Quantum Key Distribution With an Unknown and Untrusted Source

The security of a standard bi-directional "Plug & play" Quantum Key Distribution (QKD) system has been an open question for a long time. This is mainly because its source is equivalently controlled by an eavesdropper, which means the source is unknown and untrusted. Qualitative discussion on this subject has been made previously. In this paper, the authors present the first quantitative security analysis on a general class of QKD protocols whose sources are unknown and untrusted.