The Google research team recently set a new date for the migration of post-quantum cryptography: 2029. Among other things, this means that Bitcoin, along with many other cryptocurrencies, needs to use new cryptographic techniques that can withstand quantum attacks within three years.
Google announced the new timeline in a blog post. “Quantum computers will pose a serious threat to current cryptographic standards, and especially to cryptography and digital signatures,” the post said.
As for the actual science, two important papers were published on Monday. One is signed by Google researchers, the other is signed by a startup called Oratomic (with ex-Googlers and Caltech employees on board). The papers are very readable for anyone who is not a cryptography expert, but can be simplified in this way: They describe new ways to break the most important cryptographic systems using quantum computers, which have far fewer resources (10x) than previously thought.
This is relevant to Bitcoin because it makes it more likely that someone will build a quantum computer capable of deriving Bitcoin’s private key from Bitcoin’s public key. In fact, it’s very likely that Google decided not to show the quantum circuits they used to do this, instead showing mathematical proof that this is possible.
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Justin Drake, one of the researchers who co-signed the Google paper, has a good idea. “A very powerful quantum computer, the kind Google is building, could crack keys in minutes,” he wrote.
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Important point: As Adam Back, a key Bitcoin expert, pointed out, Bitcoin (the network) does not use encryption. Google’s findings do not mean that anyone can block transactions on the Bitcoin network; instead, they can crack someone’s private key, and if you have someone’s private key, you have their coins.
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In fact, it’s more complicated than that. The two papers above refer to Shor’s algorithm, a quantum algorithm developed by Peter Shor back in 1994, which makes it much faster to break certain types of encryption with quantum computers. Shor’s algorithm can be used to derive a Bitcoin private key from a public key, but only in certain cases. This includes old Bitcoin addresses, including those used by Bitcoin creator Satoshi Nakamoto himself; this is significant, as these addresses hold over a million bitcoins, meaning the potential reward for the hacker is in the tens of billions of dollars (not to mention the chaos it could cause on the network as everyone scrambles to figure out what’s next).
New addresses can also be cracked, but only until they are broadcast during the transaction, which means there is a short window (10 minutes long) where someone might use Shor’s algorithm to find that private key. No known quantum computer that can do this currently exists, even considering the optimizations found by Google and Oratomic researchers. But it is not a mystery that someone is building in the future.
Bitcoin is traditionally slow to make any changes. Adam Back, in particular, advises in 2025 that “some quantum readiness” should be added in the next five years, although he says he does not expect it to be implemented in “a few decades.”
On the contrary, new papers show that Bitcoin’s quantum threat is much closer than that, and that serious action should be taken now.
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What can be done? Google’s paper suggests ways blockchains (including Bitcoin) can mitigate the problem. This includes simple steps like moving coins from old addresses to new ones if possible, but also updating the underlying principles to include post-quantum cryptography. This process is not easy for large, established cryptocurrency networks, and it can take years to even agree on the best solution (the internal dispute over block size in the Bitcoin network took almost two years to resolve), let alone to implement it.
Other cryptocurrencies, such as Ethereum, are also vulnerable to these problems. The Ethereum Foundation, a non-profit organization responsible for the long-term growth of Ethereum, recently published a post-quantum roadmap, which aims to solve these problems before it’s too late.
