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

In the first part of this series, we explored the physics behind quantum computing and how it differs from classical computing. However, understanding the inner workings of a quantum computer doesn't reveal how it can be used to steal bitcoin. To grasp this, we need to delve into the security mechanisms of bitcoin and identify the weaknesses that a quantum computer can exploit. This piece will examine bitcoin's encryption, the role of elliptic curve cryptography, and the potential vulnerabilities that a quantum algorithm can target. We will also discuss Google's recent quantum computing paper and its implications for the timeline of a potential attack. Bitcoin's security relies on a system called elliptic curve cryptography, which uses a pair of keys: a private key and a public key. The private key is a secret number, while the public key is derived from the private key through a mathematical operation. This operation is a one-way function, meaning it's easy to generate the public key from the private key but virtually impossible to reverse the process using classical computers. The security of bitcoin hinges on this one-way function, as it allows users to prove ownership of their coins without revealing their private key. However, a quantum algorithm known as Shor's algorithm can potentially break this one-way function, rendering bitcoin's security vulnerable to attack. Shor's algorithm uses the principles of quantum mechanics to solve the discrete logarithm problem, which is the foundation of elliptic curve cryptography. By exploiting the properties of quantum superposition, entanglement, and interference, Shor's algorithm can efficiently solve the discrete logarithm problem, thereby breaking the one-way function. The implications of this are significant, as a quantum computer running Shor's algorithm could potentially derive a private key from a public key, giving the attacker control over the associated bitcoin. Google's recent paper has 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 has significant implications for the timeline of a potential attack, as it brings the possibility of a quantum computer capable of breaking bitcoin's security closer to reality. The paper also introduces a practical attack scenario, where a quantum computer can precompute the parts of Shor's algorithm that depend on the elliptic curve's fixed parameters, allowing it to sit in a primed state, waiting for a target public key to appear. Once a target public key is broadcast, the quantum computer can finish the calculation in approximately nine minutes, which is roughly the same time it takes for a bitcoin transaction to be confirmed. This creates a narrow window of opportunity for a quantum attacker to derive a private key and submit a competing transaction, potentially redirecting funds. The consequences of such an attack would be severe, with an estimated 6.9 million bitcoin (approximately one-third of the total supply) vulnerable to an 'at-rest' attack. These coins are sitting in wallets where the public key has already been permanently exposed on the blockchain, making them susceptible to a quantum computer running Shor's algorithm. The final piece in this series will explore the practical implications of this vulnerability, including which bitcoin are already exposed, the impact of Taproot, and the pace at which quantum hardware is advancing.