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%-------------------------------------------------- Title -----------------------------------------------------%

\title{Countering Detector Manipulation Attacks in
  Quantum Communication through Detector Self-testing}%

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\author{
    Lijiong~Shen\textsuperscript{(1)}, Christian~Kurtsiefer\textsuperscript{(1,2)}}%

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   \textsuperscript{(1)} Centre for Quantum Technologies, National University of Singapore, 3 Science Drive 2, Singapore 117543,
   \textcolor{blue}{\uline{}} 

   \textsuperscript{(2)} Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551,
   \textcolor{blue}{\uline{christian.kurtsiefer@gmail.com}} 

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 Detector manipulation attacks are the most critical vulnerabilities in practical quantum key distribution systems. We present a self-testing method of photodetectors to reveal manipulation by anything but single photon-level signals, which does not rely on specific assumptions about the detection or manipulation mechanism.
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    % Your 45 words abstract, which will appear in the conference program, should be an explicit summary of the paper that states the problem, the methods used as well as the major results and conclusions. It should be complementary to rather than a repeat of the title. \textcopyright2022 The Author(s)
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\section{Introduction} 
In practical quantum key distribution systems (QKD), imperfect physical devices open 
security loopholes that challenge the core premise of this technology~\cite{Scarani2009,Scarani2014,Xu2020}. Alternatively,
A critical  vulnerability of QKD systems is the detector
blinding / fake state attack family on single-photon
detectors~\cite{Makarov2009}. This class of attacks has been experimentally demonstrated
to work for detectors based on avalanche photodiodes and superconducting
nanowires~\cite{Lydersen2010,Lydersen2011,Goltsman2019}.


Countering detector manipulation attacks has been heavily investigated over
the past decade~\cite{Honjo2013,Lim2015,Qian2019}. However, most
countermeasures have drawbacks in significantly increasing the overall cost
and complexity, or reduce significantly the QKD bit rate. Here, we present a self-testing method of photodetectors to reveal manipulation by anything but single photon-level signals. 
% A detector manipulation attack, being optical or of other nature, is revealed by detecting an excess or a lack of detector events due to this self-illumination. 


\section{Self-testing strategy}


In a typical blinding attack, the adversary first measures the
photons in the quantum channel, and then replicates the result on the corresponding single-photon detector at the legitimate receiver by blinding the detector using a tailored bright illumination, and creating ``fake state'' using  stronger light pulses. 
We shows three methods of detecting a detector manipulation attack using a
simple light emitter (LE) controlled by the legitimate user coupled weakly to the single photon detectors.

\begin{figure}[!htb]
    \centering
    \includegraphics[width=1\linewidth]{figures/figure2.pdf}
    \caption{\label{fig:st-protocol}Detector self testing. Top trace: light level
    of the light emitter LE, middle trace: normal detector response (no
    manipulation), lower trace: detector response under manipulation.}
\end{figure}


When the LE is switched off (Fig.~\ref{fig:st-protocol}(a)), each detector
will detect single photons from either the legitimate sender or background
events in an unblinded scenario (labeled ``N" in
Fig.~\ref{fig:st-protocol}). The adversary will try to keep the ``fake'' state
(labeled ``F'' in Fig.~\ref{fig:st-protocol}) indistinguishable from the
normal detector response to make a successful attack. When the LE is switched
on at a low level, the optical power coupled to the single-photon detector is
much lower than the blinding power sent by the adversary. Thus, only an
unblinded detector will produce additional salt
events (``S'' in Fig.~\ref{fig:st-protocol}(b)). The legitimate user can detect the blinding attack by monitoring photon detection statistics in time interval T when the LE is switched on. 

Alternatively, the attack could also be identified by the absence of flag events (``FL'' in Fig.~\ref{fig:st-protocol}(c) ) when turning on the LE for a short pulse time interval $\delta$t. The optical power should be set at a level just enough to cause a detection event with almost unit probability on an unblinded single-photon detector. 

Further increasing the optical power of the LE leads to the third method,
where  the detector would be self-blinded. A detector not manipulated  by an
adversary  should produce a flag event  immediately after switching on the LE,
and stay silent during the rest of the self-blinding interval (see
Fig.~\ref{fig:st-protocol}(d)). Here, any positive detector manipulation will
overrule the local blinding and cause a detector event, revealing the manipulation. Both the flag event and the photon detection event in the rest of self-blinding interval could then be used to reveal the adversary's existence. 



\begin{figure}[!t]    
    \centering
    \includegraphics[width=1\linewidth]{figures/figure3a.pdf}
    % \includegraphics[width=.95\linewidth]{figures/figure3b.pdf}
     \caption{\label{fig:demosetup}
      Setup to demonstrate detector self-testing. Light from a cw
      laser diode (LD1) and a pulsed laser diode (LD2) is
      combined in a fiber beamsplitter (BS) to simulate different
      illumination scenarios. An interference filter (IF) prevents 
      LE (a LED) light leaking to the quantum channel.}
\end{figure}




 
\section{Experiment}
Figure~~\ref{fig:demosetup} illustrates the experimental setup for the countermeasure demonstration.
A low-cost LED acts as the LE for self-testing.   We consider an normal event
rate of 50\,000 s$^{-1}$ on our InGaAs detector (APD2), which is about an
order of magnitude below the maximal detection rate, and does not significantly reduce the detector efficiency. For the first method of detector self-testing, we choose a time interval of 200$\,\mu$s, which contains a mean photon number of 10 in normal operation. We switch on the LED for  more than  7000 trials  when  the detector is non-manipulated as well as when it is blinded.  
The mean photon number significantly increases to 100 for a non-manipulated detector with a minimum measured photon detection events of 79. On the other hand, the mean photon number of the blinded detector remains at 10 with a maximum photon detection events of 29. 

For the second method, a function generator drives the LED to emit 25 ns long
pulses. In more than 10000 trials, a non-manipulated detector has 93.4\%
probability  to register output photon detection events within 60\,ns, while for a blinded detector, the probability drops down to 0.3\%. Therefore, a few short test pulses can identify the attack with very high statistical significance. 

For the last self-binding method,  the non-manipulated detector has a
probability of 97.6\% in more than 7000 trials producing flag events 60\,ns after the onset
of LED light emission and a probability of 99.9\% of staying silent in the rest of the time interval T (set at 200\,$\mu$s). The blinded detector only has 0.2\% accidental flag events in the first 60\,ns, and has a close to unit probabilityto produce at least one detection event in the remaining of the time interval T.

\section{Conclusion}
We experimentally demonstrated that the self-testing concept is able to reliably reveal detector manipulation attacks on a typical InGaAs avalanche photodetector within a very short time.
This self-testing concept neither relies on specific assumptions on the detection or manipulation mechanism, nor requires the technologically more complex protocol of a measurement-device independent quantum key distribution~\cite{MDIQKD:2012}. 
 This scheme could also be easily implemented with any single-photon detector and retrofit to existing QKD systems. 
% \section*{Acknowledgements}
% This work was supported by by the National Research Foundation (NRF) Singapore
% (Grant: QEP-P1), and through the Research Centres of Excellence programme
% supported by NRF Singapore and the Ministry of Education, Singapore.
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% 
\section{Acknowledgements}

We thank S-Fifteen Instruments Pte. Ltd. providing us the InGaAs APD for testing.

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