Quantum Blockchain Security Boosted by New Protocol with Reduced Communication

Researchers are tackling a fundamental challenge in secure communication with a new approach to Quantum Byzantine Agreement (QBA), a protocol vital for blockchain technology and offering enhanced security over classical methods. Chen-Xun Weng, Ming-Yang Li, and Shi-Gen Li, working at the National Laboratory of Solid State Microstructures and School of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, in collaboration with Mengya Zhu from MatricTime Digital Technology Co. Ltd., Xiao-Ran Sun from Nanjing University, Hua-Lei Yin from the School of Physics and Key Laboratory of Quantum State Construction and Manipulation at Renmin University of China, and Zeng-Bing Chen from Nanjing University, have developed a multiparty circular QBA protocol that significantly reduces communication complexity. This innovative protocol employs a semi-decentralised architecture and circular message gathering, achieving quadratic communication complexity and improved fault tolerance, unlike previous methods hampered by exponential scaling or complex entanglement requirements. Demonstrating experimental feasibility with weak coherent states and compatibility with existing networks, their simulations on a global satellite-to-ground network confirm high consensus rates even with varying key generation protocols, paving the way for scalable and practical quantum blockchain applications and secure, decentralised services.

The newly proposed multiparty circular QBA protocol achieves quadratic communication complexity, a significant improvement that brings large-scale deployment closer to reality.

Unlike previous approaches reliant on complex multi-particle entanglement, this protocol operates using readily available weak coherent states and is designed to integrate seamlessly with existing star-shaped quantum networks. The innovation centres on a semi-decentralized architecture that leverages circular message gathering and quantum digital signatures.
This design circumvents the need for fully connected networks, a major obstacle for practical implementation in current quantum communication infrastructure. Simulations utilising a global satellite-to-ground network model demonstrate the protocol’s robustness, sustaining high consensus rates even with varying key generation methods under realistic conditions.

This achievement establishes a framework for building quantum blockchains capable of delivering secure and fault-tolerant decentralized services. This protocol directly tackles the Byzantine Generals Problem, a fundamental challenge in distributed computing where honest parties must reach agreement despite the presence of malicious actors.

While classical solutions are vulnerable to advances in quantum computing, this quantum approach guarantees security rooted in the laws of quantum mechanics. The research demonstrates a fault tolerance exceeding the classical 1/3 limit, requiring only two honest players within the network to maintain consensus.

This enhanced resilience is achieved through the use of one-time universal hashing quantum digital signatures, enabling efficient multi-bit signature generation and verification. Crucially, the protocol’s compatibility with star-shaped topologies, common in satellite-to-ground networks, simplifies practical implementation.

The satellite functions as a central certificate authority, streamlining quantum key distribution among participants. Simulations confirm the protocol’s viability in such a network, paving the way for applications like quantum-blockchain-based federated learning, secure data management, distributed finance, and the internet of things. Specifically, with ten potentially malicious nodes, designated as f = 10, the protocol requires only 132 communication rounds. This represents a reduction of over ten orders of magnitude when contrasted with 2.30 × 1015 rounds demanded by QKD-based QBA and 7.44 × 1012 rounds needed by recursive QBA, except in the case of a single malicious node (f = 1).

The architecture also minimizes quantum channel requirements, transitioning from a fully-connected mesh needing N(N −1)/2 channels to a star-shaped network utilising only N channels connecting players to a central certifying authority. Simulations of a satellite-to-ground network demonstrate high consensus rates even with varying key generation protocols and realistic conditions.

Using the homodyne protocol with ideal detectors, the consensus rate (CR) reaches 105 transactions per second (tps) at an altitude of 700km. Employing untrusted detectors under the same conditions yields a CR of approximately 104 tps at 700km. Further analysis, varying the zenith angle, reveals a consensus rate exceeding 103 tps at a zenith angle of 0 radians for both ideal and untrusted detector scenarios with three parties.

The heterodyne protocol mirrors these performance levels. Ideal detectors achieve a CR of 105 tps at 700km altitude, while untrusted detectors maintain a CR of around 104 tps at the same altitude. Consensus rate versus zenith angle analysis with the heterodyne protocol shows rates above 103 tps at 0 radians for three parties, again with both detector types. Specifically, the research leveraged both homodyne and heterodyne detection schemes to optimise performance, employing coherent states with an intensity of α = 0.72.

Post-selection parameters were carefully tuned to ∆c = 0.42, ∆a = 0.52, and ∆p = 0. These values were chosen to maximise the signal-to-noise ratio and ensure reliable key generation. A repetition rate of 109Hz was maintained throughout the simulations, and detector noise was modelled at ξhet = 2ξhom = 2ξdet = 0.02, representing realistic limitations of current detector technology.

The experimental setup simulated a satellite-to-ground network, modelling downlink transmission where a central satellite distributes quantum keys. Consensus rates were evaluated with varying numbers of participants, noise levels, and detector types, trusted versus untrusted, to assess robustness under diverse conditions.

Simulations explored the impact of satellite altitude, ranging from 200 to 900km, and zenith angle, up to 1.2 radians, on consensus rate. This range reflects typical Low Earth Orbit (LEO) satellite parameters. To further refine the analysis, the study investigated the effect of message size, utilising 1 Kb, 1 Mb, and 100 Mb data packets.

The protocol’s performance was also evaluated using the BB84 key generation protocol (KGP) adapted from asymmetric coding BB84 QKD. This adaptation incorporated a detector efficiency of 70%, a dark count rate of 10−8, a misalignment error rate of 0.02, and an error correction efficiency of 1.1. These parameters were selected to reflect the capabilities of existing satellite-based QKD systems and to provide a realistic assessment of the protocol’s feasibility. Detailed simulation parameters are available in Appendix D, ensuring full transparency and reproducibility of the work.

The Bigger Picture

The persistent challenge of establishing trust in distributed networks has long preoccupied cryptographers and, more recently, quantum information scientists. This research circumvents that limitation through a clever semi-decentralised architecture and a focus on realistic, readily available technology.

What distinguishes this approach is its compatibility with existing infrastructure. The reliance on weak coherent states, rather than exotic entangled photons, and the adaptability to star-shaped networks, mirroring current satellite communication layouts, moves QBA from the realm of fundamental physics demonstrations towards a genuinely deployable technology.

Simulations utilising a global satellite network are particularly encouraging, suggesting resilience even with imperfect key generation and realistic atmospheric conditions. However, it is crucial to acknowledge that scalability does not equate to invulnerability. The protocol’s security still hinges on the assumptions made about the trustworthiness of the digital signatures used, and the potential for side-channel attacks remains a concern.

Furthermore, while quadratic scaling is a significant improvement, it may still present limitations for extremely large networks. The next logical step will likely involve exploring hybrid approaches, combining the strengths of QBA with classical cryptographic techniques to further optimise performance and security. Ultimately, this work doesn’t deliver a finished blockchain solution, but it does provide a crucial, and refreshingly pragmatic, foundation upon which to build one.