Is quantum computing a threat to blockchain?


Some people worry that the day we see the first real, fully-functioning quantum computer is the day blockchain as we know it dies. But is that true? And, if so, why?

It all comes down to blockchain’s ‘secret sauce’ – the cryptographic methods used to secure and verify the data recorded in every block. Using today’s computers, those methods are sufficiently difficult that it’s virtually impossible to alter data in previously verified blocks without that change becoming immediately obvious to everyone on the blockchain. But with quantum computers, ‘sufficiently difficult’ suddenly becomes ‘instant’ and ‘easy’.

SVB Financial Group’s Natalie Fratto explained the difference in a recent Fortune article. Imagine, she said, a woman named Max who’s looking for her way through a forest to a cabin. She can get there by using just one path out of many, and only by choosing the correct step-by-step route through a large number of complex forks and branches.

With today’s computers, Fratto noted, finding the cabin means testing every potential path one by one until the right route is identified:

“On the blockchain, it’s a good thing that the paths to the cabin get increasingly complex – it requires many computers, each with a lot of computational power to function. This increases the blockchain’s security because it prevents one person or group from seizing too much power... But what if a new method of navigation emerged that upended the entire way Max arrived at her destination? What if Max could use a helicopter to find the path back to the cabin instead?"

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That helicopter, according to Fratto, is quantum computing.

Standard computing is currently built on two fundamental building blocks – the binary digits ‘0’ and ‘1’ – where every bit is either one or the other. But in the strange world of quantum physics, every bit becomes a ‘qubit’ that can transform from ‘0’ to ‘1’ and back again, or can even be both simultaneously. This enables quantum computers to consider many possibilities all at once and rapidly find the correct answer.

So does this mean blockchain is doomed? Not necessarily.

There are several ways in which blockchain technology could survive the advent of true quantum computers. One study, for instance, has suggested that an RSA encryption key, if large enough, could prove impractical for even quantum computers to break. And many other researchers are thinking about ’post-quantum’ blockchains and how those might work.

One possible solution is to use quantum cryptography, which generates cryptographic keys using photons and would be sufficiently difficult for quantum computers to break. Another alternative, proposed recently by researchers in New Zealand, would be to create a decentralised quantum blockchain using a concept called ‘entanglement in time’, where data is encoded using photons that are then discarded.

“In this spatial entanglement case, if an attacker tries to tamper with any photon, the full blockchain would be invalidated immediately...,” Victoria University of Wellington researchers Del Rajan and Matt Visser write in their study. “The temporal... blockchain adds a far greater benefit in that the attacker cannot even attempt to access the previous photons since they no longer exist. They can at best try to tamper with the last remaining photon, which would invalidate the full state.”

Rajan and Visser also say all the elements that could make such a quantum blockchain possible have “already been shown to be experimentally realised. Perhaps more shockingly, our encoding procedure can be interpreted as non-classically influencing the past; hence this decentralised quantum blockchain
can be viewed as a quantum networked time machine”.

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