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Advanced Quantum Technologies

Secure Decentralized Key Relaying via Random Walks in Dynamic Semi‐Trusted QKD Networks

Sergejs Kozlovičs, Krišjānis Petručeņa, Elīna Kalniņa, Juris Vīksna

Published Aug 1, 2026• 0 citations
Publisher

Abstract

ABSTRACT Due to distance and key‐rate limitations, current quantum key distribution (QKD) technology requires the use of trusted nodes, which, together with QKD links, form a graph referred to as a QKD network. We address the problem of securely routing (relaying) keys between trusted nodes, where some of them can be malicious. Existing solutions typically rely on centralized controllers, which simplify adaptive routing and congestion control but introduce single points of failure. Moreover, some proactive key‐relay protocols distribute keys to multiple nodes in the network; thus, compromising a single node results in the compromise of the shared keys. In this paper, we propose a decentralized, topology‐oblivious key‐relay algorithm based on random walks. Our algorithm provides information‐theoretic security (ITS) for QKD network topologies having at least one non‐malicious path between any pair of nodes (in particular, ‐vertex‐disjoint paths can tolerate malicious cartels of size ). Besides, it provides strong computational security guarantees for other topologies (even for graphs with malicious articulation points). We evaluate the proposed algorithm on the GÉANT GN4‐3N topology, which provides a realistic approximation of a multi‐state cross‐border topology for future QKD networks.

