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

The first part of this series explored the fundamental principles of quantum computing. However, understanding how quantum computers work is only half the story. To comprehend how they can be used to steal bitcoin, it's essential to grasp what they're attacking and where the vulnerability lies. This piece begins by examining bitcoin's encryption and then works through to the nine-minute window identified by Google's recent paper. Bitcoin's security relies on elliptic curve cryptography, which involves a one-way map. Each wallet has two keys: a private key, which is a secret number, and a public key derived from the private key using a mathematical operation. The process can be thought of as taking a private number of steps on a specific curve, with the number of steps being the private key and the endpoint being the public key. This system is virtually unbreakable for classical computers, but a quantum algorithm known as Shor's algorithm can solve the elliptic curve discrete logarithm problem efficiently. Shor's algorithm works by converting the problem into finding the period of a function and then using quantum properties to evaluate the function on all possible inputs simultaneously. 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, making the threat more feasible. The team designed two quantum circuits that implement Shor's algorithm against bitcoin's specific elliptic curve, with one using approximately 1,200 logical qubits and the other using approximately 1,450 logical qubits. The nine-minute window identified by Google refers to the time it takes to finish the second half of the calculation once a target public key appears. This introduces a practical attack scenario where a quantum computer can precompute parts of the algorithm and then finish the calculation within nine minutes. The average block confirmation time for bitcoin is 10 minutes, giving the attacker a roughly 41% chance of succeeding. The bigger concern is the 6.9 million bitcoin already exposed on the blockchain, which are vulnerable to an 'at-rest' attack that requires no time constraint.