  The security of a high speed quantum key distribution system with finite
detector dead time \tau is analyzed. When the transmission rate becomes higher
than the maximum count rate of the individual detectors (1/\tau ), security
issues affect the algorithm for sifting bits. Analytical calculations and
numerical simulations of the Bennett-Brassard BB84 protocol are performed. We
study Rogers et al.'s protocol (introduced in "Detector dead-time effects and
paralyzability in high-speed quantum key distribution," New J. Phys. 9 (2007)
319) in the presence of an active eavesdropper Eve who has the power to perform
an intercept-resend attack. It is shown that Rogers et al.'s protocol is no
longer secure. More specifically, Eve can induce a basis-dependent detection
efficiency at the receiver's end. Modified key sifting schemes that are secure
in the presence of dead time and an active eavesdropper are then introduced. We
analyze and compare these secure sifting schemes for this active Eve scenario,
and calculate and simulate their key generation rate. It is shown that the
maximum key generation rate is 1/(2\tau ) for passive basis selection, and
1/\tau for active basis selection. The security analysis for finite detector
dead time is also extended to the decoy state BB84 protocol.
