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True randomness is necessary for the security of any cryptographic protocol, including quantum key distribution (QKD). In QKD transceivers, randomness is supplied by one or more local, private entropy sources of quantum origin which can be either passive (e.g., a beam splitter) or active (e.g., an electronic quantum random number generator). In order to better understand the role of randomness in QKD, I revisit the well-known “detector blinding” attack on the BB84 QKD protocol, which utilizes strong light to achieve undetectable and complete recovery of the secret key. I present two findings. First, I show that the detector-blinding attack was in fact an attack on the receiver’s local entropy source. Second, based on this insight, I propose a modified receiver station and a statistical criterion which together enable the robust detection of any bright-light attack and thus restore security.

A homodyne detector, which is also a common element in current telecommunication, is a core component of continuous-variable quantum key distribution (CV-QKD) since it is considered the simplest setup for the distinguishing of coherent states with minimum error. However, the theoretical security of CV-QKD is based on the assumption that the responses of the homodyne detector are always linear with respect to the input, which is impossible in practice. In the real world, a homodyne detector has a finite linear domain, so the linearity assumption is broken when the input is too large. Regarding this security vulnerability, the eavesdropper Eve can perform the so-called homodyne detector-blinding attack by saturating the homodyne detector and then stealing key information without being detected by the legitimate users. In this paper, we propose a countermeasure for the homodyne detector-blinding attack by using an adjustable optical attenuator with a feedback structure. Specifically, we estimate the suitable attenuation value in the data processing of CV-QKD and feed it back to the adjustable optical attenuator before the detector in real time. Numerical simulation shows that the proposed countermeasure can effectively defend against homodyne detector-blinding attacks and ensure the security of the Gaussian-modulated coherent state protocol with finite-size effect.