## 6 Discussion and Conclusions  
In this work, I investigate the merit of internal entropy sources for the security of QKD. To that end, I study a particularly vicious \"detector blinding attack\" (DBA) which was successfully demonstrated on the most frequently used architectures of QKD receivers used for the BB84 protocol, as described in the literature. By virtue of a DBA, an attacker ([PERSON]) is able to obtain a perfect copy of the key agreed upon between two legitimate parties ([PERSON] and [PERSON]) while remaining undetected. While most of the defenses described in the literature consist essentially of detecting a strong light incoming from the quantum channel, the authors of the DBA have shown that such countermeasures can be defeated, as discussed in the introduction. Therefore, I took a different path. Noting that the necessary condition for the success of a DBA is [PERSON]'s ability to gain control over the internal entropy source of the receiver station (Bob), I modified the setup such that the mentioned condition cannot be met. We show that even though the successfully attacked passive (Figure 2) and semi-active (Figure 3) receiver configurations, as well as the fully active (Figure 4) configuration proposed herein, are all seemingly functionally equivalent, only the latter is secure against attacks on the internal entropy source. Namely, a circularly polarized light incident on the passive or semi-active setup will distribute its power evenly between all detectors present in the setup. This gives [PERSON] an opportunity to blind the detectors and consequently manipulate their detections. On the other hand, in the proposed fully active receiver, light never falls on all detectors simultaneously; it can only fall upon one detection base at a time. Whenever the blinding light is switched from one base to another which was previously in the dark, the two detectors belonging to that base will generate a coincident detection with a very high probability which is orders of magnitude higher than in the normal (undisturbed) quantum communication, in which correlations of that sort are highly suppressed by design. In essence, my attack defense consists of comparing the measured coincidence probability to the level expected from the undisturbed quantum communication; if it is higher, then the system is under attack. In the passive and the semi-active setups, an information-theoretically perfect random number generator (QRNG), which is required for selecting the receiving base, is realized via a front-end non-polarizing beam splitter which can be directly accessed by [PERSON] via the quantum channel. A subtle difference of my proposed setup is that it makes use of an explicit QRNG which collapses the wave function within itself and outputs entropy in form of classical information. Such an entropy source does not accept any information from the communication channels and consequently cannot be controlled nor predicted by [PERSON]. Thus, I deter the DBA, and in fact any strong-light attack, at the conceptual level by disabling for [PERSON] the possibility of manipulating or reading out the internal entropy source of Bob. The modified receiver setup and the statistical criterion for recognizing a strong-light attack are the main contributions of this work. From the rigorous mathematical treatment and examples, it is clear that the proposed statistical criterion has an overwhelming power to discern between a genuine quantum communication and an attack using a strong light. I believe that my approach of ensuring provable security, which is non-specific to any particular version of a DBA, should have greater resilience against modified DBAs that may be conceived in the future. Even so, I think that neither this, nor any other defense strategy, voids the necessity of monitoring the proper functioning of single-photon detectors, as well as all other active or passive components of both QKD stations, Alice and Bob, alike, and the characteristics of the quantum channel.