Kevin Han Yong Loh

Client authentication (CA) is a cryptographic protocol where a server tries to validate the identity of a client. Fehr et. al. proposed a CA protocol with pre-shared basis information between the client and server which has a nice key recycling property, where secrets including the pre-shared basis can be securely reused after each successful round. We extend the protocol to a practical setting by including decoy state and error correction, but the leakage of pre-shared basis information via multi-photon events limits the performance of such a protocol. As such, we propose the use of a pseudorandom number generator (PRNG), assumed to be secure only during each run of the protocol, to perform basis selection to reduce information leakage. A formal proof of the protocol security is provided by modifying the entropic uncertainty relation to account for basis generated by a PRNG, which could be of independent interest as it may be applicable to other protocols such as quantum key distribution. An experimental implementation of the protocol, with appropriate post-selection, was performed to demonstrate its feasibility. We also designed a CA protocol secure in the practical setting with only two rounds of communication: a challenge by the server and a response by the client.