctuates randomly in the link, providing a random source for key generation. Eve close to the legitimate party can easily cause information disclosure because the PMD can only produce a favorable effect when the distance is long enough. Based on the key distribution scheme of the optical fiber interferometer [19; 20], the interferometer is exposed to the public and vulnerable to active intrusion attacks. As a result, other solutions that can guarantee high security and easy implementation have not been fully explored.  
Recently, researchers have proposed a key generation scheme based on the bit error rate (BER) measurement of physical channel characteristics [21; 22]. The scheme takes advantage of the physical channel's randomness to ensure the security of the key and does not change the structure of the existing optic fiber link. However, this scheme requires a high BER in order for the normal transmission to be affected. We propose a key distribution measurement scheme based on the SNR characteristics of the physical layer of the optical fiber channel without affecting the normal transmission. The combination of key distribution and encryption transmission is realized by simulation. The final KGR can reach 25 kbps, and the KCR can reach 98%. Fingerprint SNR is used as the random source of key extraction in the system, and the uniqueness of the fiber channel ensures the high security of the generated key.