1 Navigating the Quantum Computing Threat Landscape for Blockchains: A Comprehensive Survey Hassan Khodaiemehr, Khadijeh Bagheri and Chen Feng Abstract—Quantum computers pose a significant threat to blockchain technology’s security, which heavily relies on public- key cryptography and hash functions. The cryptographic algo- rithms used in blockchains, based on large odd prime numbers and discrete logarithms, can be easily compromised by quantum computing algorithms like Shor’s algorithm and its future qubit variations. This survey paper comprehensively examines the im- pact of quantum computers on blockchain security and explores potential mitigation strategies. We begin by surveying the existing literature on blockchains and quantum computing, providing insights into the current state of research. We then present an overview of blockchain, highlighting its key components and functionalities. We delve into the preliminaries and key definitions of quantum computing, establishing a foundation for understanding the implications on blockchain security. The ap- plication of blockchains in cybersecurity is explored, considering their strengths and vulnerabilities in light of evolving quantum computing capabilities. The survey focuses on the quantum security of blockchain’s fundamental building blocks, including digital signatures, hash functions, consensus algorithms, and smart contracts. We analyze the vulnerabilities introduced by quantum computers and discuss potential countermeasures and enhancements to ensure the integrity and confidentiality of blockchain systems. Furthermore, we investigate the quantum attack surface of blockchains, identifying potential avenues for exploiting quantum computing to strengthen existing attacks. We emphasize the need for developing quantum-resistant defenses and explore solutions for mitigating the threat of quantum computers to blockchains, including the adoption of quan- tum and post-quantum blockchain architectures. By examining vulnerabilities and discussing mitigation strategies, we aim to guide researchers, practitioners, and policymakers in developing robust and secure blockchain systems capable of withstanding advancements in quantum computing technology. Index Terms—Blockchain technology, quantum computing, hyper ledger, postquantum cryptography, consensus algorithm, zero-knowledge proof. I. I NTRODUCTION T HE advent of quantum computing is poised to revolu- tionize a multitude of industries, including the world H. Khodaiemehr is with Department of Computer Science and Statistics, Faculty of Mathematics, K. N. Toosi University of Technology, Tehran, Iran and also with the Faculty of Applied Science, School of Engineering, The University of British Columbia (UBC), Okanagan Campus, Kelowna, BC, Canada (e-mail: ha.khodaiemehr@kntu.ac.ir). K. Bagheri is with Electronics Research Institute, Sharif University of Technology, Tehran, Iran (e-mail: kh.bagheri@sharif.edu). C. Feng is with the Faculty of Applied Science, School of Engineering, The University of British Columbia (UBC), Okanagan Campus, Kelowna, BC, Canada (e-mail: chen.feng@ubc.ca). This work was supported in part by Public Safety Canada under Grant with Contract No. 4248090-1-NS-5001-22170 of blockchain technology. At the heart of blockchains lies a complex web of cryptographic algorithms guaranteeing the security and unchangeability of transactional data. However, the power of quantum computing threatens to upend this delicate balance by rendering many of these algorithms vul- nerable to attack. The two fundamental cryptographic features employed by blockchains, namely public-key cryptography and hash functions, rely on mathematical operations that can be impacted by quantum computers. Part of these algorithms rely on large prime numbers and discrete logarithms, which can be solved exponentially faster by a quantum computer using Shor’s algorithm. Once the private-keys used to secure blockchain transactions are compromised, the entire integrity of the system is thrown into question. However, researchers are not taking this threat lightly, and are working tirelessly to find ways to mitigate this risk. By exploring the use of post- quantum cryptographic algorithms in blockchain technology, these researchers aim to create a system that is resistant to quantum attacks. Such algorithms are designed to withstand the computational power of quantum computers, and could provide a new level of security and resilience to blockchains in the face of this technological revolution. As quantum computing technology continues to advance, the security of blockchain technology will need to evolve accordingly. With the right approach, however, the future of blockchains could be brighter than ever, ushering in a new era of secure and decen- tralized data management. Blockchain and quantum computing are two technologies that hold great promise in various fields and have the potential to influence one another. Therefore, the focus of this study is to explore the possible impact of quantum computing on blockchain technology. The architecture of blockchain technology is transparent and entirely distributed among peers which renders it suit- able for applications in cryptocurrencies [1], [2], smart con- tracts [3], [4], the Internet of Things (IoT) [5]–[7], com- munication systems [8], [9], healthcare [10], [11], financial systems [12], [13], electronic voting [14], [15], censorship resistance [16], and distributed provenance [17], amongst the rest. The blockchain’s append-only model ensures that accepted transactions cannot be modified [18], [19], offering significant advantages to these applications. For example, the blockchain’s transparency allows for the storage of records that are publicly verifiable and unchangeable [20]. Addition- ally, the blockchain’s peer-to-peer framework offers a way to uphold a verifiable ledger without requiring a central entity. This approach effectively addresses concerns related