## 2 Key Distribution Scheme  
The scheme proposed in this paper is a single-ended measurement scheme. Firstly, [PERSON] needs to use Basis B to solve the mapping and convert it into a digital signal; then, the digital signal is mapped to an electrical signal using Basis B; finally, it is modulated to an optical signal and sent to [PERSON]. Figure 1 shows the comparison of the difference between Basis A and Basis B. The level in the figure is the level. The physical realization is that the 1 and 0 corresponding to Basis A respectively represent the high and low level, and the 0 and 1 corresponding to Basis B respectively represent the high and low level. The high and low level of Basis A are slightly different from the high and low level. This small difference is easily obscured by noise, and it is impossible to distinguish Basis A from Basis B simply by measuring levels. After transmission, states a and b are overwhelmed by noise and cannot be distinguished. [PERSON] and [PERSON] can quantify the differences between bases. Because the signals produced by [PERSON] and [PERSON] through the DA converter have no optical noise, this difference can be resolved and quantified. [PERSON] and [PERSON] can distinguish the difference. This signal difference is equivalent to the displacement of the signal. The larger the displacement of the signal, the greater the error rate. Because the signal is masked by noise after electro-optic conversion, [PERSON] cannot tell the difference between the bases. [PERSON] cannot quantify the difference between the bases. This small difference has an accumulated effect. [PERSON] uses 10,000 bits to calculate a BER, and the accuracy of BER is relatively high. Therefore, BER can be affected by small differences. The difference between high and low levels of Basis A and Basis B is very small. The quantum noise is completely random, and Basis A and Basis B are masked by quantum noise, thus they cannot be distinguished. [PERSON] can only eavesdrop on the optical fiber. [PERSON] eavesdrops by splitting the light. Even if he amplifies the signal, he does not have the key Basis, and thus he will introduce more quantum noise from the amplifier. Then, the eavesdropping results are different from [PERSON] and [PERSON]. This article assumes that [PERSON] cannot send data. [PERSON] can only eavesdrop on fiber signals in the middle and cannot send data because [PERSON] and [PERSON] will block [PERSON]'s loopback measurements. [PERSON] is unable to measure bases and the basis is masked. This basis is pre-defined and the basis is kept secret from [PERSON]. Figure 2 depicts the process of key distribution based on the masking effect of fiber channel noise. The original data is random and no information is transmitted. [PERSON] generates random data DA and random basis SA. After basis conversion, the random data DA is converted to TA. TA is emitted by the optical transmitter and reaches the Erbium Doped Fiber Amplifier (EDFA) at the receiver side through the optical fiber. After being amplified by EDFA, the optical signal reaches the optical receiver. [PERSON] uses the random basis SB to convert the received signal and make a decision. Then, [PERSON] converts the decision using basis SB to get TB. After TB is transmitted, the optical signal passes through the optical fiber and EDFA and reaches the [PERSON] side. [PERSON] uses basis SA to convert the received data to get data DA', and compares DA with DA' to calculate the BER. The BER of the optical fiber loop can be measured by this method. [PERSON] uses the masking effect to encrypt the data. The original data is a pseudorandom number, the copy sent is a 10,000-bit random number and the pseudo-random number is repeatedly sent; these data are different. This paper negotiates the key by measuring the [PERSON] and [PERSON] channel characteristics BER of the optical channel transceiver. The specific steps are as follows:  
Step 1. [PERSON] and [PERSON] randomly select a key, which is defined as Basis A or Basis B.