Recently, Google's Quantum AI team revealed that a potential future quantum computer could extract a bitcoin private key from its corresponding public key in approximately nine minutes. This revelation sent shockwaves across social media and financial markets. But what are the practical implications of this claim?
To understand, it's essential to grasp how bitcoin transactions function. When a bitcoin transaction is initiated, the sender's wallet utilizes a private key - a secret number confirming ownership of the coins - to sign the transaction.
This signature also discloses the public key, a shareable address that gets broadcast to the network, waiting to be included in a block by a miner, a process that typically takes around 10 minutes. The private and public keys are connected through a complex mathematical problem known as the elliptic curve discrete logarithm problem, which classical computers cannot solve within a practical timeframe. However, a sufficiently powerful future quantum computer, leveraging an algorithm like Shor's, could potentially reverse this math.
The concept of 'priming' a quantum computer by pre-computing parts of the attack not specific to any public key is where the nine-minute factor comes into play. Once a public key appears in the mempool, the quantum computer would only need about nine minutes to derive the private key, given that bitcoin's average confirmation time is 10 minutes.
This gives an attacker roughly a 41% chance of redirecting funds before the original transaction is confirmed. It's akin to a thief pre-building a universal safe-cracking machine that, while taking hours to construct, only needs a few minutes of final adjustments to crack any safe. This 'mempool attack' is alarming but requires a quantum computer that does not yet exist, with estimates suggesting it would need fewer than 500,000 physical qubits, far beyond the capabilities of today's largest quantum processors. A more immediate concern is the 6.9 million bitcoin, roughly a third of the total supply, that are already vulnerable due to permanently exposed public keys.
These include older bitcoin addresses that used pay-to-public-key formats and wallets that have reused addresses, thereby exposing public keys. Such coins are at risk of being cracked by a sufficiently powerful quantum computer without any time constraint. The 2021 Taproot upgrade inadvertently expanded the pool of vulnerable wallets by making public keys visible on-chain by default. While the bitcoin network would continue to function, as mining uses the SHA-256 algorithm that quantum computers cannot significantly speed up, the ability to derive private keys from public keys would undermine the ownership guarantees that give bitcoin its value.
The solution lies in post-quantum cryptography, which would replace vulnerable algorithms with ones resistant to quantum computing. Ethereum has been working towards this migration for eight years, while bitcoin has yet to begin.