  Round-robin-differential-phase (RRDPS) quantum key distribution (QKD)
protocol has attracted intensive studies due to its distinct security
characteristic, e.g., information leakage in RRDPS can be bounded without
learning error rate of key bits. Nevertheless, its implementation is still far
from practical due to the complication of its measurement device. Moreover, on
the theoretical side, its security is still not clear in view of optimal
attack. Here, by observing a potential phase randomization of the encoding
states and its connection with eavesdropper's ancilla, we develop a theory to
bound information leakage quite tightly and differently. Our theory is
applicable for both with and without monitoring signal disturbance scenarios,
which is significant for the understanding of RRDPS. Based on our novel
security proof, the practicality and performance of RRDPS can be both improved
dramatically. Furthermore, we realize a proof-of-principle experiment up to 140
km fiber distance which is the longest achievable distance of RRDPS system
until now, while the original security proof predicts no secret key can be
generated in our experiment. Our results pave an avenue towards practical
RRDPS.
