The Quantum Threat to Bitcoin: How Your Coins Can Be Stolen in Under 10 Minutes

The first part of this series delved into the physics behind quantum computing, explaining how these machines differ fundamentally from classical computers. However, understanding the inner workings of a quantum computer doesn't reveal how it can be used to steal bitcoin. To grasp this, one must comprehend what a quantum computer attacks and how the security of bitcoin is structured, including its weaknesses. This piece begins by examining bitcoin's encryption, leading up to the nine-minute window identified by Google's recent quantum computing paper, in which a bitcoin's security can be compromised. Bitcoin utilizes elliptic curve cryptography, a system that proves ownership. Each wallet has 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 math leads to the public key. While moving forward is straightforward, reversing the process is virtually impossible for classical computers. The security model relies on this one-way trapdoor, with the private key proving ownership and the public key being safe to share because classical computers cannot reverse the math. However, Shor's algorithm, discovered in 1994, breaks this trapdoor by solving the discrete logarithm problem efficiently, something that would take a classical computer longer than the age of the universe. Shor's algorithm works by converting the problem into finding the period of a function, which quantum computers can solve by representing all possible values simultaneously and then applying a quantum operation to find the correct period. This period allows for the recovery of the private key, thus compromising the bitcoin's security. The reason bitcoin remains secure is that running Shor's algorithm requires a quantum computer with a large number of stable qubits, which has been beyond current capabilities. Google's recent paper reduced the estimated number of qubits needed from millions to fewer than 500,000, making the threat more feasible. The paper introduced a practical attack scenario where parts of Shor's algorithm can be precomputed, leaving the quantum computer in a primed state, ready to finish the calculation once a target public key appears. This could potentially give an attacker a nine-minute window to derive a private key and submit a competing transaction, with a roughly 41% chance of succeeding before the original transaction confirms. More alarming is the vulnerability of the 6.9 million bitcoin already exposed on the blockchain, which can be attacked without a time constraint. As quantum computing advances, the gap between the capability to attack and the security of bitcoin narrows, posing a significant threat to the cryptocurrency's future.