The first quantum-resistant Bitcoin transaction has been confirmed on mainnet, moving the debate over Bitcoin's long-term cryptographic defenses from theory to on-chain reality.
The transaction, confirmed Wednesday in block 964,199, spent a 10,000-satoshi output protected by the Quantum Safe Bitcoin (QSB) scheme developed by StarkWare researcher Avihu Levy. MARA Pool mined the block after receiving the transaction through its Slipstream service, which accepts transactions directly from large submitters rather than the public relay network.
How the Scheme Works
QSB combines hash-based one-time signatures with computational searches that bind an authorization to one specific transaction. The construction is designed to prevent forgery even if a sufficiently powerful quantum computer breaks the elliptic-curve cryptography Bitcoin relies on today.
The demonstration shows that Bitcoin's existing consensus rules can accommodate at least one form of quantum-resistant spending without any protocol change. That matters because the threat is no longer hypothetical: in March, Google researchers estimated that a capable quantum computer could theoretically derive a Bitcoin private key within nine to 12 minutes of its public key becoming visible — fast enough to replace a pending transaction during the confirmation window.
The Catch: Cost and Relay Policy
QSB remains expensive and impractical for everyday use. Generating the transaction required hours of computation and cost roughly $150 to $200, in line with Levy's April estimate of $75 to $150 in GPU computation when he first proposed the scheme. The transaction is also classified as nonstandard under Bitcoin Core's default relay policies, meaning ordinary nodes would not propagate it — hence the direct submission through MARA's Slipstream.
StarkWare positions QSB as a last-resort safety net rather than a substitute for protocol-level protection. "A soft fork should happen, and I believe it will," CEO Eli Ben-Sasson said, referring to the longer-term need to upgrade Bitcoin's cryptography at the base layer.
Bitcoin developers are separately evaluating proposals including BIP-360, a draft soft fork that would introduce a Pay-to-Merkle-Root output type while removing Taproot's quantum-vulnerable key-path spend. The BIP-360 repository and Levy's full paper and code are public.
For now, the confirmed transaction stands as proof that quantum-safe spending is possible on Bitcoin today — for anyone willing to pay hundreds of dollars and bypass the standard relay network to move a fraction of a cent.