# Quantum Key Distribution in the Presence of the Intercept-Resend with Faked States Attack
## 2 Quantum Hacking in \(QKD\) Systems  
A variety of attacks have been conceived of as exploiting the security of \(BB84\)-based systems, either theoretically or technologically. The Photon Number Splitting (\(PNS\)) attack can be included in the first category. In the second class, commonly referred to as quantum hacking, the Intercept Resend with Faked States (\(IRFS\)) attack can be mentioned, which exploits loopholes in the Avalanche Photo Diodes (\(APDs\)) of the electronic detection system. In the following section, we will describe the Intercept Resend (\(IR\)) attack and the Intercept-Resend with Faked States (\(IRFS\)). (a) Intercept Resend (\(IR\)) attack  
In the intercept resend attack, the eavesdropper measures each photon pulse sent by [PERSON], which she replaces with another pulse prepared in the quantum state that she has already measured. [PERSON] is successful \(50\%\) of the time in measuring the pulse in the correct measurement basis, while [PERSON] chooses half of the times the same basis as her; thus over time, she generates a Quantum Bit Error Rate (\(QBER\)) of \(50\%\times 50\%=25\%\) (see Figure 1 and [4]). (b) Intercept Resend with Faked States (\(IRFS\)) attack  
In the Intercept Resend with Faked States (\(IRFS\)) attack, the eavesdropper is not interested in reconstructing the original states, but in producing instead pulses of that light that can be detected by [PERSON] in a way that is controlled by her while she passes unnoticed in the quantum channel. The eavesdropper exploits imperfections of their optical system, so that [PERSON] and [PERSON] can assume that they are detecting the original quantum states while they are in fact detecting light pulses generated by [PERSON]. These light pulses are called faked states [7]. To mount an \(IRFS\) attack, [PERSON] can exploit some weaknesses of [PERSON]'s detector, such as time shift [8; 9; 10] or quantum blinding [7; 8; 9]. The quantum blinding attack is an \(IRFS\) attack where the \(QKD\) system is controlled by an eavesdropper using bright photon pulses during the linear mode operation of the \(APDs\). In such attacks, [PERSON] can eavesdrop on the full secret key without increasing the \(QBER\) of the protocol. In order  
Figure 1: The Intercept Resend (\(IR\)) attack against the \(BB84\) produces a detectable Quantum Bit Error Rate (\(QBER\)) of \(25\%\). In the figure, [PERSON] sends a \(|0_{Z}\rangle\) state to [PERSON]. However, [PERSON] is in the middle of the quantum channel, and she applies the \(X\) basis measurement getting \(|1_{X}\rangle\). Thus, she prepares a copy of such a state, and she resends it to [PERSON], who obtains \(|1_{Z}\rangle\) because he used the \(Z\) basis measurement. This introduces a secret bit error because [PERSON] expects that [PERSON] obtained \(|0_{Z}\rangle\). to achieve this, the eavesdropper sends bright pulses to [PERSON]'s station that will be detected by the \(APD\), which would operate like a classical photo diode, instead of operating in Geiger mode. [PERSON] now can use the \(IRFS\) attack to obtain the key [11; 12]. As a result, as depicted in Figure 2a, if [PERSON] chooses the same measurement basis as [PERSON], he gets a detection event in the corresponding \(APD\) detector. Otherwise, if [PERSON] measures with the opposite basis as shown in Figure 2b, the optical power is distributed over the two detectors, and no event is detected. Thus, [PERSON] blinds [PERSON]'s \(APDs\) detectors to make them operate as classical photo diodes. At the final stage of the protocol, [PERSON] exploits the announcements revealed by [PERSON] over the public channel to perform the classical post-processing, getting the same secret bits as [PERSON] and [PERSON]. A simple countermeasure that can be applied in the electronic detection system is a watchdog detector that detects bright faked states [13]. The intercept-resend with faked states attack and quantum blinding were first implemented over a commercial \(QKD\) system at the University of Singapore [12].