# Quantum computing and blockchain technology

It is popularly stated that quantum computing could destroy and disrupt blockchain technology completely. How is quantum computing a threat to blockchain technology?

• We have a couple of blocked questions on this topic, which is seemingly popular: 1,2. I suggest editing to improve the question (and/or un-closing+editing the closed ones) rather than blocking all of them. Surely a quality Q&A can be composed on this topic. – agaitaarino Apr 21 '18 at 6:45
• @agaitaarino If you think a question should be reopened, then it is best if you cast a 'reopen vote' on that question. The common problem of these questions seems to be that blockchain technology can neither be 'broken by quantum' or 'quantum resistant', because those are properties of specific implementations of the technology. If you can address this by editing the closed question, then I think that question could be reopened as long as it isn't a duplicate. – Discrete lizard Apr 21 '18 at 11:59

My crude understanding of blockchain (derived mainly from the Wikipedia article) is that it gets its security from two sources:

• Individual communications are performed using a public key cryptography scheme

• Information is stored in a decentralised manner across many different computers, meaning that there are many different copies of the same information.

The level of security provided by these two items differs, I suspect. Public key cryptography has an exponential form of security against classical attacks: it's based on a mathematical problem, and you add one bit to the problem size, and the difficulty (roughly) doubles. It's really easy to add a few bits, and put the problem completely out of anybody's reach. Meanwhile, for the decentralised part, I imagine that adding one extra computer to the network doesn't significantly increase the resources required to monitor all the communications; for a network of $N$ nodes, there are only $\binom{N}{2}$ communication links to monitor (a polynomial in $N$, not exponential). So, while adding a few more computers to the network might make monitoring a daunting task for an individual, state-level interference is unlikely to be eliminated. Thus, the security is heavily dependent upon the security of the public key cryptosystem being used.

The point is that quantum computers will be good at breaking existing public key cryptography systems such as RSA. RSA, for example, is secured by the assumption that it is difficult to find the prime factors of a large number (the person who is allowed to decript a message proves that they can by giving the factors of a particular number). To the best of our knowledge, this is true for classical computers, but Shor's algorithm makes this an easy task for a quantum computer. This means that, in principle, individual communications can be read an manipulated by a quantum computer.

Researchers are working on replacement public key cryptography systems that will be resiliant to attack by a quantum computer (called post-quantum cryptography) but these are not yet in place.

This answer assumes that you do not have a technical background in cryptography or quantum physics.

Most current implementations of the blockchain rely on two math concepts: (1) Public key encryption. (2) Hash keys.

Quantum computers can break the public key encryption part, through a famous method known as Shor's algorithm (For technical details: see page 8 of: https://arxiv.org/pdf/1710.10377.pdf). This is a powerful threat. But since the digital security of the modern world is built on public key encryption, this would be a broader problem (as opposed to a blockchain specific one).

Quantum computers can also break the hash key component, through a method known as Grover search. But this part is relatively resistant to the attack (For technical details: see page 4 of the above link).

There are other ways to build a blockchain to protect against the above attack:

Some would be based on math ideas. This is known as post-quantum blockchains. Since quantum computers have already shown to break math-based cryptography systems and researchers are working on new algorithms of this future computer (see: https://www.nature.com/news/first-quantum-computers-need-smart-software-1.22590), it casts a doubt on the long-term durability of such a cryptographic system.

A research group based a blockchain on a cryptographic system that uses quantum physics (For technical details see: https://arxiv.org/abs/1705.09258); this uses the properties of quantum particles as opposed to math ideas. It's known as quantum cryptography and it is resistant to attacks from a quantum computer. The weakness in that blockchain system is that it makes technical assumptions that may not be viable in the real world.

Another research group made the blockchain itself into a quantum system (For technical details, see: https://arxiv.org/abs/1804.05979). This uses a property of quantum particles known as entanglement in time. The weakness is that the research only presents a conceptual design.

In summary, quantum computers pose a threat to the current implementations of the blockchain; future implementations may not suffer. Furthermore, it would be incorrect to single out just blockchains for such a threat; quantum computers pose a threat to other systems protected by current digital security methods.

Hope that helps.

• Really thanks for this answer most informative answer i ever got on this question. – Chetan Warke Apr 22 '18 at 12:23