Abstract

We reply to the comment on our robust countermeasure against the detector control attack [Optica 6, 1178 (2019) [CrossRef]  ] and explain that the concerns arising from an over-interpretation of our countermeasure are one-sided and untenable.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

We appreciate the attention paid by Wu et al. [1] to our robust countermeasure against the detector control attack [2]. However, we have to point out that all four comment points are based on a one-sided depiction of the security of quantum key distribution (QKD) and an over-interpretation of our countermeasure. The countermeasure against detector control attacks is an additional strategy to enhance the practical security of a QKD system. The eavesdropping behavior that steals no information of the secure key makes no sense.

In the process of BB84 QKD, after Alice transmits and Bob measures all the quantum states, they first reveal part of their raw key to estimate critical parameters like quantum bit error rate (QBER), and in turn Eve’s potential information that is upper bounded according to the security proof. For a certain proposed attack, if the information intercepted by Eve $I_{{\rm{AE}}}^{{\rm{eve}}}$ is lower than the estimated upper bound calculated by the security proof $I_{{\rm{AE}}}^{{\rm{est}}}$, Alice and Bob can perform a post-processing procedure to distill the secure key, and reduce Eve’s information to an arbitrarily low value. Based on the parameters of the partial attack [1], $I_{{\rm{AE}}}^{{\rm{est}}}$ is always greater than $I_{{\rm{AE}}}^{{\rm{eve}}}$ for all pass rates, and the value of $({I_{{\rm{AE}}}^{{\rm{est}}} - I_{{\rm{AE}}}^{{\rm{eve}}}})/I_{{\rm{AE}}}^{{\rm{eve}}}$ rapidly increases with pass rate, which means Eve cannot eavesdrop any information of the final secure key shared between Alice and Bob, and the strategy only attacking partial states does not bring any benefits for Eve.

Thus, it is unwarranted to claim that Eve “indeed eavesdrops a certain amount of key” just by employing an intercept-resend strategy with a simple detector control attack that introduces an error rate about 24.75%. In order to verify this thought, we recalculated the key rate based on the scenario and parameters provided in Ref. [1]. Without the attack, Alice and Bob estimate the secure key rate with the measured overall gains and QBERs, the result is 0.002 bit/pulse, which is consistent with Wu et al. Whereas under the partial attack proposed in Ref. [1], Alice and Bob still estimate the key rate with the newly measured overall gains and QBERs, and the result is ZERO! Obviously, Eve cannot eavesdrop any amount of key and definitely exposes her existence.

The second comment point is about the efficiency mismatch between two single photon detectors (SPDs). Surely, no two SPDs are exactly alike in every respect. Under the detector control attack scenario, we assume two SPDs belonging to the same basis are identical. This assumption is the most beneficial to Eve since she can perfectly control two SPDs with the same operations. Otherwise, Eve would leave unconcealable fingerprints if she can only control one SPD entirely every turn due to the mismatch of the SPDs. So even in Wu et al.’s own attack model [3], all of Bob’s SPDs are assumed identical. However, as a small change, two coefficients above 0 can be added into the quadratic terms of the QBER formulas to remove the identical assumption, and the corresponding quadratic terms are indifferent to the final inequality. Thus, our countermeasure is still valid without assuming that two SPDs have the same efficiency.

The last two comment points reflect the inadequate understanding of Bob’s autonomy. Though Alice and Bob are assumed to be much weaker than Eve in QKD research, based on the random numbers they still have “free will” to carry out their actions. In our countermeasure, the attenuation—including the value and execution moment—of the variable attenuator (VA) in front of the SPD is randomly changed according to Bob’s random number, which is unavailable to Eve in QKD’s assumptions.

As Wu et al. said in the third point, any electronic signal “undergoes a rising edge or a falling edge” [1]. If “gate signals under these edges” happened, Bob could only count the detection events outside the switching edges. It has no impact on our conclusion. Furthermore, the countermeasure has no special requirement on the switching edges, which can be wider than the gate or even several repetition periods.

