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arXiv:0704.3253 · Physical Review A

Quantum hacking: Experimental demonstration of time-shift attack against practical quantum-key-distribution systems

Yi Bing Zhao, Chi‐Hang Fred Fung, Bing Qi, Christine Chen, Hoi‐Kwong Lo

Published Oct 28, 2008• 598 citations
arXivHTMLPDF

Abstract

Quantum-key-distribution (QKD) systems can send quantum signals over more than $100\phantom{\rule{0.3em}{0ex}}\mathrm{km}$ standard optical fiber and are widely believed to be secure. Here, we show experimentally a technologically feasible attack---namely, the time-shift attack---against a commercial QKD system. Our result shows that, contrary to popular belief, an eavesdropper, Eve, has a non-negligible probability $(\ensuremath{\sim}4%)$ to break the security of the system. Eve's success is due to the well-known detection efficiency loophole in the experimental testing of Bell's inequalities. Therefore, the detection efficiency loophole plays a key role not only in fundamental physics, but also in technological applications such as QKD systems.

DOI
10.1103/physreva.78.042333

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Quantum-key-distribution (QKD) systems can send quantum signals over more than 100km100\phantom{\rule{0.3em}{0ex}}\mathrm{km}100km standard optical fiber and are widely believed to be secure. Here, we show experimentally a technologically feasible attack---namely, the time-shift attack---against a commercial QKD system. Our result shows that, contrary to popular belief, an eavesdropper, Eve, has a non-negligible probability (\ensuremath∼4(\ensuremath{\sim}4%)(\ensuremath∼4 to break the security of the system. Eve's success is due to the well-known detection efficiency loophole in the experimental testing of Bell's inequalities. Therefore, the detection efficiency loophole plays a key role not only in fundamental physics, but also in technological applications such as QKD systems.
Quantum key distribution (QKD) can, in principle, provide unconditional security based on the fundamental laws of physics. Unfortunately, a practical QKD system may contain overlooked imperfections and violate some of the assumptions in a security proof. Here, we report two types of eavesdropping attacks against a practical QKD system. The first one is "time-shift" attack, which is applicable to QKD systems with gated single photon detectors (SPDs). In this attack, the eavesdropper, Eve, exploits the time mismatch between the open windows of the two SPDs. She can acquire a significant amount of information on the final key by simply shifting the quantum signals forwards or backwards in time domain. Our experimental results in [9] with a commercial QKD system demonstrate that, under this attack, the original QKD system is breakable. This is the first experimental demonstration of a feasible attack against a commercial QKD system. This is a surprising result. The second one is "phase-remapping" attack [10]. Here, Eve exploits the fact that a practical phase modulator has a finite response time. In principle, Eve could change the encoded phase value by time-shifting the signal pulse relative to the reference pulse.
The rules of quantum mechanics makes it possible to exchange a secret key at a distance. This is called quantum key distribution (QKD). In theory the key exchange can be made completely secure. Real QKD implementations however, has numerous imperfections. Luckily one has also been able to prove the security of QKD with a large variety of imperfections. The field of QKD has matured over the recent years, and it has now reached commercial applications with photons as the quantum bits, and optical fibers as the quantum channel. Today there are at least three commercial vendors of QKD-systems. We live in the times of quantum hacking. Researchers has begun the task of breaking the security of QKD-systems. Many new imperfections has been discovered, some of which might be used to break the security of QKD. This thesis is a study of the detector efficiency mismatch loophole. Most QKD-systems require two detectors, and it is virtually impossible to make two identical detectors with the exact same efficiency. What is worse, it turns out that the eavesdropper can often control the relative efficiencies of the two detectors trough some domain, for instance by controlling the timing, the frequency or the spacial mode of the photons. This can in turn be used by the eavesdropper to gain information about the secret key. Previously the best known attack would compromise security if the detector efficiency mismatch of about 1:15. Here the current attacks on systems with detector efficiency mismatch are improved to compromise security for a mismatch of about 1:4. This is less than the mismatch found in a commercial QKD-system, so the attack could in principle be used to eavesdrop on this QKD-system. One might try to close the loophole by modifying the implementation. One suggestion is the four state Bob. The problem is that this patch will in turn open other loopholes, and one of these loopholes reopen the detector efficiency mismatch loophole. One can remove Eves information about the key by doing a sufficient amount of extra privacy amplification. Here a general security bound is presented, quantifying the required amount of extra privacy amplification to remove Eve's information about the key. The proof is more general than the previous security proof, and is valid for any basis dependent, possibly lossy, linear optical imperfections in the channel and receiver/detectors. Since this is more realistic assumptions for a QKD-implementation, the proof represents a major step of closing the loophole in real devices.
