Bitcoin investors and users might need to be more wary of their security than they think. As blockchain continues to gather headlines, it becomes easy to get caught up in the hype and ignore looming threats. It might be easier than ever to buy bitcoin online, but it should not be forgotten that alongside regulatory challenges, blockchain also faces potential threats from another emerging and powerful technology that has serious implications for cyber security and, by extension, blockchain: quantum computing. In this article we’ll take a look at how the workings of blockchain and quantum computing relate to each other, what threat the latter poses to the former and what potential solutions to the issue are being worked on.

## Public Key Cryptography

To understand the root of the threat posed by quantum computing to blockchain it’s worth reviewing the basics of the encryption underpinning blockchain’s workings – public key cryptography. The system requires the user to have two different encryption keys – a private key held only by its owner and a public key that’s accessible to anyone. Anyone using the technology can encrypt information for the user with their public key, but only that particular user will be able to decrypt it with their private key. The more complex the relationship between the public and private keys, the more secure the encryption is. The best current public key cryptography systems link private and public keys with the factors of a number that’s the product of massively large prime numbers.

Using classical computing, it’s functionally impossible to determine a user’s private key from their public key alone. The prime numbers in question are so large that even testing a trillion keys a second, it’s believed it would take 785 million times longer than the current 14 billion year age of the universe to come to the right key. Quantum computing could, however, narrow that gap significantly.

## Quantum Computing

Quantum computing offers processing orders of magnitude faster than classical computing. Non-quantum computers rely on bits existing in a state that’s either 0 or 1 to handle computations. The threat from quantum computing comes from the fact that it uses quantum bits – or ‘qubits’ – that can exist in any superposition between the values of 0 and 1, enabling them to process much higher volumes of information than classical computing. Furthermore, the effect is compounded as you add more qubits, leading to exponentially faster processing speeds.

It’s been demonstrated by Google that its D-Wave quantum computer is 100 million times faster than conventional computers at particular tasks. Meanwhile, multiple different companies are working on their own quantum computers. One limitation of the technology is that they require special quantum computing algorithms that need extremely specialized knowledge, and only a handful currently exist. While bedroom hackers aren’t likely to be creating their own quantum algorithms in the immediate future, however, one of the most famous ones already in existence already poses a potential threat. Shor’s algorithm enables the fast factoring of large primes, which means that a quantum computer could theoretically crack today’s public key cryptography.

The hardware to run it doesn’t exist yet, but sooner or later it will and quantum computing will pose an existential threat to public key cryptography, challenging not only blockchain but also every other technology based on it. A report from the Global Risk Institute suggested that there is a 50 per cent chance that current public key cryptography tools will be undermined as soon as 2031.

## Responses

The good news is that quantum resistant cryptographic systems, such as lattice-based algorithms, code-based algorithms and hash-based algorithms are already been worked on to solve the looming problem. What makes so-called ‘post quantum’ technologies effective is the use of systems much more complex than conventional prime factorization to generate private keys from public keys.

The Quantum Resistant Ledger team, for example, is currently working on a blockchain that will use a hash-based cryptography system to future proof the technology against quantum attacks. Hash based cryptography uses extremely complex hash based cryptographic structures to generate private keys from their public counterparts. This makes the relationship much more complicated than traditional public key encryption and as such far less vulnerable to attacks from quantum computers running Shor’s algorithm.

Another potential (and more far reaching) solution proposed by the University of Wellington would see data encoded on quantum particles, with quantum entanglement used to create a truly immutable blockchain. It would make changes to the system far more fundamental level than the consensus mechanism and make it much harder to attack. It’s a far out solution that would require a network very different to today’s internet, but is nevertheless another potential response to the quantum threat.

## Conclusion

It’s clear that the threat posed by quantum computing to public key encryption is very real, but solutions are being worked on already. So long as developers keep pace with the issue, blockchain does not have to be undermined by the emergence of the new technology.