The strength of the Bitcoin network is based on very strong cryptography. But what if one day in the not-too-distant future, computers advanced to such a level that current cryptographic standards were no longer good enough?

That's the promise and the danger of quantum computing, a technology that allows computers to utilize the rules of quantum physics to speed up calculations and process massive amounts of data at speeds that today’s computers are simply not capable of doing. 

Right now, quantum computers are in their infancy and aren't available on the mainstream market. But they're perhaps closer than many realize.

Cracking open a Bitcoin private key?

One of the most-cited ways to attack a Bitcoin private key—though it has never been done successfully—is to “brute force” attack a Bitcoin private key itself. 

A brute force attack is simply an exhaustive, computer-automated search of possible combinations to discover a private key, which would thus grant an external actor access to another person's crypto. 

The problem for today’s would-be hackers is that a private key is a number between 1 and 2^256, aka 115 quattuorvigintillion. Yes, that’s a real number, and it’s estimated to be a figure greater than the total number of atoms in the universe. 

That is a level of number-crunching today’s computers are simply not capable of performing—unless you've got a few hundred years on your hands.

How far along is quantum computing? 

Quantum computing technology is already well off the ground. 

In February 2021, Microsoft announced the opening of a service called Azure Quantum, which was designed to bring quantum technology directly to Microsoft computers. In China, computer manufacturer SpinQ is working on a quantum computer with one eye on releasing it mainstream for just $5,000. And in March 2022, the NATO Cyber Security Center announced that it successfully tested secure communication flows in what was described as a “post-quantum world.” 

Is Bitcoin really at risk?

Bitcoin would be most at risk as transactions queue up to be processed.

That's not very long. New blocks are mined on the Bitcoin blockchain every 10 or so minutes (though not all eligible transactions are included in the first-available block). Once that’s happened, they can no longer be tampered with. However, before that happens, a private key could theoretically be replicated, allowing a hacker to steal funds from a user’s wallet before a new block on the blockchain is confirmed. 

Quantum Computing CEO Andersen Cheng told Decrypt, “Once that public key is exposed, a quantum computer can work out the private key relatively quickly, in minutes or hours at most.”

But Cheng says the danger is not even necessarily about tampering with actual transactions—it's about trust.

"The main threat is not whether quantum computers can ‘open up’ private key information," he said in April 2022. "It’s more about the power of a quantum computer to replicate a private key without you knowing, undercutting trust in the entire signature process."

We're not there yet, though.

According to Mark Webber at the University of Sussex in the U.K., breaking this level of encryption would reportedly require a quantum computer with 1.9 billion “qubits." This is a staggeringly high figure, especially when you consider that IBM’s best quantum computer boasts a mere 127 quibits in comparison. 

As Cheng previously told Decrypt, quantum computing is not something that will hit the mainstream market any time soon. But that doesn’t mean the crypto industry can turn away from quantum computing and the risks this technology may present. 

“When people say that we don’t need to worry about quantum computers—[that] they are still 10, 20, 30 years away—they are often speaking about a commercial quantum computer,” Cheng said, adding that “in the cybersecurity world, the threat is much closer.” 

In fact, according to some estimates, the functional reality of a quantum computer cracking encrypted systems may only be five years away. 

“We are concerned with a massive, poorly built prototype in a basement, which is all that’s needed to break current encryption and lead to the risks in crypto that I outlined,” Cheng said, concluding that, inevitably, “the entire crypto ecosystem will need to become quantum-safe.”

What can be done? 

Others are thinking about how to avoid this potential problem, which would affect not only Bitcoin and cryptocurrency but also other systems that rely on cryptography—such as banks.

Imperial College researchers have suggested a soft fork of the Bitcoin blockchain that would allow the "secure transition of funds to quantum-resistant wallets." Others have proposed increasing the size of Bitcoin keys.

Crypto asset management firm CoinShares writes that the very structure of Bitcoin may make quantum computing, if not irrelevant, then at least unwise: "We believe that combining the development costs and the technical capability to run a quantum system suggests it remains technically and economically unviable to compete with ASIC miners at present, and perhaps not ever."

Nonetheless, it recommends taking the next decade to "modify existing cryptographic infrastructure" to avoid the threat.