The Quantum Threat to Bitcoin: How a Powerful Computer Can Steal Your Cryptocurrency in Under 10 Minutes

A quantum computer can potentially steal your bitcoin in a matter of minutes, but to understand how, you need to know what it's attacking and where the weakness lies. Bitcoin uses elliptic curve cryptography, a one-way map that makes it easy to generate a public key from a private key but virtually impossible for classical computers to reverse the process. However, a quantum algorithm known as Shor's algorithm can break this security code efficiently. The algorithm works by finding the period of a function, which is a task that quantum computers are well-suited for due to their ability to exist in multiple states simultaneously and perform calculations on all possible inputs at once. Google's recent paper reduced the estimated number of qubits needed to run Shor's algorithm against bitcoin's elliptic curve from millions to fewer than 500,000, making the threat more real. The paper also introduced a practical attack scenario where a quantum computer can precompute parts of the algorithm and then 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 bigger concern is the 6.9 million bitcoin already exposed on the blockchain, which are vulnerable to an 'at-rest' attack that requires no time constraint.