## 2 Key Distribution Scheme
The 0 and 1 levels corresponding to Basis A and Basis B are opposite, and there is a small difference between them. Figure 1: Comparison of differences between Basis A and Basis B. Step 2. [PERSON] encrypts the original data with Basis A and sends it to [PERSON]. [PERSON] uses Basis B to receive signals and send data to [PERSON]. [PERSON] uses Basis A to decrypt the signals after receiving them. Step 3. If Basis A is equal to Basis B, the BER calculated by [PERSON] is relatively low. If Basis A is not equal to Basis B, the calculated BER is higher. Step 4. According to the aforementioned analysis, when the BER calculated by [PERSON] is relatively low, it can be considered that Basis A is equal to Basis B. When the BER calculated by [PERSON] is relatively high, it can be considered that Basis A is not equal to Basis B. Step 5. Discard the case with high BER and select the case with low BER. At this time, [PERSON] and [PERSON] will quantize and encode the BER to reach a consensus key. [PERSON] and [PERSON] must discard at the same time. They also discard the inconsistent positions of the basis. [PERSON] will think that the BER obtained is low, which means that the basis is consistent. At the same time, [PERSON] will tell [PERSON] the location, and [PERSON] will use the Basis B of the corresponding location as the key. [PERSON] needs to measure through the communication loopback. [PERSON] uses its local data and the data sent by [PERSON] to calculate BER. If Basis A and Basis B are different, BER may be higher. SA and SB are a key base randomly selected by [PERSON] and [PERSON]. We define them as Basis A and Basis B. The corresponding 0 and 1 levels of Basis A and Basis B are opposite, and there is a small difference between them. Modify Figure 2, Basis B (Basis A = Basis B), Basis B (Basis A \(\
eq\) Basis B). TA is the encrypted signal sent from the [PERSON] optical transmitter, and TB is the encrypted signal sent from the [PERSON] optical transmitter. There is quantum noise in the fiber channel, which is mainly generated by amplifiers and lasers. Because the amplitude difference between Basis A and Basis B is very small, it is easily covered by optical quantum noise. Because [PERSON] at the unusual receiving end cannot distinguish Basis A from Basis B, the security of the system uses the masking effect of noise. [PERSON] and [PERSON] can distinguish Basis A and Basis B by measuring BER; [PERSON] can only steal signals by splitting light. It cannot measure the loopback BER of the channel. Moreover, due to the masking effect of quantum noise, [PERSON] cannot directly measure Basis A and Basis B, so the system is safe. As shown in Figure 3, [PERSON] uses the key base (ABAABBABAB) to send data (\(d_{0}d_{1}d_{2}\cdots d_{9}\)) to [PERSON], and [PERSON] uses the randomly generated key base (BBABBAAAB) to receive signals, and then uses the key base to send signals to [PERSON]. Since the bases of [PERSON] and [PERSON] are 0 and 1, suppose that [PERSON] uses key base A when receiving signals and key base B when sending signals, which is equivalent to two reversals of 0 and 1, and has no effect on the signals. However, the difference between [PERSON] and [PERSON] will cause the BER of  
Figure 2: Key distribution scheme based on the masking effect of fiber channel noise. the signal to increase. Finally, [PERSON] compares the received data (\(d_{0}^{\prime}d_{1}^{\prime}d_{2}^{\prime}\cdots d_{9}^{\prime}\)) with the sent data. For example, when BER > 0.3 is compared and calculated, we discard the position 0 at this time, as shown in the blue area in Figure 3. Because the basis is inconsistent, the BER increases. When BER < 0.3 is calculated by comparing \(d_{0}\) and \(d_{0}^{\prime}\), we retain the current position 1 and send the current position information to [PERSON]. Each piece of position information corresponds to a base, as shown in the red area of Figure 3; [PERSON] and [PERSON] have the same base. They both know that the base is B, thus the key KEY = 1. [PERSON] and [PERSON] do not need to declare a basis.