How Will Quantum Computing Impact Blockchain Technology?

Shaan Ray
Blockstreet HQ
Published in
5 min readJul 10, 2018


Cryptographers often voice the concern that blockchain technology could become obsolete upon the development of quantum computers (probably 10 to 15 years from now). They fear that once developed, quantum computers will be able to break the security mechanisms blockchains use (asymmetric cryptography and hashing). Though these concerns are somewhat justified, blockchain developers have time to proactively address the threat.

What is Quantum Computing?

To understand the security threat posed by quantum computers, it is important to understand how they differ from classical computers. A classical computer uses a base 2 numerical system: it understands, processes and communicates data exclusively using bits. Each bit can hold only one of two positions: 0 or 1. We can think of each bit as a light switch: it can either be on or off.

Quantum computers, on the other hand, use qubits which can hold two positions at once. (For example, a qubit could hold both a 0 and 1 at the same time). This phenomenon, known as superposition, is the secret weapon of quantum computing. Superposition decreases the number of operations required for the computer to solve a problem. Thus, quantum computers can handle multiple complex queries simultaneously.

A quantum computer’s ability to engage in more computations per second than a classical computer explains why many computer scientists believe that a quantum computer will be able to crack current cryptographic protocols with relative ease.

A Classical bit compared to a Quantum Bit or ‘Qubit’

Asymmetric Cryptography, Symmetric Cryptography, and Hashing

To gauge how, specifically, quantum computers will impact cryptography, it is worth understanding a few kinds of cryptography widely in use today. I have discussed cryptography and hashing previously. Here is a quick recap:

· Symmetric cryptography uses the same cryptographic key for encryption and decryption. This is similar to using the same key to lock the front door to your house in the morning, and to unlock it in the evening.

· Asymmetric cryptography, also called public-key cryptography, uses a pair of keys for each transaction: a public key and a private key. The public key encrypts a message, while the private key decrypts it. They must be used together to execute a transaction.

· Hashing algorithms scramble data into a unique string of letters and numbers that represents that specific data. Any given piece of untampered data will always produce the exact same hash.

Blockchains use two primary security tools: encryption via asymmetric cryptography, and hashing algorithms.

Asymmetric Cryptography Depends on the Difficulty of Factoring Large Numbers

Asymmetric cryptography algorithms, which secure blockchains, are based on prime numbers and factors. In every blockchain protocol, the public and private keys are both extremely large numbers that have been hashed into a series of smaller letters and numbers. Since finding the prime factors of an enormous number is virtually impossible for classical computers, blockchains are tamper-proof.

Prime Factors of ‘321,232’ shown above.

Quantum computers specialize in the optimization of combinations and probabilities. A quantum computer will be able to perform combinatorial calculations on larger numbers at a much deeper level than classical computers can. This means that quantum computers will be able to factor large numbers in fewer arithmetic steps (though not necessarily faster) than classical computers. Thus, finding the prime factors of an enormous number will be a relatively easy task for a quantum computer.

We can therefore expect that quantum computers will be able to use superposition to crack the asymmetric cryptography securing blockchains.

Hashing is not as Vulnerable

Blockchains are secured not just by encryption (via asymmetric cryptography), but also by hashing. A key difference between encryption and hashing is that hashing cannot be reversed. Quantum computers will therefore pose less of a security threat to hashing algorithms than they will to encryption.

Bitcoin’s Defense Against Quantum Computing

Bitcoin’s protocol was designed with quantum computers in mind, and therefore has some built-in resistance. Bitcoin uses a SHA-256 hashing algorithm, which is considered partly quantum-proof. Since Bitcoin addresses are only used once, a user’s public key is revealed only at the moment at which the user sends Bitcoin. In theory, a this would give a quantum computer only a very narrow window, between when the transaction is sent and validated, to find the private key.

Other than Bitcoin and Bitcoin-based blockchains, other platforms and developers are also taking the issue seriously, because blockchains can only inspire trust if they are tamper-proof. Blockchain developers should take further precautions to build upon their platforms’ native security mechanisms to fully address potential vulnerability to quantum computing.

IBM Q’s hardware on the left & a set of DWave Quantum Computers on the right.

The Quantum Threat

Quantum computers currently only exist in very rudimentary forms. Many commentators believe that today’s so-called quantum computers are not truly worthy of the name. Experts predict that it will take 15 to 20 years for quantum computers to become capable enough to hack a blockchain or cryptocurrency. Skeptics have even questioned whether building a quantum computer is even possible.

However, if true quantum computers are developed, they could pose a very real threat to the core security mechanisms powering blockchain technology and cryptocurrencies today.

Real quantum computing would render useless not just blockchain security protocols, but also the majority of other digital security protocols in use today, including the encryption and cryptography securing traditional online banking.

We can therefore expect that engineers and developers, on the blockchain and off, will create exponentially more powerful, quantum-proof security protocols well before quantum computers become a reality. Still, blockchain platforms developers would be wise to create blockchains that seek to be quantum-proof by design, rather than to potentially play catch-up later.

Next: Two quantum algorithms which will disrupt blockchain tech.

Shaan Ray

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