Circular mediated semi-quantum key distribution.

Mediated semi-quantum key distribution (M-SQKD) allows two "semi-quantum" or "classical" users to establish a secret key with the help of a third party (TP), in which TP has full quantum power and possibly adversarial. It is the basis of the multi-party semi-quantum cryptography protocol for securing the private data of classical users. In this work, we propose a M-SQKD protocol based on circular transport structure and prove the protocol is unconditional security. Even in the worst case (i.e., TP is untrusted), the protocol's noise tolerance is also close to the BB84 protocol. The results show that our protocol can reach the similar level of security as the full quantum protocol. Besides, due to the scalability of the circular transport structure, our protocol is easily extended to multi-party scenarios, which offers an approach to realizing multiple "classical" users' key distribution. Therefore, it can be used to protect the private data of multiple classical users and may have potential application scenarios in the future. [ABSTRACT FROM AUTHOR]