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.
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.
We propose a new definition for quasi-trusted relays. Our quasi-trusted relays are defined as follows: (1) being honest enough to correctly follow a given multi-party finitetime communication protocol; (2) however, being under the monitoring of eavesdroppers. From the new definition, we first develop a simple 3-party quasi-trusted model called Quantum Quasi-Trusted Bridge (QQTB) model. In this model, the origin Alice and the destination Bob are assumed out of range of Quantum Key Distribution (QKD). Carol is a quasi-trusted relay that can share QKD links with Alice and Bob. We show that QQTB protocol allows Alice and Bob, in cooperation with Carol, to securely establish secret keys. The originality of QQTB protocol is that we do not need invoke entangled photon pairs. Then, we extend QQTB model to Quantum Quasi-Trusted Relay (QQTR) model that is capable of securely distributing secret keys over arbitrarily long distances. Although QQTB model requires entangled photon sources, the originality is that we do not invoke entanglement swapping and entanglement purification as in [5, 6, 15]. Keywords: Controlled-NOT (C-NOT) gate, quantum circuit, quantum Key Distribution (QKD), QKD relaying model, quasi-trusted model, unconditional security 1
We investigate Quantum Key Distribution (QKD) relaying models. Firstly, we propose a novel quasi-trusted QKD relaying model. The quasi-trusted relays are defined as follows: (i) being honest enough to correctly follow a given multi-party finite-time communication protocol; (ii) however, being under the monitoring of eavesdroppers. We develop a simple 3-party quasi-trusted model, called Quantum Quasi-Trusted Bridge (QQTB) model, to show that we could securely extend up to two times the limited range of single-photon based QKD schemes. We also develop the Quantum Quasi-Trusted Relay (QQTR) model to show that we could securely distribute QKD keys over arbitrarily long distances. The QQTR model requires EPR pair sources, but does not use entanglement swapping and entanglement purification schemes. Secondly, we show that our quasi-trusted models could be improved to become untrusted models in which the security is not compromised even though attackers have full controls over some relaying nodes. We call our two improved models the Quantum Untrusted Bridge (QUB) and Quantum Untrusted Relay (QUR) ones. The QUB model works on single photons and allows securely extend up to two times the limited QKD range. The QUR model works on entangled photons but does not use entanglement swapping and entanglement purification operations. This model allows securely transmit shared keys over arbitrarily long distances without dramatically decreasing the key rate of the original QKD schemes.
Remote distribution of secret keys is a challenging task in quantum cryptography. A significant step in this direction is the measurement-device independence quantum key distribution (MDI-QKD). For two remote (or independent) parties Alice and Bob who initially no share secret information, the MDI-QKD enables them to share a secret key by the measurement of an untrusted relay. Unfortunately, the MDI-QKD yields the assumption that the devices of both Alice and Bob have to be trusted. Here, we show that QKD between two independent parties can also be realized even if the device of either Alice or Bob is untrusted. We tackle the problem by resorting to the recently developed one-sided device-independent QKD protocol. We derive conditions on the extracted secret key to be unconditionally secure against arbitary attacks in the limit of asymptotic keys. In the presence of Gaussian states and measurements, we theoretically demonstrate our scheme is feasible, which could be an attractive candidate for long distance secret communication.
Modern quantum key distribution (QKD) network designs are based on sending photons from one node to another and require free-space or dedicated fiber optic cables between nodes. The purpose of this is to co-generate secret key material on both sides of the quantum channel. In addition to this quantum link, there are several insecure classical channels that allow QKD algorithms to exchange book-keeping information and send symmetrically encrypted data. The attenuation of photons transmitted through fiber becomes too high to practically generate key material over fiber at distances of more than 100 km. Free-space transmission through the atmosphere or the vacuum of space can reduce attenuation, but at the cost of system complexity and sensitivity to other impairments, such as weather. To extend the effective range of QKD networks, we present a method that combines QKD algorithms with post-quantum, homomorphic key-switching to allow multiple parties to effectively share secret key material over longer distances through semi-trusted relay nodes. We define how such a system should work for arbitrary network topologies and provide proofs that our scheme is both correct and secure. We assess the feasibility of this solution by building and evaluating two implementations based on lattice-based cryptography: learning with errors.
This paper presents an investigation of practical quantum secure communication using multi-photon tolerant protocols. Multi-photon tolerant protocols loosen the limit on the number of photons imposed by currently used quantum key distribution protocols. The multi-photon tolerant protocols investigated in this paper are multi-stage protocols that do not require any prior agreement between a sender Alice and a receiver Bob. The security of such protocols stems from the fact that the optimal detection strategies between the legitimate users and the eavesdropper are asymmetrical, allowing Bob to obtain measurement results deterministically while imposing unavoidable quantum noise to the eavesdropper Eve's measurement. Multi-photon tolerant protocols are based on the use of transformations known only to the communicating party applying them i.e. either Alice or Bob. In this paper multi-photon tolerant protocols are used in order to share a key or a message between a sender Alice and a receiver Bob. Thus such protocols can be either used as quantum key distribution (QKD) protocols or quantum communication protocols. In addition, multi-stage protocols can be used to share a key between Alice and Bob, followed by the shared key used as a seed key to a single-stage protocol, called the braiding concept. This paper presents a practical study of multi-photon tolerant multi-stage protocols. Security aspects as well as challenges to the practical implementation are discussed. In addition, secret raw key generation rates are calculated with respect to both losses and distances over a fiber optical channel. It is well-known that raw key generation rates decreases with the increase in channel losses and distances. In this paper, coherent non-decoying quantum states are used to transfer the encoded bits from Alice to Bob. Raw key generation rates are calculated for different average photon numbers µ and compared with the case of µ=0.1, which is the average number of photons used in most single-photon based QKD protocols. Furthermore, an optimal average number of photons to be used within the secure region of the multi-photon tolerant protocols is calculated. It is worth noting that, with the increased key generation rates and distances of communication offered by the multi-photon tolerant protocols, quantum secure communication need not be restricted to quantum key distribution; it can be elevated to attain direct quantum secure communication.
In a cyber–physical system, quantum key distribution (QKD) protocols can provide a continuous supply of common secret key bits for secure communications between the control center and a remote device. QKD protocols operate on a peer-to-peer basis between two adjacent nodes and are distance-limited. Hence, trusted nodes are installed between two communicating end nodes for a better QKD range and key rate. These trusted nodes relay in a hop-by-hop manner, key bits generated in the first hop to the other end node. At each trusted node, the first hop key bits are encrypted by other locally generated key bits before being relayed. Since QKD process is stochastic, secret key bits are generated at different rates at different hops. Thus, buffer is needed at a trusted node to temporally hold some key bits while waiting for other key bits. There is no existing work that accounts for such a realistic relaying operation in deriving the key rate. This article contributes to developing a key rate model to determine accurately the end-to-end rates across multiple trusted nodes. We have evaluated accuracy of the proposed model through extensive simulations. The results show that there exist an optimal number of trusted nodes for a given distance between two end nodes. For example, the optimal number of relays increases from four for 100 km to six for 200 km.