The last comment point is about their own newly proposed attack [3]. For the pulse illumination attack with several hundred bright pulses [3], even their homemade SPD (with an abnormal “comparator that only works on the timing” when the gate is loaded [3]) does not click when the bright pulse train randomly modulates by VA. According to the measured data in Ref. [3], both the blinded and controlled periods are very sensitive to the blinding energy, and therefore will certainly be changed by the VA. These random changes will inevitably introduce extra errors, in addition to the “huge instantaneous photocurrent hill” observed by Wu et al. in Ref. [3]. These abnormal performances will definitely reveal Eve’s existence. At the very least, if none of the above happens, all VA switching actions occur in the fully controlled period without “blinding pulses”, since the VA still worked on “trigger pulses.” Then we would be back to our countermeasure model against the detector control attack without blinding light. So our countermeasure is still effective.

In summary, all of the four comment points in Ref. [1] are one-sided and untenable. Our countermeasure provides an effective approach to enhance the security of realistic QKD systems. In addition, for a specific QKD system with a specific transmission loss, the ranges of ${R_0}/\!{R_3}$ and $\{{{e_0},{e_3}} \}$ should be further optimized to enhance the practicability of the countermeasure.

Funding

National Key Research and Development Program of China (2018YFA0306400).

Disclosures

The authors declare no conflicts of interest.

REFERENCES

1. Z. Wu, A. Huang, X. Qiang, J. Ding, P. Xu, X. Fu, and J. Wu, “Robust countermeasure against detector control attack in a practical quantum key distribution system: comment,” Optica 7, 1391–1393 (2020). [CrossRef]  

2. Y.-J. Qian, D.-Y. He, S. Wang, W. Chen, Z.-Q. Yin, G.-C. Guo, and Z.-F. Han, “Robust countermeasure against detector control attack in a practical quantum key distribution system,” Optica 6, 1178–1184 (2019). [CrossRef]  

3. Z. Wu, A. Huang, H. Chen, S.-H. Sun, J. Ding, X. Qiang, P. Xu, and J. Wu, “Hacking single-photon avalanche detector in quantum key distribution via pulse illumination,” arXiv 2002.09146 (2020).

References

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  1. Z. Wu, A. Huang, X. Qiang, J. Ding, P. Xu, X. Fu, and J. Wu, “Robust countermeasure against detector control attack in a practical quantum key distribution system: comment,” Optica7, 1391–1393 (2020).
    [Crossref]
  2. Y.-J. Qian, D.-Y. He, S. Wang, W. Chen, Z.-Q. Yin, G.-C. Guo, and Z.-F. Han, “Robust countermeasure against detector control attack in a practical quantum key distribution system,” Optica 6, 1178–1184 (2019).
    [Crossref]
  3. Z. Wu, A. Huang, H. Chen, S.-H. Sun, J. Ding, X. Qiang, P. Xu, and J. Wu, “Hacking single-photon avalanche detector in quantum key distribution via pulse illumination,” arXiv 2002.09146 (2020).

2019 (1)

Chen, H.

Z. Wu, A. Huang, H. Chen, S.-H. Sun, J. Ding, X. Qiang, P. Xu, and J. Wu, “Hacking single-photon avalanche detector in quantum key distribution via pulse illumination,” arXiv 2002.09146 (2020).

Chen, W.

Ding, J.

Z. Wu, A. Huang, X. Qiang, J. Ding, P. Xu, X. Fu, and J. Wu, “Robust countermeasure against detector control attack in a practical quantum key distribution system: comment,” Optica7, 1391–1393 (2020).
[Crossref]

Z. Wu, A. Huang, H. Chen, S.-H. Sun, J. Ding, X. Qiang, P. Xu, and J. Wu, “Hacking single-photon avalanche detector in quantum key distribution via pulse illumination,” arXiv 2002.09146 (2020).

