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

The first part of this series introduced the basics of quantum computing, highlighting its unique capabilities compared to classical computers. However, understanding the inner workings of a quantum computer is only half the story; the real concern is how it can be utilized to compromise bitcoin's security. To grasp this, we must examine the encryption methods bitcoin employs and where the vulnerabilities lie. This piece will dissect bitcoin's encryption, the role of elliptic curve cryptography, and how a quantum algorithm can exploit it. Bitcoin's security relies on a one-way function, akin to a map, where a private key is used to derive a public key. This process is easy to perform in one direction but virtually impossible to reverse using classical computers. The security model is based on this principle, ensuring that only the owner of the private key can access and transfer the associated coins. The public key can be shared without risk because it cannot be used to deduce the private key. However, the introduction of Shor's algorithm, a quantum algorithm discovered by mathematician Peter Shor, changes this dynamic. Shor's algorithm can efficiently solve the discrete logarithm problem, which is the backbone of bitcoin's encryption. By leveraging the principles of quantum mechanics, such as superposition, entanglement, and interference, the algorithm can find the private key associated with a given public key, effectively breaking the encryption. The process involves converting the problem into finding the period of a function, which quantum computers are particularly adept at solving. Once the period is found, the private key can be deduced, giving access to the associated bitcoin. The implications are profound, as a sufficiently powerful quantum computer could potentially steal bitcoin from vulnerable wallets. The main obstacle to this attack has been the requirement for a large number of stable qubits to maintain coherence throughout the calculation. Recent research by Google, in collaboration with other experts, has significantly lowered the estimated number of qubits needed, from millions to fewer than 500,000. This reduction makes the threat more tangible, as the development of such a quantum computer becomes more feasible. The research also outlined a practical attack scenario, where parts of the calculation can be precomputed, leaving only the final steps to be completed once a target public key is identified. This could potentially be done within a nine-minute window, which is alarmingly close to the average time it takes for a bitcoin transaction to be confirmed. For bitcoin users, this means that if a transaction is broadcast and the public key is visible, there is a narrow window in which a quantum attacker could derive the private key and intercept the transaction. Furthermore, a significant portion of the bitcoin supply, approximately 6.9 million coins, is already vulnerable to an 'at-rest' attack because their public keys have been exposed on the blockchain. These coins can be targeted without the urgency of a time-sensitive attack, as the attacker can take as long as needed to compute the private key. The advent of Taproot, a privacy upgrade on Bitcoin, has also changed the landscape, as it makes public keys visible for post-upgrade transactions. In conclusion, the threat posed by quantum computers to bitcoin's security is real and growing. As quantum technology advances and the gap between the required computational power and current capabilities narrows, the urgency to address these vulnerabilities becomes more pressing. The final piece in this series will explore the practical implications of these findings, the current state of quantum computing in relation to bitcoin, and potential strategies for mitigating this emerging threat.