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 of quantum computing, explaining how these machines differ fundamentally from regular computers. However, understanding the mechanics of quantum computers doesn't directly reveal how they can be used to steal bitcoin. To grasp this, one must comprehend what quantum computers are attacking, how bitcoin's security is structured, and where the vulnerability lies. This piece begins by examining bitcoin's encryption, leading up to the nine-minute window identified by Google's recent quantum computing paper, within which a quantum algorithm can potentially break bitcoin's security. Bitcoin utilizes elliptic curve cryptography, a system that involves two keys: a private key, which is a secret number, and a public key derived from the private key through a mathematical operation. This 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 mathematics leads to the public key. While moving forward in this map is straightforward, reversing the process is virtually impossible for classical computers due to the elliptic curve discrete logarithm problem. However, in 1994, mathematician Peter Shor discovered a quantum algorithm that can efficiently solve this problem, thereby breaking the security model of bitcoin. Shor's algorithm leverages the properties of quantum mechanics, including superposition, entanglement, and interference, to find the period of a function related to the elliptic curve, which in turn reveals the private key. The significance of this algorithm is that it can potentially allow a quantum computer to derive a private key from a public key in a relatively short period, specifically within the timeframe of bitcoin's block confirmation. Google's recent paper has reduced the estimated number of qubits needed 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, waiting for a target public key to appear. Once a public key is visible, the quantum computer can finish the calculation in about nine minutes, which is close to bitcoin's average block confirmation time. This creates a window of opportunity for a quantum attacker to derive a private key and submit a competing transaction. The implications are alarming, especially for the 6.9 million bitcoin that have their public keys exposed on the blockchain, making them vulnerable to an 'at-rest' attack without the need for a race against the clock.