Fu, X.

Z. Wu, A. Huang, X. Qiang, J. Ding, P. Xu, X. Fu, and J. Wu, “Robust countermeasure against detector control attack in a practical quantum key distribution system: comment,” Optica7, 1391–1393 (2020).
[Crossref]

Guo, G.-C.

Han, Z.-F.

He, D.-Y.

Huang, A.

Z. Wu, A. Huang, X. Qiang, J. Ding, P. Xu, X. Fu, and J. Wu, “Robust countermeasure against detector control attack in a practical quantum key distribution system: comment,” Optica7, 1391–1393 (2020).
[Crossref]

Z. Wu, A. Huang, H. Chen, S.-H. Sun, J. Ding, X. Qiang, P. Xu, and J. Wu, “Hacking single-photon avalanche detector in quantum key distribution via pulse illumination,” arXiv 2002.09146 (2020).

Qian, Y.-J.

Qiang, X.

Z. Wu, A. Huang, X. Qiang, J. Ding, P. Xu, X. Fu, and J. Wu, “Robust countermeasure against detector control attack in a practical quantum key distribution system: comment,” Optica7, 1391–1393 (2020).
[Crossref]

Z. Wu, A. Huang, H. Chen, S.-H. Sun, J. Ding, X. Qiang, P. Xu, and J. Wu, “Hacking single-photon avalanche detector in quantum key distribution via pulse illumination,” arXiv 2002.09146 (2020).

Sun, S.-H.

Z. Wu, A. Huang, H. Chen, S.-H. Sun, J. Ding, X. Qiang, P. Xu, and J. Wu, “Hacking single-photon avalanche detector in quantum key distribution via pulse illumination,” arXiv 2002.09146 (2020).

Wang, S.

Wu, J.

Z. Wu, A. Huang, X. Qiang, J. Ding, P. Xu, X. Fu, and J. Wu, “Robust countermeasure against detector control attack in a practical quantum key distribution system: comment,” Optica7, 1391–1393 (2020).
[Crossref]

Z. Wu, A. Huang, H. Chen, S.-H. Sun, J. Ding, X. Qiang, P. Xu, and J. Wu, “Hacking single-photon avalanche detector in quantum key distribution via pulse illumination,” arXiv 2002.09146 (2020).

Wu, Z.

Z. Wu, A. Huang, X. Qiang, J. Ding, P. Xu, X. Fu, and J. Wu, “Robust countermeasure against detector control attack in a practical quantum key distribution system: comment,” Optica7, 1391–1393 (2020).
[Crossref]

Z. Wu, A. Huang, H. Chen, S.-H. Sun, J. Ding, X. Qiang, P. Xu, and J. Wu, “Hacking single-photon avalanche detector in quantum key distribution via pulse illumination,” arXiv 2002.09146 (2020).

Xu, P.

Z. Wu, A. Huang, X. Qiang, J. Ding, P. Xu, X. Fu, and J. Wu, “Robust countermeasure against detector control attack in a practical quantum key distribution system: comment,” Optica7, 1391–1393 (2020).
[Crossref]

Z. Wu, A. Huang, H. Chen, S.-H. Sun, J. Ding, X. Qiang, P. Xu, and J. Wu, “Hacking single-photon avalanche detector in quantum key distribution via pulse illumination,” arXiv 2002.09146 (2020).

Yin, Z.-Q.

Optica (1)

Other (2)

Z. Wu, A. Huang, H. Chen, S.-H. Sun, J. Ding, X. Qiang, P. Xu, and J. Wu, “Hacking single-photon avalanche detector in quantum key distribution via pulse illumination,” arXiv 2002.09146 (2020).

Z. Wu, A. Huang, X. Qiang, J. Ding, P. Xu, X. Fu, and J. Wu, “Robust countermeasure against detector control attack in a practical quantum key distribution system: comment,” Optica7, 1391–1393 (2020).
[Crossref]

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