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, revealing how it differs fundamentally from classical computing. However, understanding the mechanics of quantum computing doesn't necessarily explain how it can be used to compromise bitcoin security. To grasp this, it's essential to comprehend what quantum computers are attacking and where the weaknesses in bitcoin's security lie. This piece begins by examining bitcoin's encryption method, known as elliptic curve cryptography, which is used to verify ownership. Every bitcoin wallet has a private key, a secret number, and a public key derived from the private key through a mathematical operation on a specific curve. The process can be thought of as a one-way map, where starting from a known point, taking a certain number of steps defined by the curve's math leads to a public key. While it's straightforward to calculate the public key from the private key, reversing this process is virtually impossible for classical computers due to the nature of the elliptic curve discrete logarithm problem. In 1994, mathematician Peter Shor discovered a quantum algorithm that can efficiently solve this problem, potentially breaking the security model of bitcoin. Shor's algorithm works by converting the problem of finding the private key into a problem of finding the period of a function related to the elliptic curve. Quantum computers are particularly adept at finding such periods due to their ability to process multiple inputs simultaneously through superposition, entanglement, and interference. Despite Shor's algorithm being known for over 30 years, the lack of a sufficiently powerful quantum computer has kept bitcoin safe. Recent estimates by Google, in collaboration with researchers from the Ethereum Foundation and Stanford, suggest that the number of qubits required to run Shor's algorithm against bitcoin's elliptic curve could be significantly lower than previously thought, possibly fewer than 500,000. This development introduces a practical attack scenario where parts of the calculation can be precomputed, allowing a quantum computer to derive a private key in about nine minutes once a target public key is identified. Given bitcoin's average block confirmation time of 10 minutes, this poses a significant risk, especially for the 6.9 million bitcoin whose public keys have already been exposed on the blockchain, making them vulnerable to an 'at-rest' attack without any time constraint.