The Photonic Side Channel

Shahin Tajik

Chi Zhang, Xiao-Long Hu, Cong Jiang +10 more

Quantum key distribution can provide unconditionally secure key exchange for remote users in theory. In practice, however, in most quantum key distribution systems, quantum hackers might steal the secure keys by observing the side channels in the emitted photons, such as the photon frequency spectrum, emission time, propagation direction, spatial angular momentum, and so on. It is hard to prevent such kinds of attacks because side channels may exist in many dimensions of the emitted photons. Here we report an experimental realization of a side-channel-secure quantum key distribution protocol which is not only measurement-device independent, but also immune to all side-channel attacks to the photons emitted from Alice's and Bob's labs. We achieve a secure key rate of 1.73×10^{-6} per pulse through 50 km fiber spools.

Бабухин Данила Валерьевич, Dmitry Anatolevich Kronberg, Сыч Денис Васильевич

The distinguishability of photons in nonoperational degrees of freedom compromises the unconditional security of quantum key distribution since an eavesdropper can improve attack strategies by exploiting this distinguishability. However, the optimal eavesdropping strategies in the presence of light-source side channels are not known. Here we provide several explicit attack strategies on the Bennett-Brassard 1984 (BB84) protocol with partially distinguishable photons. In particular, we consider the phase-covariant cloning attack, which is optimal in the absence of side channels, and show that there are even better strategies in the presence of side channels. The improved strategies exploit a measurement of the side-channel state followed by an attack on the signal photon. Our results explicitly demonstrate a reduction of the critical error rate and set an upper bound on the practical secret key rate.

S. N. Molotkov

Abstract The implementation of quantum cryptography systems significantly differs from idealized models used to prove the secrecy of distributed keys. Without taking into account the imperfections of real systems, it is impossible to seriously talk about cryptographic strength. The practical use of quantum cryptography systems requires taking into account all the real factors affecting the secrecy of distributed keys. In this paper, in fact an analytical method is proposed that takes into account the nonstrict single-photon nature of information states, different quantum efficiencies of detectors, fluctuations of parameters due to finite transmitted sequences, information leakage through side channels during both passive detection of side radiation and active probing of the components of the system. The states in the side channels may have extremely low intensity; therefore, they are considered in quantum terms. The maximum possible total information leakage to the eavesdropper in all channels is achieved in joint collective measurements of both quantum information states and quantum states in side channels. The method is applicable to any spectral distribution of the number of photons in side channels.

Vittorio Degiorgio, Ilaria Cristiani