Google LLC today published a paper that indicates a quantum computer with 500,000 qubits could be used to steal cryptocurrency.

The cybersecurity risks posed by quantum computers were already well-known to researchers. However, it was believed that breaching cryptocurrencies such as bitcoin would require a machine with millions of qubits. Such computers may be decades away. One of the main conclusions of Google’s new paper is that cryptocurrency encryption could potentially be bypassed much sooner.

Many digital coins use an encryption method known as ECC, or elliptic curve cryptography, to protect users’ funds. ECC is powered by a mathematical object that can be visualized as a curved line on a two-dimensional plane. The line comprises a series of points that all share certain mathematical properties. Those properties are encapsulated in a relatively simple equation.

ECC works by effectively hiding the user’s password behind one of the points that make up the curved line. In theory, a hacker could deduce the hidden password by analyzing the point. However, doing so requires carrying out such a large number of calculations that the task is impossible in practice.

A future large-scale quantum computer could potentially bypass ECC encryption. A set of interconnected qubits can complete a large number of computations in a short amount of time. As a result, a quantum computer can quickly work through the calculations needed to reconstruct an ECC-protected password.

Bitcoin and several other cryptocurrencies use an implementation of ECC called secp256k1. According to Google, its researchers determined that the technology could be broken in a few minutes by a quantum machine with 500,000 superconducting qubits.

A superconducting qubit is a set of transistors cooled close to absolute zero. At such temperatures, electrons travel through transistors in pairs rather than one after another as they usually do. The presence of electron pairs gives rise to quantum phenomena that speed up calculations.

Previously, researchers believed that breaking secp256k1 would require a quantum computer with about 10 million qubits, or 20 times more than Google’s estimate. A quantum machine with 500,000 qubits may be ready much sooner. However, such technology is likely still years away. The fastest superconducting quantum processor that Google has built to date, a device called Willow (pictured), features 105 qubits.

Besides a large-scale quantum computer, bypassing secp256k1 also requires an algorithm that Google developed as part of the study. To prevent misuse, the search giant has not made the algorithm publicly available. It instead released a so-called zero-knowledge proof. The proof enables external researchers to verify the algorithm without finding out how it works.

“We want to raise awareness on this issue and are providing the cryptocurrency community with recommendations to improve security and stability before this is possible, including transitioning blockchains to post-quantum cryptography,” Google executives Ryan Babbush and Hartmut Neven wrote in a blog post today.

Post-quantum cryptography algorithms fend off cyberattacks by hiding data in more complicated mathematical objects than ECC. Many use objects called lattices that can be visualized as a grid of points. Usually, the grid spans more than three dimensions and contains deliberate errors designed to make it more difficult for hackers to extract the user’s data.

Image: Google