There is a lot of talk about the fact that blockchain can’t be hacked, it is impossible to change, and so on. There is much talk about such problems as slow transactions, energy-intensive mining. But few people speak about such problem as the insecurity of blockchain. Meanwhile, the problem exists, and there is a high probability that there will soon be stolen all the money of Satoshi Nakamoto, Vitalik Buterin, etc. This moment we call “Quantum Apocalypse”.

In this article we tell you what quantum computers are and why they are dangerous for blockchain.

**Blockchain and encryption**

The core of blockchain is cryptography. This is the science of how to ensure the confidentiality of information and its integrity. The basis of cryptography is encryption, that is, changing data with a certain cipher, which is accessible only to a limited circle of persons. The simplest example of a cipher: each letter of the alphabet corresponds to a certain number. This primitive method of encryption has been known since ancient times.

It is a bit more difficult in blockchain.

Transactions in blockchain are based on cryptographic hash function. Hash function transforms any data to the specific set of letters and/or numbers. This alphanumeric set * always has the same length*. For example, Bitcoin’s hash function includes 64 characters. No matter how long the original message is – three letters or three pages. The number of characters in the hash will always be the same. But the set of characters in the hash will be different for each message.

Hash function is an irreversible encryption. If something is encrypted with hash function, it is only possible to verify the identity of initial data but not to decrypt initial data. Therefore, the result of hash function is also called “imprint”. And transactions are signed with such imprint. Then this imprint can be used only by the person who signs the next transaction with a public key, the imprint of which matches the imprint in the output of previous transaction. That is why the chain of transactions in blockchain cannot be falsified. Addresses in Bitcoin are organized in the same way, they are an imprint (hash) of a public key.

Digital signatures based on asymmetric cryptography are used to verify the authentication of messages in blockchain. There are two keys – private and public. On the example of Alice and Bob, the permanent heroes of encryption algorithms, we describe how this works.

– So, Alice wants to send something to Bob (money, files, etc.);

– Alice hashes her message, gets its hash (imprint) and signs (encrypts) this hash with her private key. In this way the encrypted part of digital signature is created;

– Alice attaches a public key for Bob to the encrypted part of signature;

– Then Bob having received the message and encrypted signature decrypts (opens) signature with the public key. Yes, the opposite is true in the algorithm of digital signature. A hash is encrypted by private key and decrypted by public key;

– Bob calculates the hash (imprint) of Alice’s message and compares the result with the hash that is inside the encrypted part of signature;

– If Bob has the hash of Alice’s message matching the hash he has found inside the encrypted part of Alice’s signature, this means the message came from Alice.

**Quantum computer and classic computer. What’s the difference?**

Now let’s talk about quantum computers. Quantum computer is a new generation of computing devices that is capable of solving some problems radically faster than classical computer due to much higher computing power. Some calculations that classical computer would do for years, quantum supercomputer can do in seconds. What is the secret of such a super power of quantum computer? The fact is that quantum computer is based on phenomena of quantum physics describing the behavior of ultra-small particles. And microscopic particles behave quite differently than ordinary material bodies in classical physics. Quantum computer uses the phenomena of quantum superposition and quantum entanglement. Now let’s see into it.

* Quantum entanglement.
*Let’s imagine that we have two suitcases, and there is a shirt in each of them. One suitcase you give to a friend, and another one carry away to your home. Reaching home you open the suitcase and see that the shirt is green. At this time, the shirt in another suitcase left at your friend automatically becomes red to keep the system balance. That is, one object determines the characteristic of another object. Both quantum particles have no definite characteristics and are in the mode of waves. As soon as an observer appears, a wave “turns” into a perceptible object. Tricky, volatile particles ?

* Quantum superposition* means that any quantum particle can be in several different states and in several different points of space simultaneously. Do you realize how many variants are possible? These quantum tricks are what gives the advantage to quantum computer over classical computer. So, the processor of classical computer makes calculations based on bits which can be only in one of two states – either 0, or 1. Quantum computer operations are based on quantum bits (qubits) that can be in the state of 1, 0, or in the state of 0 and 1 simultaneously.

