Google’s Quantum Update Shrinks the Safety Window for Bitcoin

Researcher Craig Gidney said in a May 23 blog post that a much smaller quantum computer could do the job.

Previously, Gidney estimated that factoring a 2,048-bit RSA key would take about eight hours on a machine with 20 million error-prone qubits. In the latest paper, he stated that fewer than one million noisy qubits could do it in under a week, which is a 20-times drop in the number of qubits needed.

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The reduced requirement comes from two main changes. First, the researchers found a faster way to perform the kind of calculations used in RSA encryption. Second, they improved error correction by packing more logical operations into the same space, which means they can do more with the same number of physical qubits.

They also used a technique called "magic state cultivation", which helps the computer carry out key steps more efficiently without using extra resources.

Although Bitcoin does not use RSA, it relies on elliptic curve cryptography, which is based on similar math. Bitcoin uses 256-bit keys, which are harder to break than RSA-2048, but not by a huge margin when dealing with quantum systems that grow in power very quickly.

If RSA can be cracked sooner than expected, Bitcoin’s defenses may be on a shorter timeline than previously thought.

Meanwhile, the Bitcoin Core team has decided to remove the 80-byte limit on OP_RETURN data in its next software update. Why? Read the full story.