The Quantum Threat to Bitcoin: How a Powerful Computer Can Steal Your Cryptocurrency in Under 10 Minutes

In the first part of this series, we explored the physics of quantum computing. Now, we dive into how a quantum computer can be used to compromise bitcoin's security. To understand the threat, we need to examine how bitcoin's encryption works and where its weaknesses lie. Bitcoin relies on elliptic curve cryptography, which uses a one-way function to derive a public key from a private key. This function is virtually impossible for classical computers to reverse, but a quantum algorithm called Shor's algorithm can break it. Shor's algorithm exploits the properties of quantum mechanics to solve the discrete logarithm problem efficiently. The algorithm works by converting the problem into a function that repeats in a cycle, and then using quantum operations to find the period of this cycle. Once the period is known, the private key can be recovered. Google's recent paper reduced the estimated number of qubits required to run Shor's algorithm against bitcoin's elliptic curve from millions to fewer than 500,000. The team designed two quantum circuits that implement Shor's algorithm, one using approximately 1,200 logical qubits and the other using approximately 1,450 logical qubits. The paper also introduced a practical attack scenario, where the quantum computer can precompute parts of the calculation and then finish the second half in about nine minutes. This means that if a user broadcasts a transaction and their public key is visible in the mempool, a quantum attacker has roughly nine minutes to derive a private key and submit a competing transaction. The math gives the attacker a roughly 41% chance of finishing before the original transaction confirms. Furthermore, approximately 6.9 million bitcoin are sitting in wallets where the public key has already been permanently exposed on the blockchain, making them vulnerable to an 'at-rest' attack that requires no race against the clock.