Two of the most significant arising technological advancements currently underway that are showing an ever‐increasing spread both in industrial and academic areas, are the blockchains and the advent of quantum technologies. Since, blockchains have dramatically advanced in the recent years and have found numerous applications with the expectation to significantly enhance their security, the conundrum related to the quantum threat and the implementation of post-quantum signatures in a blockchain platform is a trending topic in nowadays scientific community.
Most of the blockchains’ benefits arise from its security allures, since the blockchain is an innovative technology that has proved to bring considerable advantages in many areas by inherently providing security, interoperability and sustainability. Therefore, it can be used as an independent root-of-trust in a distributed (possibly adversarial) setting to allow a set of entities mutually trust each other. For this reason, the blockchain is considered as a decentralized technology for the sharing of information and performing transactions in a secure manner.
At present, classical cryptographic algorithm is still being used in blockchain technology. The security of a classical cryptographic algorithm mainly depends on intractability of elliptic curve discrete logarithm problem or integer factorization problem. However, as any product that is based on cryptographic primitives, blockchains are influenced by the advent of quantum computing. Shor proposed quantum algorithms to find discrete logarithms and factoring integers on a quantum computer that can break the most secure algorithms such as the RSA, DSA and ECDSA. Therefore, as blockchain technology is based on these algorithms, its security is affected by the advent of quantum computing.
Due to the importance of the post-quantum cryptography, NIST initiated in 2016 a process to solicit, evaluate, and standardize one or more quantum-resistant public-key cryptographic algorithms1. The ultimate goal is that the new public-key cryptography standards will specify specific algorithms for digital signatures, public-key encryption (PKE), and key-establishment algorithms (KEA), so as to be available worldwide and provide security into the foreseeable future, including after the advent of quantum computers. In the first round of this competition, 69 algorithms were submitted; the security of each of them rests with the difficulty of one of the hard-mathematical problems. After a rigorous process of evaluation, 26 of them “passed” to the second round of evaluation in January 2019. The second round was completed in July 2020. Currently, we are in the third round of the process, in which 15 submissions still continue. NIST has now classified these 15 algorithms into two categories, as shown in Table 1 and 2. The first category consists of 7 algorithms, being the main family of candidate post-quantum cryptographic and the second refers to alternate algorithms.
To tackle the quantum attack in the blockchain technology, several researchers have proposed post-quantum enabled blockchain solutions or even some adjustments to popular distributed leaders, in order to address the threat stemming from the quantum computing.
Such solutions are provided in ,  for the Ethereum platform, in , , ,  for the Bitcoin platform and very recently in  for Hyperledger Fabric. Commercial blockchains have also analyzed and addressed the impact of quantum computers. These include the Quantum Resistant Ledger (QRL) which uses XMSS, the IOTA which uses WOTS and Corda which uses BPQS.
As a concluding remark, it should be also mentioned that, apart from port-quantum cryptography, quantum cryptography has been started to be considered as a primitive in blockchains. In this direction, quantum cryptography has been proposed to implement smart contracts. Furthermore, more research is necessary on key establishment physics-based methods that are collectively known as Quantum-Key Distribution (QKD).
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