The security of quantum noise stream cipher against collective attacks

Quantum noise stream cipher (QNSC), where signal states are masked by intrinsic quantum noise to directly encrypt data, provides a physical layer security. This paper firstly discusses the security of the QNSC system under the assumption that eavesdropper is restricted to collective attacks. The maximum security capacity for the encrypted data, the running key, and the entire system is derived, respectively. Our simulation results allow a positive information capacity when data transmission distance is no more than 300 km for the number of bases $$M_\mathrm{{b}} = 31$$ and any mean photon number $$\alpha ^2$$ by assuming state-of-the-art technology. We find that the security of whole system depends on the security of running key for shorter distances. However, the security of whole system is related to the security of data for longer distances. Furthermore, it is important to improve security of data in QNSC system for longer distances. Besides, this new method proposes a concrete solution for physical encryption systems and paves the way for a wider implementation of the QNSC system.