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

The first part of this series delved into the principles of quantum computing, exploring its unique properties and capabilities. However, understanding how quantum computers work is only half the story - to grasp their potential impact on bitcoin, we need to examine the target: the encryption that secures the cryptocurrency. This piece will dissect how bitcoin's encryption functions, why a quantum algorithm can break it, and the implications of Google's recent research on the timeline of a potential quantum attack. Bitcoin's security relies on a system called elliptic curve cryptography, which uses a pair of keys: a private key, kept secret, and a public key, derived from the private key through a mathematical operation on a specific curve. This process creates a one-way map, making it easy to generate a public key from a private key but virtually impossible for classical computers to reverse the process. The security model is based on this one-way trapdoor, with the private key serving as proof of ownership and the public key being safe to share. When sending bitcoin, the wallet uses the private key to create a digital signature, verifying ownership without revealing the secret number. However, a quantum algorithm known as Shor's algorithm can break this trapdoor, solving the discrete logarithm problem efficiently. This algorithm works by converting the problem into finding the period of a function, which is a task that quantum computers are well-suited for. By using superposition, entanglement, and interference, a quantum computer can find the period and subsequently recover the private key. Although Shor's algorithm has been known for over 30 years, its implementation has been hindered by the need for a large number of stable qubits to maintain coherence throughout the calculation. Recent research by Google has reduced the estimated number of qubits required, making the prospect of a quantum attack more feasible. The study 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 around 1,450 logical qubits. The introduction of a practical attack scenario has significant implications, as it changes how we think about the threat. The parts of Shor's algorithm that depend on the elliptic curve's fixed parameters can be precomputed, allowing the quantum computer to sit in a primed state, waiting for a target public key to appear. Once a public key is visible, the machine only needs to finish the second half of the calculation, which Google estimates takes around nine minutes. This creates a narrow window for a quantum attacker to derive a private key and submit a competing transaction, with a roughly 41% chance of succeeding before the original transaction confirms. 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. A quantum computer running Shor's algorithm can turn a bitcoin public key into the private key that controls the coins, posing a significant threat to the security of the cryptocurrency.