Calibration Attack and Defense in Continuous Variable Quantum Key Distribution

Abstract

We have found neew attacks agains Continuous Variable Quantum Key Distribution based on the accessibility of the phase reference beam by the adversary. We then give easy countermeasures to this attack and prove their security.Over the past few years, quantum continuous variables (CV) have been explored as an alternative to qubits for quantum key distribution (QKD). More specifically, protocols using coherent states and homodyne or heterodyne measurements have been proposed and experimentally demonstrated [1]. Relying on technologies allowing much higher rates than allowed by the single photon detectors used in qubit based QKD, those protocols are the only ones which could allow key rates in the GHz range in the foreseeable future.Furthermore, their security proofs have recently been extendend to generic attacks in the case of gaussian modulation. However, to our knowledge, all security analyses up to know have implicitely assumed Bob's measurement setup to use a homo-or heterodyne detection with an external classical phase reference, while the experimental realizations use a lightbeam for this role.This beam doesn't carry any information useful to the spy, and any attack on its phase could equally be carried on the signal beam. However, the intensity of this beam is used in experimental implementations to calibrate the quantum noise level. This calibration is then used to determine the channel gain, which is an important parameter of Eve's attack.We show that this allows Eve to implement an intercepte resend attack forbiding QKD as soon as the losses are above 3dB. We also give simple countermeasures to this new class of attacks. * Electronic address: frederic.grosshans@ens-cachan.fr [1] F. Grosshans, G. Van Assche, J. Wenger, R. Brouri, N.J. Cerf and Ph. Grangier, Nature (London) 421, 238 (2003), arXiv quant-ph/0312016.

Long-distance continuous-variable quantum key distribution by controlling excess noise

“…Since the shot noise variance is proportional to the LO power, we use a photodiode (PD) to monitor the LO at Bob’s side which is transmitted through the insecure quantum channel and it could be manipulated by a potential eavesdropper. In addition, a recent robust shot noise measurement scheme 23 can also be employed in our experiment to prevent some attacks targeting the shot noise, such as LO fluctuation attacks 24 and LO calibration attacks 25 26 . The potential risk of other attacks can also be resisted by additionally inserting optical devices.…”

One-Time Shot-Noise Unit Calibration Method for Continuous-Variable Quantum Key Distribution

“…This way, the actual slope of the response curve will be decreased, then if Bob still employs the original response curve to evaluate the SNU, the SNU will be overestimated, which further will lead to the underestimate of the excess noise. Also, attack against the intensity of the LO [28] during the key distribution stage can also cause the misestimate of the SNU, the noise induced by Eve's attack may be underestimated if Bob applies the calibrated SNU to normalise its data. Thus a security loophole may be turned on.…”

“…And since the SNU is not measured directly, it will certainly bring in more inaccuracy. Notwithstanding, such calibration scheme can open security loopholes that the eavesdropper Eve can utilize to procure the key information [28,29]. Eve can take actions to change the SNU during the key distribution procedure, then the SNU used to normalize the measured quadratures will not be the same as the real SNU, in this way Alice and Bob are prone to underestimate the channel excess noise that further threats the security of the CV-QKD system.…”

Preventing calibration attacks on the local oscillator in continuous-variable quantum key distribution

“…Wavelength dependent beamsplitter attacks targeting CVQKD schemes using heterodyne detection have recently been studied [10,11]. Finally, attacks specific to CVQKD [12,13] typically involve manipulation of the power of the local oscillator, which is the phase reference classical signal required for the coherent detection and is usually sent from Alice to Bob together with the quantum signal [14].…”