## 5 Improved Setup with a Fully Active Base Selection
### Proposed Defense Strategy against Strong-Light Attacks  
To defend against strong-light attacks, I focus on two pairs of equations: Equations (5) and (8), which model the in- and out-of-gate coincidence probabilities during the execution of a pristine BB84 QKD protocol, and Equations (10) and (11), which model the same coincidence probabilities but under a strong-light attack. The huge increase in the coincidence probabilities under a strong-light attack, even for an injected light level far less than what is required to mount an actual blinding attack, presents a robust figure of merit for discerning the undisturbed quantum communication from an attack condition. In modern QKD systems, the output of each detector is connected to a dedicated channel of a time-dagger, and time stamps are recorded; thus, the above probabilities can be evaluated using a simple data analysis. The defense strategy, which is valid for the receiver in Figure 4, is outlined by the flow chart in Figure 5, and the details are described below. **In the first step**, one collects information about [PERSON]'s system quantum efficiency \(\epsilon_{B}\), the dark count rates \(f_{DCR}^{(k)}\) of all four detectors, where the index \(k\in\{0,1,2,3\}\) marks the detector dedicated to measuring the k-th polarization (namely, the linear polarization of an angle \(\frac{\pi}{4}k\)), and estimates their respective statistical uncertainties \(\sigma_{DCR}^{(k)}\). As before, in order to simplify treatment but with an obvious extension to the general case, I henceforth assume that all detectors exhibit the same dark count parameters: \(f_{DCR}\) and \(\sigma_{DCR}\). All these parameters may be known in advance. Alternatively, one may disconnect [PERSON] from the quantum channel and perform an appropriate set of measurements (calibration) to obtain them. I also assume that the parameters \(\langle n\rangle\), \(f_{G}\), \(t_{G}\) are known or negotiated before the start of the QKD protocol. While [PERSON] may know all of these parameters (by Kerchoff's principle), she may not influence them. **In the second step**, during the quantum communication session (namely, the first phase of the BB84), one measures [PERSON]'s in-gate and out-of-gate coincidence probabilities related to the reception of qubits, namely, \(p^{\prime}_{Cln}\) and \(p^{\prime}_{COnt}\). For example, \(p^{\prime}_{Cln}\) is evaluated as the sum of rates of coincidences between pairs of detectors (0, 2) and (1, 3) that happen only during gates divided by the rate of gates \(f_{G}\). The evaluation of \(p^{\prime}_{COnt}\) is similar except that the coincidences are now counted out of gates. Furthermore, if the quantum channel transmissivity \(\epsilon_{QCh}\) is not known/calibrated beforehand, it should be estimated. To that end, one measures the total rate of detections for all four detectors within a coincidence window \(f_{QInTot}\) and estimates its variance \(\sigma_{QInTot}^{2}\). The qubit reception probability is then equal to  
\[p_{QInTot}=\frac{f_{QInTot}-f_{G}t_{G}\Sigma_{k=0}^{3}f_{DCR}^{(k)}}{f_{G}}= \frac{f_{QInTot}}{f_{G}}-4f_{G}f_{DCR} \tag{12}\]  
According to Equation (10), for an undisturbed BB84, it should be equal to  
\[p_{QInTot}=1-\left(1-\epsilon_{B}\epsilon_{QCh}\right)^{\langle n\rangle}. \tag{13}\]  
Solving for \(\epsilon_{QCh}\) gives  
\[\epsilon_{QCh}=\frac{1}{\epsilon_{B}}\bigg{(}1-\left(1-p_{QInTot}\right)^{ \frac{1}{\langle n\rangle}}\bigg{)}. \tag{14}\]  
Note that [PERSON] does not control the variables \(\epsilon_{B}\) or \(\langle n\rangle\), but she can manipulate the apparent \(p_{QInTot}\) to a certain extent. On one hand, she could lower \(p_{QInTot}\) by randomly omitting qubits from [PERSON], but that would not enable her to increase her information about the resulting key, while the key would only become shorter for all three parties.