The Quantum Threat to Bitcoin: How a 9-Minute Window Can Steal Your Coins

This series began by explaining the principles of quantum computing. However, understanding how it works doesn't reveal how it can be used to steal bitcoin. To grasp this, we need to know what a quantum computer is attacking and where the vulnerability lies. This piece explores bitcoin's encryption, from its foundation to the nine-minute window required to break it, as outlined in Google's recent quantum computing paper. Bitcoin utilizes elliptic curve cryptography, which involves a private key and a public key derived from it. The public key is obtained by performing a mathematical operation on the private key, creating a one-way map. Anyone can verify the public key, but classical computers cannot reverse-engineer the private key due to the complexity of the elliptic curve discrete logarithm problem. In 1994, mathematician Peter Shor discovered a quantum algorithm that efficiently solves this problem, breaking the security model. Shor's algorithm converts the problem into finding the period of a function and uses quantum properties to find the private key. The algorithm requires a quantum computer with a large number of stable qubits to maintain coherence throughout the calculation. Google's paper reduced the estimated number of required qubits from millions to fewer than 500,000. The team designed quantum circuits that implement Shor's algorithm against bitcoin's elliptic curve, using approximately 1,200 and 1,450 logical qubits. The paper introduced a practical attack scenario, where parts of the algorithm can be precomputed, allowing the quantum computer to finish the calculation in about nine minutes once a target public key appears. This creates a 41% chance of the attacker deriving the private key and submitting a competing transaction before the original transaction confirms. The bigger concern is the 6.9 million bitcoin vulnerable to an 'at-rest' attack, where the public key has already been exposed on the blockchain. A quantum computer running Shor's algorithm can turn a bitcoin public key into the private key, controlling the coins.