Cryptographic security concerns on timestamp sharing via a public channel in quantum-key-distribution systems

Quantum-key-distribution protocols are known to be vulnerable against a side channel attack that exploits the time difference in detectors??? 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. A common notion is that using a large bin width reduces the resolution of detectors??? 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 detectors??? 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 examine the effect of full width half maximums (FWHMs) of the detectors??? responses on the mutual information and show that decreasing the FWHM increases the mutual information. Lastly, the start time of binning is also shown to be important in the binning process and the mutual information fluctuates periodically with respect to it.