A New Proposal for QKD Relaying Models

We investigate a quasi-trusted QKD relaying model. This is a three-party communication model. Two nodes Alice and Bob want to establish a secret key. However, the distance between them exceeds the limited range of QKD. Carol is a intermediate node that could share QKD links with Alice and Bob. Carol is assumed to be quasi-trusted: she correctly follows the given three-party protocol. However, she may be eavesdropped by the malicious person Eve. We propose two single-photon based protocols that allow Alice, Carol and Bob to achieve the goal. As a result, these protocols could be applied to securely extend up to two times the limited range of QKD without invoking entangled photons.