Quantum Advantage on Blockchain Technologies

This thesis will be a discussion of the intersection of two emerging technologies that have the potential to have a major impact on the world over the coming decades, blockchain technologies and quantum computation.

Blockchain technologies have exploded in popularity since their inception in the early 2010s. There are thousands of different projects and millions of users globally. Cryptocurrencies as a product of blockchains are owned and used by hundreds of millions of people globally.

Quantum devices pose a significant threat to the future of many of our most relied upon cryptographic schemes. This has the potential to impact all aspects of our lives on the internet. While it is a technology still in its infancy, with only small-scale circuits capable of very little real-world application, major investment and research over recent years has accelerated the industries' growth.

Within this thesis, it is shown how quantum computers could dramatically affect the entire blockchain industry. Some blockchains will be left vulnerable in such a way that users' cryptocurrency and ownership of their cryptographic keys could be stolen, while others could be rendered almost completely useless. It will be demonstrated that this has an impact on all blockchains save the few that have been designed with quantum resistance in mind. While attacks to digital signature schemes such as RSA and ECDSA have been widely described and are well understood, attacks on the Proof-of-Work mechanism blockchains use for gaining consensus were less well understood. It will be shown how within a medium term time frame, PoW could be attacked by a single quantum entity, potentially leading to 51% attacks against some of the most secure blockchain networks.

This thesis also explores the potential positive impacts that quantum devices could have on blockchain technologies. At present, only a handful of near-term, real-world applications exist for small, error-prone quantum hardware. One promising candidate is the concept of quantum cryptocurrency miners-devices analogous to classical ASICs but engineered specifically to mine Proof-of-Work networks. Introducing such miners can boost profitability and create a self-reinforcing feedback loop that attracts even more quantum miners. Furthermore, it will be quantified how their potential effect on the Bitcoin network's energy footprint, demonstrating that widespread deployment could reduce total consumption by up to 99.999%.

Throughout this thesis, it will be hypothesised what the blockchain industry could look like over the next two decades as quantum devices become more accessible and more computationally powerful. This includes both the positives and the negatives that this could bring for the industry. The importance of this is evident as there exists currently over $1 Trillion in market cap of cryptocurrencies. For most of these funds, the underlying ledgers that they are held on are at least in some way vulnerable. Blockchains due to their decentralised governance structure will be slower to move towards post-quantum security. Furthermore, they are entirely reliant and dependent on quantum vulnerable cryptographic primitives.