Secure Multi-Hop QKD Protocol with Honest-but-Curious Relay Nodes

Abstract

: Quantum key distribution (QKD) is a promising secret key exchange protocol that can replace the currently used public key cryptography, which is vulnerable to quantum computers. The main limitation of QKD is communication distance constrained by the attenuation in optical fibers. One solution to this limitation is classical relay using trusted relay nodes. This technology extends the communication distance by securely relaying a secret key from one node to the next using QKD-shared keys. The trusted relay node assumption incurs significant security costs. Therefore, relaxing this trust assumption is desirable. This paper proposes a QKD protocol that relaxes the reliability of relay nodes to honest-but-curious and enables long-distance communication through multi-hop transmission, which has no constraint on the number of relay nodes. Our protocol utilizes BB84 quantum states to transmit a bit sequence via relay nodes, and employs a secure classical channel implemented with post-quantum cryptography (PQC) to prevent relay nodes from deriving the shared secret key. We then analyze the security of our protocol against a possible attack involving the inference of the bit sequence by relay nodes, which arises from the relaxation of the reliability. We finally show that our protocol ensures the security against honest-but-curious relay nodes by discarding insecure secret keys with a high inference probability.