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

The first part of this series explored the physics behind quantum computing, but understanding how it works is only half the story. To comprehend how a quantum computer can be used to steal bitcoin, it's essential to grasp what it's attacking and where the weakness lies. This piece delves into bitcoin's encryption, the nine-minute window it takes to break it, and the significance of Google's recent quantum computing paper. Bitcoin relies on elliptic curve cryptography, which involves a one-way map that makes it easy to go forward but virtually impossible to go backward for classical computers. However, Shor's algorithm, discovered in 1994, can break this trapdoor using quantum properties. The algorithm converts the problem into finding the period of a function, which is what quantum computers are designed for. By using superposition, entanglement, and interference, the algorithm can find the period and recover the private key. Although Shor's algorithm has been known for over 30 years, running it requires a quantum computer with a large number of stable qubits. Google's paper reduced the estimated number of qubits needed from millions to fewer than 500,000, making the threat more tangible. The paper also introduced a practical attack scenario, where the quantum computer can precompute parts of the algorithm and finish the calculation in about nine minutes once a target public key appears. This means that if a user broadcasts a transaction, a quantum attacker has roughly nine minutes to derive the private key and submit a competing transaction. The math gives the attacker a roughly 41% chance of succeeding. Furthermore, approximately 6.9 million bitcoin are vulnerable to an 'at-rest' attack, as their public keys have already been exposed on the blockchain. A quantum computer running Shor's algorithm can turn these public keys into private keys, giving the attacker control over the coins. The implications of this threat and the potential consequences for bitcoin are significant and will be explored in the next piece in this series.