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Twin-field quantum key distribution (TF-QKD) and its variants can overcomethe fundamental rate-distance limit of QKD which has been demonstrated in thelaboratory and field while their physical implementations with side channelsremains to be further researched. We find the external modulation of differentintensity states through the test, required in those TF-QKD with post-phasecompensation, shows a side channel in frequency domain. Based on this, wepropose a complete and undetected eavesdropping attack, named passive frequencyshift attack, on sending or not-sending (SNS) TF-QKD protocol given anydifference between signal and decoy states in frequency domain which can beextended to other imperfections with distinguishable decoy states. We analyzethis attack by giving the formula of upper bound of real secure key rate andcomparing it with lower bound of secret key rate under Alice and Bob'sestimation with the consideration of actively odd-parity pairing (AOPP) methodand finite key effects. The simulation results show that Eve can get fullinformation about the secret key bits without being detected at long distance.Our results emphasize the importance of practical security at source and mightprovide a valuable reference for the practical implementation of TF-QKD.

Sending or not sending twin-field quantum key distribution with distinguishable decoy states