For example, the task is to find a number that meets certain criteria among the numbers from 0 to one billion. Classical computer will iterate through all the numbers sequentially, checking them for compliance with the conditions of the problem. Quantum supercomputer will do the same task using qubits which are in the states of 0 and 1 simultaneously (and in different percentage proportions!). It will check all numbers simultaneously, immediately discarding the wrong ones. The output is the number that most closely corresponds to the conditions of the problem. To achieve accuracy the operation can be repeated several times.

Being enabled to process all the possible states at the same time quantum supercomputer can solve certain problems with a huge advantage over classical computer. As for blockchain cryptography, this will look as follows. Supercomputer will be able to check trillions of possible options for private key at the same time. So, having matched private keys quantum computer will simply hack into a blockchain and will get the access to all information inside. This will be the Quantum Apocalypse.

Today quantum computer is been developed by such giants as Google and IBM. As well as National Security Agency of the United States.We can periodically see press reports about creating the next version of quantum computer. Chinese scientists announced in 2017 that they had created quantum computer with a chain of 10 qubits, which is 24 times higher than that of similar quantum systems. And Russian and American scientists at the international conference on quantum technologies ICTT-2017 in July 2017 said that they had already developed 51-qubit quantum computer. This pace of development makes Quantum Apocalypse an inevitable reality.

**Expert opinion**

When you had learnt what quantum computer is and how dangerous it is for cryptocurrencies, it wouldn’t be superfluous to listen to the expert opinion in this matter. We bring to your noticean** interview with Dmitry Gerasimov, a specialist in blockchain and cryptography, the author of DapCash project.**

**What do you think about blockchain vulnerability?**

There have already been break-ins of digital signatures similar to those in blockchain and even without quantum computers. In particular RSA break-in is practically realized. More than 700 bits of keys were hacked on an ordinary cluster. And it will soon be possible to hack the algorithms of elliptic encryption ECDSA, on the basis of which in particular Bitcoin and most of cryptocurrencies were built.

**What cryptocurrencies in your opinion are the most vulnerable to quantum computer?**

Almost all cryptocurrencies are vulnerable to quantum computer with rare exceptions. I can note NEO and IOTA as exceptions. Unfortunately, the algorithms of digital signature in crypto-currencies are not very diverse. ECDSA algorithm makes digital signature quite compact unlike other algorithms and this makes it possible to save space in blockchain. And this feature has determined the choice of ECDSA as the standard for digital signature.

**When do you think Quantum Apocalypse will come?**

As soon as the processing power of quantum computer reaches 1000 qubits or more it will be possible to hack all the existing blockchains of cryptocurrencies and steal all the money. Judging by the pace of quantum computer development this will happen in the coming years.

**Do you think blockchain developers, users and miners are aware of the impending threat?**

Blockchain developers are mostly practical programmers without deep understanding of cryptography basics. But even specialists in cryptography rarely have knowledge in the field of quantum mechanics and are not familiar with the situation in such areas as photonics, solid state physics, semiconductor physics, low temperature physics and rarely have at least superficial knowledge in engineering. At the same time, experts in physics rarely have knowledge in such branches of mathematics as cryptography. And it is very rare for specialists in physics to know the nuances of blockchain programming. As a result, to realize the looming threat it requires a nontrivial set of skills. All the others have to rely on other people’s opinions. And the opinions of others are very different, often contradictory because of the above circumstances. You can easily find physicists who are familiar with cryptography, but not very familiar with engineering and the situation in photonics and semiconductors physics and therefore they deny Quantum Apocalypse. And you can also find experts in photonics, who are sure that Apocalypse will come soon or has already came. Moreover, they due to the poor knowledge of cryptography are also sure that post — quantum algorithms are a fiction and they will not give any advantage. There are physicists who make blockchain on quantum keys but using trusted nodes for data transmission, which destroys the main idea of blockchain. In general the industry is in a mess, and it is very difficult for an ordinary person to understand all the nuances.