How Quantum Computers Can Compromise Bitcoin Security in Under 10 Minutes

The first part of this series provided an in-depth explanation of quantum computing's underlying physics. However, understanding the mechanism of quantum computers is only half the story; the other half involves grasping what they can attack and how they can be used maliciously, such as stealing bitcoin. This piece focuses on the target of these attacks: the encryption used by bitcoin, why quantum algorithms can break it, and the significant shift in timeline presented by Google's recent paper. The security of bitcoin is based on a system called elliptic curve cryptography, which utilizes a pair of keys: a private key and a public key. The public key is generated from the private key through a mathematical operation on a specific curve known as 'secp256k1.' This process can be thought of as a one-way map, where starting from a known point on the curve, one can easily move forward to find the public key, but reversing the process to find the private key is virtually impossible for classical computers. This one-way function is the cornerstone of bitcoin's security model, ensuring that while the public key can be shared safely, the private key remains secure. The private key serves as proof of ownership of coins, and when sending bitcoin, the wallet uses the private key to create a digital signature, proving knowledge of the secret number without revealing it. However, in 1994, mathematician Peter Shor discovered a quantum algorithm that can efficiently break this one-way trapdoor. Shor's algorithm solves the discrete logarithm problem, which classical computers cannot, by converting it into a problem of finding the period of a function. This is achieved by exploiting the properties of quantum mechanics, such as superposition, entanglement, and interference, allowing the algorithm to evaluate a function for all possible inputs simultaneously and find the period, from which the private key can be derived. The reason bitcoin remains secure today is that running Shor's algorithm requires a quantum computer with a sufficiently large number of stable qubits to maintain coherence throughout the calculation. Recent research by Google, in collaboration with the Ethereum Foundation and Stanford cryptographer Dan Boneh, has significantly reduced the estimated number of qubits needed, from millions to fewer than 500,000. This reduction introduces a practical attack scenario, where parts of Shor's algorithm can be precomputed and the quantum computer can be primed, waiting for a target public key to appear. Upon sighting the public key, the machine only needs to complete the second half of the calculation, which Google estimates takes approximately nine minutes. Given that bitcoin's average block confirmation time is 10 minutes, this creates a narrow window for a quantum attacker to derive a private key and submit a competing transaction. The concern is not only for future transactions but also for the approximately 6.9 million bitcoin that have already exposed their public keys on the blockchain, making them vulnerable to attack without any time constraint.