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 physics behind quantum computing, but understanding how it works is only half the story. To grasp the threat it poses to bitcoin, we need to examine what it's attacking – the encryption that secures the cryptocurrency. Bitcoin relies on elliptic curve cryptography, a system that uses a pair of keys: a private key, which is a secret number, and a public key, derived from the private key through a complex mathematical operation. This operation is akin to a one-way map, where it's easy to move forward but virtually impossible to reverse-engineer, at least for classical computers. However, the advent of quantum computers, particularly with the discovery of Shor's algorithm, changes this dynamic. Shor's algorithm can efficiently solve the discrete logarithm problem, which is the backbone of bitcoin's security. By leveraging quantum properties such as superposition, entanglement, and interference, a quantum computer can potentially break the encryption and steal bitcoin. The recent paper by Google's Quantum AI division, in collaboration with researchers from the Ethereum Foundation and Stanford, has significantly reduced the estimated number of qubits required to run Shor's algorithm against bitcoin's elliptic curve, from millions to fewer than 500,000. This reduction, along with the introduction of a practical attack scenario, has shifted the timeline for potential quantum threats to bitcoin. The scenario involves precomputing parts of Shor's algorithm that depend on the elliptic curve's fixed parameters, allowing a quantum computer to sit in a primed state, ready to finish the calculation once a target public key appears. Google estimates this could take about nine minutes, which is alarming given bitcoin's average block confirmation time of 10 minutes. This creates a window of opportunity for a quantum attacker to derive a private key and submit a competing transaction. Furthermore, the attack is not limited to newly broadcast transactions but also poses a significant threat to the approximately 6.9 million bitcoin that have had their public keys exposed on the blockchain. These coins are vulnerable to an 'at-rest' attack, where the time constraint is not a factor. The implications of this quantum threat, including what it means for the security of bitcoin and how the development of quantum-resistant cryptography is becoming increasingly urgent, will be explored in the next part of this series.