Google's Quantum AI team said breaking the elliptic curve cryptography used by Bitcoin and Ethereum wallets could require fewer than 500,000 physical qubits, about 20 times below prior estimates in the millions. The paper said a quantum computer could crack a Bitcoin private key in about nine minutes after a transaction exposes a public key, giving an attacker a 41% chance of beating Bitcoin's 10-minute confirmation window. Google estimated that about 6.9 million BTC sit in wallets with already exposed public keys.

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