  This paper presents a multi-stage, multi-photon quantum key distribution
protocol based on the double-lock cryptography. It exploits the asymmetry in
the detection strategies between the legitimate users and the eavesdropper. The
security analysis of the protocol is presented with coherent states under the
intercept-resend attack, the photon number splitting attack, and the
man-in-the-middle attack. It is found that the mean photon number can be much
larger than one. This complements the recent interest in multi-photon quantum
communication protocols that require a pre-shared key between the legitimate
users.
