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

The first part of this series explored the fundamental principles of quantum computing, a technology that operates under the unique rules of physics at the atomic and subatomic level. However, understanding how quantum computers work 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 will dissect the encryption mechanism used by bitcoin, known as elliptic curve cryptography, and explain how a quantum algorithm, specifically Shor's algorithm, can compromise this security. The security of bitcoin is based on a one-way function, where a private key (a secret number) is used to derive a public key, which can be shared without fear of the private key being deduced. This is due to the elliptic curve discrete logarithm problem, which is virtually unsolvable for classical computers. However, Shor's algorithm, discovered in 1994, offers a quantum solution to this problem, potentially allowing a quantum computer to derive a private key from a public key in a relatively short period. The recent paper by Google's Quantum AI division, in collaboration with other researchers, has provided new insights into the feasibility of such an attack, suggesting that the number of qubits required to perform the calculation might be lower than previously estimated. They designed quantum circuits that could implement Shor's algorithm against bitcoin's specific elliptic curve, requiring fewer than 500,000 physical qubits, a significant reduction from previous estimates. The attack scenario introduced by Google involves precomputing parts of Shor's algorithm that depend on the elliptic curve's fixed parameters, allowing the quantum computer to be in a 'primed' state, ready to finish the calculation once a target public key appears. This could potentially give an attacker a nine-minute window to derive a private key and submit a competing transaction, a timeframe that is alarming given bitcoin's average block confirmation time is 10 minutes. Furthermore, the study highlights that approximately 6.9 million bitcoin, where the public key has already been exposed on the blockchain, are particularly vulnerable to such attacks, with no time constraint for the attacker to derive the private key. The implications of these findings and the race to develop quantum-resistant cryptography will be explored in the next part of this series.