The Quantum Threat to Bitcoin: How Your Cryptocurrency Could Be Stolen in Under 10 Minutes

The first part of this series delved into the principles of quantum computing, but understanding how it works is only half the story. To comprehend the threat it poses to bitcoin, we need to examine the cryptocurrency's security framework and identify the weaknesses that a quantum computer can exploit. Bitcoin's security relies on elliptic curve cryptography, a system that utilizes a pair of keys: a private key, which is a secret number, and a public key, derived from the private key through a mathematical operation. The relationship between these keys is based on a one-way function, making it easy to generate the public key from the private key but virtually impossible to reverse the process using classical computers. However, the advent of quantum computing and the development of Shor's algorithm have changed the landscape. Shor's algorithm can efficiently solve the discrete logarithm problem, which is the foundation of bitcoin's security, by leveraging the principles of superposition, entanglement, and interference. This means that a sufficiently powerful quantum computer could potentially break the encryption and steal bitcoin. The recent paper by Google's Quantum AI division, in collaboration with researchers from the Ethereum Foundation and Stanford, has significantly reduced the estimated number of qubits required to run Shor's algorithm against bitcoin's specific elliptic curve, from millions to fewer than 500,000. The team designed two quantum circuits that implement Shor's algorithm, one using approximately 1,200 logical qubits and the other using around 1,450 logical qubits. The practical implication of this breakthrough is a reduced timeframe for a potential attack. Google's paper introduced a precomputation step that allows a quantum computer to sit in a primed state, waiting for a target public key to appear. Once a public key is broadcast, the machine only needs to finish the second half of the calculation, which Google estimates takes around nine minutes. This timeframe is alarming because it is comparable to bitcoin's average block confirmation time of 10 minutes, giving an attacker a roughly 41% chance of deriving a private key and submitting a competing transaction before the original transaction confirms. The bigger concern is the 6.9 million bitcoin, roughly one-third of the total supply, that are already vulnerable to an 'at-rest' attack because their public keys have been permanently exposed on the blockchain. These coins can be targeted without any time constraint, as the attacker can take as long as needed to derive the private key. The race to develop a quantum computer capable of running Shor's algorithm against bitcoin's encryption has significant implications for the cryptocurrency's security and the future of quantum computing.