Russian scientists future-proof blockchain from quantum attacks
The Russian Quantum Center has developed a blockchain system that secures data through quantum distribution. By using photons to send information, users of the system can be sure of each other's identity. While blockchains that use a digital signature could be altered with large-scale attacks from quantum computers, a quantum signature is able to secure transactions over long distances. The system has been proven to work in a small test, in which one user attempted to game the system by double-spending.
And therein lies the problem. Brute force attacks are difficult for classical computers but will be easy for the next generation of quantum computers. The vast number-crunching power of these devices mean that as soon as they are available, cryptocurrencies will be suddenly more vulnerable to attack.

So a way of securing blockchain technology against quantum attack would be hugely useful.

Enter Evgeny Kiktenko at the Russian Quantum Center in Moscow and a few pals who have designed, built, and tested a quantum blockchain system in which the security is guaranteed by quantum mechanics. They’ve built it using a standard quantum cryptography system of the kind that is already commercially available.

First some background. Blockchains record a list of transactions in a way that prevents dishonest use, such as tampering or double spending. They allow any computer to keep track of this list by compiling them into a block, which is then encrypted to form a number called a hash.

The encryption process is important. It is an algorithm that is easy to calculate but hard to do in reverse (like factorization). The hash value it produces is a unique property of the block, and any tampering with the records would be immediately obvious because this would change the hash.

New transactions are next gathered together into a new block and added to the existing hash value. This is then encrypted to create a new hash for the new block. This is added to the next list of transactions when they are encrypted, and so on. The result is a chain of blocks that each contain the hash values of all preceding blocks—hence the term blockchain.

All the computers that store these blocks regularly compare their hash values to ensure that they are all in agreement. Any computer that does not agree, discards the records that are causing the problem.

This approach is good but it is not perfect. One way to game this system is for a dishonest user to change the list of transactions in their favor, but in a way that leaves the hash unchanged. This can be done by brute force, in other words by changing a record, encrypting the result, and seeing whether the hash value is the same. And if not, trying again and again and again until it finds a hash that matches.