DOI
10.1002/qute.70393

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ABSTRACT Due to distance and key‐rate limitations, current quantum key distribution (QKD) technology requires the use of trusted nodes, which, together with QKD links, form a graph referred to as a QKD network. We address the problem of securely routing (relaying) keys between trusted nodes, where some of them can be malicious. Existing solutions typically rely on centralized controllers, which simplify adaptive routing and congestion control but introduce single points of failure. Moreover, some proactive key‐relay protocols distribute keys to multiple nodes in the network; thus, compromising a single node results in the compromise of the shared keys. In this paper, we propose a decentralized, topology‐oblivious key‐relay algorithm based on random walks. Our algorithm provides information‐theoretic security (ITS) for QKD network topologies having at least one non‐malicious path between any pair of nodes (in particular, ‐vertex‐disjoint paths can tolerate malicious cartels of size ). Besides, it provides strong computational security guarantees for other topologies (even for graphs with malicious articulation points). We evaluate the proposed algorithm on the GÉANT GN4‐3N topology, which provides a realistic approximation of a multi‐state cross‐border topology for future QKD networks.
Quantum key distribution (QKD) networks require relaying when distant key management entities share no direct quantum link. Most relay strategies, however, rely on centralized control or globally maintained routing state. This paper asks whether useful security and efficiency can still be obtained with topology-oblivious stochastic forwarding. It studies the security-overhead trade-off in a model in which fragmented key material is relayed via random-walk variants and reconstructed under privacy amplification. The analysis asks whether strictly local forwarding can retain useful information-theoretic security (ITS). Evaluation on the GÉANT topology, representing a European academic backbone network, shows clear differences between random-walk variants. The proposed highest-score-neighbor local path-diversification heuristic reduces the probability that relayed key material passes through a compromised node. The evaluation also shows that scouting-based loop erasure significantly shortens sampled routes and improves throughput in the model. Against one- to three-node cartels, random flow protects slightly more source-target pairs than a static disjoint-multipath method on the evaluated topologies. These findings position topology-oblivious stochastic forwarding as a simpler decentralized design for QKD relaying without centralized orchestration or gossip protocols.
Abstract Secure key distribution is crucial for maintaining communication confidentiality in networks with multiple communicating partners. Asymmetric key algorithms are vulnerable to quantum attacks because of the computational advantage offered by quantum computers, whereas symmetric key algorithms with sufficiently large keys remain comparatively more resistant. Quantum Key Distribution (QKD) has emerged as a promising quantum-secure key distribution technique based on the principles of quantum physics. However, practical QKD networks often rely on relay nodes to extend communication distance, and malicious relay nodes can compromise quantum keys during the relaying process. This study proposes a secure key relaying approach based on Shamir's Secret Sharing to mitigate this vulnerability. The proposed method uses a probabilistic behavioural model to assess relay nodes according to their curiosity and collaboration tendencies. By splitting a quantum key into multiple shares and routing them through paths with minimum compromise risk, the approach reduces exposure to malicious or colluding relay nodes. The proposed algorithm assigns risk to network paths by evaluating node-level curiosity and collaboration potential, determines the appropriate number of shares, and selects secure routing paths to minimise key compromise. The study further analyses the parameters influencing the ideal number of shares for a given network, thereby offering a resilient mechanism for quantum-safe communication in QKD networks with malicious relay nodes.
Quantum key distribution (QKD) networks promise information-theoretic security for multiple nodes by leveraging the fundamental laws of quantum mechanics. In practice, QKD networks require dedicated routing protocols to coordinate secure key distribution among distributed nodes. However, most existing routing protocols operate under the assumption that all relay nodes are honest and fully trustworthy, an assumption that may not hold in realistic scenarios. Malicious nodes may tamper with routing updates, causing inconsistent key-state views or divergent routing plans across the network. Such inconsistencies increase routing failure rates and lead to severe wastage of valuable secret keys. To address these challenges, we propose a distributed routing framework that combines two key components: (i) Causal Consistency Key-State Update, which prevents malicious nodes from propagating inconsistent key states and routing plans; and (ii) Trust-Aware Multi-path Flow Optimization, which incorporates trust metrics derived from discrepancies in reported states into the path-selection objective, penalizing suspicious links and filtering fabricated demands. Across 50-node topologies with up to 30% malicious relays and under all three attack modes, our protocol sustains a high demand completion ratio (DCR) (mean 0.90, range 0.81-0.98) while keeping key utilization low (16.6 keys per demand), decisively outperforming the baselines-Multi-Path Planned (DCR 0.48, 30.8 keys per demand) and OSPF (DCR ≤0.12, 296 keys per demand; max 1601). These results highlight that our framework balances reliability and efficiency, providing a practical and resilient foundation for secure QKD networking in adversarial environments.
Quantum key distribution (QKD) technology can establish unconditional secure keys between two communicating parties. Although this technology has some inherent constraints, such as the distance and point-to-point mode limits, building a QKD network with multiple point-to-point QKD devices can overcome these constraints. Considering the development level of current technology, the trust relaying QKD network is the first choice to build a practical QKD network. However, the previous research didn’t address a routing method on the trust relaying QKD network in detail. This paper focuses on the routing issues, builds a model of the trust relaying QKD network for easily analysing and understanding this network, and proposes a dynamical routing scheme for this network. From the viewpoint of designing a dynamical routing scheme in classical network, the proposed scheme consists of three components: a Hello protocol helping share the network topology information, a routing algorithm to select a set of suitable paths and establish the routing table and a link state update mechanism helping keep the routing table newly. Experiments and evaluation demonstrates the validity and effectiveness of the proposed routing scheme.
Quantum communication networks promise information-theoretic security but this security collapses if intermediate relay nodes are compromised, a risk that grows sharply when compromised nodes cannot be identified. We address this twofold: first, we show that multi-path routing provides quantifiable protection against unidentified compromised nodes. Second, we provide a comprehensive statistical and graph-theoretic analysis of network and node security under compromise, covering full and partial node compromise, optimal placement of additional edges via an iterative method, and comparative hacking-probability analysis across network topologies. Together, these results identify the topology and multi-path routing strategy that most improves security for a given network. We apply this framework to quantum key distribution across multiple network topologies, demonstrating a scalable route to secure, reliable quantum communication under realistic, unidentified adversarial threats, without relying on quantum memory.
There is a need to ensure unconditionally secure communications between different sites of an organization. We use quantum key distribution (QKD) protocol to provide a continuous stream of secret key bits to each communicating pair for message encryption through one-time-pad (OTP) cryptography. Due to long distance between different sites, trusted nodes are used to relay each service key in a hop-by-hop manner from one end node to another end node of a communicating pair. In a QKD network, routes for key relaying can be affected by availability of relay key bits at trusted nodes. Also, relay key allocation among different routes can impact security. We consider the scenario of supporting multiple communicating pairs, where each pair demands delivery of a certain service key rate across the QKD network, and trusted nodes may not have sufficient relay key rates to perform bitwise encryption to protect the service key. Hence, there is a need to allocate the limited relay key bits to meet all service key delivery demands while considering its effects on security. We propose to perform jointly routing and relay key allocation to minimize attack success probability. We have formulated the joint problem as an optimization and further proposed an efficient algorithm to find the optimal solutions. We have evaluated the proposed algorithm to confirm its ability and efficiency in finding optimal routes and relay key allocations. For the specific network topology that we have adopted, results show that the minimum number of required trusted nodes increases linearly when end nodes are added to the system. Furthermore, we can achieve a higher security level by adding more trusted nodes in a QKD network. But increasing the number of trusted nodes beyond the minimum requirement may not continue to reduce significantly the attack success probability.