Cryptographic Security Concerns on Timestamp Sharing via Public Channel in Quantum Key Distribution Systems

TL;DR

This paper investigates how the choice of bin width and detector response characteristics in quantum key distribution systems affects the security against timing side channel attacks, revealing that larger bin widths do not always reduce information leakage.

Contribution

It challenges the conventional wisdom by showing that mutual information varies non-monotonically with bin width and depends on detector response features and bin start time.

Findings

Mutual information fluctuates with bin width, not always decreasing as bin width increases.

Decreasing the FWHM of detector responses increases mutual information.

The start time of binning significantly affects the mutual information.

Abstract

Quantum key distribution protocols are known to be vulnerable against a side channel attack that exploits the time difference in detector responses used to obtain key bits. The recommended solution against this timing side channel attack is to use a large time bin width instead of high resolution timing information. Common notion is that using a large bin width reduces the resolution of detector responses, hence supposedly minimizes the information leakage to an eavesdropper. We challenge this conventional wisdom, and demonstrate that increasing the bin width does not monotonically reduce the mutual information between the key bits and the eavesdropper's observation of detector responses. Instead of randomly increasing the bin width, it should be carefully chosen because the mutual information fluctuates with respect to the bin width. We also examined the effect of full width half maximums (FWHMs) of the detectors responses on the mutual information and showed that decreasing the FWHM increases the mutual information. Lastly, the start time of binning is also shown to be important in binning process and the mutual information fluctuates periodically with respect to it.