Unconditionally secure communication, which has been pursued for thousands of years hasn't, however, yet been reached due to a continuous competition between encryption and hacking. Quantum key distribution (QKD), harnessing the quantum mechanical nature of superposition and noncloning, may promise unconditional security by incorporating the one-time pad algorithm rigorously proved by Claude Shannon. Massive efforts have been made in building practical and commercial QKD systems; in particular, decoy states have been employed to detect photon-number splitting attacks against a single-photon source loophole, and measurement-device-independent (MDI) QKD has further closed all loopholes on the detection side, which leads to a seemingly real-life application. Here, we propose and experimentally demonstrate a MDI-QKD hacking strategy on the trusted-source assumption by using an injection-locking technique. An eaves-dropper (Eve) injects near off-resonance photons with randomly chosen polarization into a sender's laser, where injection locking in a shifted frequency can happen only when Eve's choice matches the sender's state. By setting a shifted window and switching the frequency of photons back afterwards, Eve in principle can obtain all the keys without terminating the real-time QKD. We observe the dynamics of a semiconductor laser with injected photons, and obtain a hacking success rate reaching $60.0\mathrm{%}$ of raw keys. Our results suggest that the spear-and-shield competition of unconditional security may continue until all potential loopholes are discovered and closed ultimately.
We demonstrate a practical countermeasure against a well-known class of attacks on quantum key distribution (QKD) systems that exploit detection efficiency mismatch, where the receiver's detectors do not exhibit identical responses to incoming photons across all degrees of freedom. This class of quantum hacking strategies is broad and significantly includes the time-shift attack, which targets an arrival-time-dependent side channel at the receiver. The four-state countermeasure, previously only proven to be secure in theory, is implemented here on a GHz-clocked prototype QKD system and evaluated for its security and performance. We show that its presence enables almost complete recovery of the system's ideal secret key rate. Our results provide strong justification for adopting this countermeasure as a standard component in future scalable and practical QKD systems.
Security verification for the physical implementation of a cryptography system is an important step to ensure the security level promised by theory. As has been shown many times, any physical device has characteristics and behavior that deviate from theoretical expectations. Frequently, those lead to new security loopholes. \n \nThis thesis presents three experimental studies of attacks on quantum key distribution (QKD) systems. The first is the detection efficiency mismatch on free-space systems, which takes advantage of alignment imperfections in Bob's detector to control detection efficiencies. The experiment was done on a polarization-encoding free-space receiver to find the detection efficiencies of each detector for different spatial modes of an incoming photon. Those results were put into an optimization program, which modeled an intercept-and-resend attack on a non-decoy Bennett-Brassard 1984 (BB84) protocol. The result shows that an adversary is able to gain information about the key without being detected by Alice and Bob. The second study is an experimental test of reliability of a spatial filter (a pinhole), which is proposed as a countermeasure for the previous attack. The result shows that, by sending a high-power laser beam focused on the pinhole, the pinhole can be widened without affecting other components in the receiver. Thus, the ability to perform a spatial mode detection efficiency-mismatch attack is recovered. The last experiment is a demonstration of Eve's ability to force a commercial system to distill a key from a raw key of a short length, where the asymptotic assumption of security claimed by the manufacturer might not hold. It was shown that this could be done by inducing transmission loss in the channel at an appropriate time.
Recently, a new type of attack, which exploits the efficiency mismatch of two single photon detectors (SPD) in a quantum key distribution (QKD) system, has been proposed. In this paper, we propose another ``time-shift'' attack that exploits the same imperfection. In our attack, Eve shifts the arrival time of either the signal pulse or the synchronization pulse or both between Alice and Bob. In particular, in a QKD system where Bob employs time-multiplexing technique to detect both bit "0'' and bit "1'' with the same SPD, Eve, in some circumstances, could acquire full information on the final key without introducing any error. In addition, we prove that if Alice and Bob are unaware of our attack, the final key they share is insecure. We emphasize that our attack is simple and feasible with current technology. Finally, we discuss some counter measures against our and earlier attacks.