The notion that a quantum computer could potentially 'crack' bitcoin in a matter of minutes has been making headlines, but what does this actually mean in practical terms? To grasp the implications, it's essential to understand how bitcoin transactions work. When a bitcoin transaction is made, the sender's wallet uses a private key to sign the transaction, which is then paired with a public key that is shared publicly.
This public key is linked to the private key through a complex mathematical problem known as the elliptic curve discrete logarithm problem, which classical computers are unable to solve efficiently. However, a sufficiently powerful quantum computer could potentially solve this problem, thereby deriving the private key from the public key.
According to Google's study, a quantum computer could be 'primed' in advance by pre-computing parts of the attack, and then, once a public key is made available, it could derive the corresponding private key in approximately nine minutes. This timeframe is concerning because it falls within the average confirmation time for bitcoin transactions, which is around 10 minutes. As a result, an attacker would have a roughly 41% chance of successfully deriving the private key and redirecting the funds before the original transaction is confirmed. It's analogous to a thief constructing a universal safe-cracking machine that can be used to crack any safe, with the final adjustments taking around nine minutes.
This type of attack, known as a mempool attack, is alarming but requires a quantum computer that does not yet exist. A more pressing concern is the approximately 6.9 million bitcoins that are already vulnerable due to exposed public keys. This includes early bitcoin addresses that used a pay-to-public-key format, as well as wallets that have reused addresses, making their public keys visible on the blockchain. These coins are at risk of being cracked by a sufficiently powerful quantum computer without any time pressure.
The bitcoin network itself would continue to function, as the mining process uses a different algorithm that is not vulnerable to quantum computer attacks. Nevertheless, if private keys can be derived from public keys, the security guarantees that make bitcoin valuable would be compromised, putting ownership at risk and undermining trust in the network's security model. The solution to this issue is to implement post-quantum cryptography, which would replace the vulnerable mathematical algorithms with ones that are resistant to quantum computer attacks.
While Ethereum has been working towards this migration for eight years, bitcoin has yet to initiate this process.