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

The first part of this series delved into the physics behind quantum computing, explaining how it differs from classical computing. However, understanding the mechanics of quantum computing doesn't necessarily reveal how it can be used to compromise bitcoin's security. To grasp this, it's essential to comprehend what quantum computers are attacking and where the vulnerability lies. This piece will explore bitcoin's encryption, how it can be broken by a quantum algorithm, and the implications of Google's recent quantum computing paper. Bitcoin utilizes elliptic curve cryptography, which involves a private key and a public key. The private key is a secret number, while the public key is derived from the private key through a mathematical operation. This process can be thought of as a one-way map, where it's easy to move forward but virtually impossible to reverse the process using classical computers. The security of bitcoin relies on this one-way trapdoor, where the private key proves ownership, and the public key can be shared without compromising security. However, a quantum algorithm known as Shor's algorithm can break this trapdoor. Shor's algorithm solves the discrete logarithm problem efficiently, which would take a classical computer an impractically long time to solve. The algorithm works by finding the period of a function, which is a problem that quantum computers are well-suited to solve. This is achieved through the use of superposition, entanglement, and interference, allowing the quantum computer to evaluate the function for all possible inputs simultaneously and find the correct answer. The reason bitcoin remains secure today is that running Shor's algorithm requires a quantum computer with a large number of stable qubits. Recent estimates suggest that the number of qubits required has been reduced to fewer than 500,000. Google's paper introduced a practical attack scenario, which involves precomputing parts of Shor's algorithm that depend on the elliptic curve's fixed parameters. This allows the quantum computer to sit in a primed state, waiting for a target public key to appear. Once the public key is visible, the machine only needs to finish the second half of the calculation, which is estimated to take around nine minutes. This creates a narrow window for a quantum attacker to derive a private key and submit a competing transaction. The bigger concern is the large number of bitcoin that are already vulnerable to an 'at-rest' attack, where the public key has been permanently exposed on the blockchain. A quantum computer running Shor's algorithm can turn a bitcoin public key into the private key that controls the coins, making these coins highly susceptible to theft.