The Quantum Threat to Bitcoin: How a Powerful Computer Can Crack Encryption in Under 10 Minutes

The first part of this series explored the fundamental principles of quantum computing, highlighting its distinction from classical computing. However, understanding how quantum computers work is only half the story; the other half involves grasping what they can attack and how. Bitcoin's security relies on a complex system of cryptography, specifically elliptic curve cryptography, which is used to verify ownership of coins. Every bitcoin wallet has a private key, a secret number, and a public key derived from this private key through a mathematical operation on the 'secp256k1' curve. This process can be thought of as a one-way map, where starting from a known point on the curve, taking a certain number of steps defined by the curve's mathematics leads to a public key. The crucial aspect of this system is that while moving forward (from private to public key) is straightforward, reversing the process (from public to private key) is virtually impossible for classical computers. This impossibility is what secures bitcoin transactions, as only the owner of the private key can create a digital signature to send bitcoins. However, the advent of quantum computing, particularly through Shor's algorithm discovered in 1994, poses a significant threat to this security model. Shor's algorithm can efficiently solve the discrete logarithm problem, which is the backbone of bitcoin's encryption, by exploiting quantum properties such as superposition, entanglement, and interference. The algorithm works by converting the problem of finding a private key into finding the period of a function related to the elliptic curve, a task that quantum computers are uniquely suited for. Despite Shor's algorithm being known for over 30 years, its application has been hindered by the requirement for a large number of stable qubits. Recent advancements, including a paper by Google's Quantum AI division, have significantly reduced the estimated number of qubits needed, from millions to fewer than 500,000, bringing the threat closer to reality. The paper introduced a practical attack scenario where parts of Shor's algorithm can be precomputed, allowing a quantum computer to sit in a 'primed' state, ready to finalize the calculation once a target public key is identified. This leads to a 'nine-minute window' for attacking bitcoin transactions, where an attacker has roughly nine minutes to derive a private key and submit a competing transaction after a user broadcasts a transaction. More alarmingly, approximately 6.9 million bitcoins are already vulnerable to an 'at-rest' attack, as their public keys have been exposed on the blockchain. The implications of these findings and the race to develop quantum-resistant cryptography will be explored further in the next piece of this series.