Google Says Quantum Threat to Bitcoin May Need Fewer Qubits

TLDR

• Google’s Quantum AI team said breaking Bitcoin may require fewer qubits than previously estimated.
• Researchers found that fewer than 500,000 physical qubits could crack Bitcoin and Ethereum cryptography.
• The team outlined two attack methods that require about 1,200 to 1,450 high-quality qubits.
• Google said a quantum system could complete a live transaction attack in about nine minutes.
• The report estimated a 41% chance of intercepting a Bitcoin transaction before confirmation.

Google’s Quantum AI team said quantum computers could break Bitcoin with fewer qubits than earlier estimates. The team published its findings in a blog post and whitepaper on Monday. Researchers linked part of the exposure to Bitcoin’s 2021 Taproot upgrade.

Bitcoin Security Model Faces Lower Quantum Threshold

Google researchers said attackers could break Bitcoin and Ethereum cryptography with fewer than 500,000 physical qubits. They challenged past claims that such attacks would require millions of qubits. The team stated that reduced hardware demands could narrow the gap between research systems and practical attacks.

The whitepaper described two attack methods that require about 1,200 to 1,450 high-quality qubits. Researchers said they could precompute part of the algorithm before targeting a live transaction. They added that a quantum system could complete the final step in about nine minutes.

Bitcoin transactions usually take around 10 minutes to confirm on the network. Therefore, the researchers estimated a 41% chance of redirecting funds before confirmation. They said Ethereum may face lower exposure because it confirms transactions faster.

The team explained that an attacker could monitor the network for new Bitcoin transfers. When a user sends funds, the transaction briefly reveals a public key. A fast quantum computer could derive the private key and redirect the funds.

Google stated that it used zero-knowledge proofs to confirm its results. The team avoided publishing step-by-step attack details in public. Researchers said this approach allows verification without exposing sensitive methods.

Taproot Upgrade and Exposed Wallets Increase Bitcoin Exposure

Google’s report also examined how Taproot changed Bitcoin’s address structure. Taproot improved efficiency and privacy when it launched in 2021. However, it made public keys visible on the blockchain by default.

Older Bitcoin address formats concealed public keys until users spent funds. Taproot removed that layer by exposing keys during standard transactions. Researchers said this design choice may widen the pool of vulnerable wallets.

The paper estimated that about 6.9 million Bitcoin already sit in wallets with exposed public keys. That figure equals roughly one-third of the total supply. It includes about 1.7 million Bitcoin mined in the network’s early years.

The report also counted funds affected by address reuse practices. In contrast, CoinShares recently estimated that only about 10,200 Bitcoin sit in highly concentrated wallets. Google’s figures, therefore, exceed earlier public estimates.

Google has previously cited 2029 as a possible milestone for useful quantum systems. The new research indicates that required computing power may fall below earlier assumptions. The company published the whitepaper and blog post on Monday.

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