Experimental quantum-safe Bitcoin spend reaches mainnet without a fork

An experimental Bitcoin transaction designed to remain spendable securely against a future quantum adversary was confirmed on mainnet on Aug. 26, according to StarkWare, the project’s public code repository and Bitcoin block data.

The transaction used Quantum-Safe Bitcoin, or QSB, a construction developed by StarkWare researcher Avihu Levy. It operated under Bitcoin’s existing consensus rules rather than requiring a soft fork or hard fork. That makes the demonstration technically significant now, but much narrower than making Bitcoin itself quantum-resistant.

The “quantum-safe” description is the researchers’ security characterization. The blockchain proves that Bitcoin accepted the transaction; it does not independently prove the construction’s claimed resistance to every quantum attack.

What the blockchain confirms

The identified spending transaction, whose ID begins `305a24ff`, was included in block 964,199. Mempool.space records the block at 20:48:34 UTC on Aug. 26 and attributes it to MARA Pool. Reporting by The Block and Unchained says the transaction spent a 10,000-satoshi output protected by QSB after being submitted through MARA’s Slipstream service.

That miner-direct route was necessary because the construction creates a nonstandard transaction that ordinary Bitcoin nodes would not normally relay. “Nonstandard” is a network policy classification, not the same as invalid under consensus. A miner can include such a transaction when every consensus rule is satisfied.

The distinction explains how the experiment reached mainnet without changing Bitcoin. It also limits immediate usability: the method is not a normal wallet transaction that can simply be broadcast across the peer-to-peer network.

How QSB changes the spending path

Conventional Bitcoin authorization depends on elliptic-curve signatures. A sufficiently capable quantum computer running Shor’s algorithm could theoretically derive a private key from a revealed public key, undermining that signature security.

QSB adds a hash-based authorization structure using existing Bitcoin Script behavior. Its public repository says the construction combines one-time hash commitments with a computational search that binds authorization to a particular transaction. The claimed security rests on hash preimage resistance rather than the elliptic-curve problem vulnerable to Shor’s algorithm.

Producing the spend is deliberately expensive in computation. The repository estimates $75 to $150 of cloud GPU expense for its configured search, while StarkWare’s announcement and subsequent reports describe hours of computation. Those are developer estimates, not an audited cost benchmark, and they will vary with hardware and cloud pricing.

Why this is not a network-wide fix

QSB protects only coins deliberately placed behind its special locking construction. It does not retroactively protect addresses whose public keys are already exposed, automatically migrate existing wallets or change Bitcoin’s standard signature system.

The construction also cannot establish that a cryptographically relevant quantum computer exists today. No such machine was used in the demonstration. Instead, the transaction shows that one proposed defensive path can satisfy current consensus rules under the researchers’ stated security assumptions.

Levy and StarkWare continue to favor a protocol-level upgrade as the broader response. QSB is presented as a possible emergency or migration tool while that debate develops, not a substitute for network-wide post-quantum standards.

What remains uncertain

The main unresolved question is independent cryptographic scrutiny. The project provides a paper, source code and transaction artifacts, but the public repository still describes parts of the implementation status inconsistently, including language saying an on-chain broadcast had not yet been attempted. That documentation may be stale after the Aug. 26 test and should be reconciled.

Event time and reporting time are separate: the block timestamp was Aug. 26 at 20:48:34 UTC; StarkWare dated its announcement Aug. 26; The Block published at 10:24 p.m. EDT that day; and Coinburn’s edition date is Aug. 27 in America/New_York. No cryptocurrency price or percentage-performance claim is made. The next meaningful checks are independent review of the construction, complete publication of reproducible transaction artifacts and any formal Bitcoin improvement proposal for broader quantum migration.