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

Part one of this series explored the fundamentals of quantum computing, but understanding how it works is only half the story. To grasp the threat it poses to bitcoin, we need to examine the target: the encryption that secures bitcoin transactions. Bitcoin utilizes a system called elliptic curve cryptography, which relies on a one-way function to prove ownership. Each wallet has a private key, a secret number, and a public key derived from it through a mathematical operation. The public key can be shared without compromising security, as classical computers cannot reverse the math to obtain the private key. However, a quantum algorithm known as Shor's algorithm can efficiently break this one-way function, potentially allowing an attacker to derive a private key from a public key. This algorithm exploits the principles of superposition, entanglement, and interference in quantum mechanics to find the period of a function, which is crucial for solving the discrete logarithm problem. The recent paper by Google's Quantum AI division has reduced the estimated number of qubits required to run Shor's algorithm against bitcoin's elliptic curve, making the threat more tangible. The paper introduced a practical attack scenario where parts of the algorithm can be precomputed, allowing a 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 approximately nine minutes. This timeline poses a significant threat, particularly to the 6.9 million bitcoin already exposed on the blockchain, which are vulnerable to an 'at-rest' attack. The implications of this threat and the potential consequences for bitcoin's security are profound, and the gap between the required hardware and current technology is narrowing.