StarkWare has announced that Bitcoin has reached a major milestone in its race against quantum computing. The company confirmed that a quantum-safe transaction was mined on the Bitcoin mainnet on August 26, marking the first live demonstration of its Quantum Safe Bitcoin (QSB) construction.

The transaction shows that Bitcoin can support a quantum-resistant spending method without any changes to its consensus rules. The development, however, does not make the Bitcoin network fully quantum-safe.

Quantum Risk Meets a Live Bitcoin Transaction

Bitcoin currently relies on elliptic-curve cryptography for transaction signatures. A sufficiently powerful quantum computer running Shor's algorithm could eventually derive private keys from exposed public keys, potentially putting some Bitcoin holdings at risk.

The exposure is uneven. Public keys are already visible on-chain for coins held in the pay-to-public-key (P2PK) format used in Bitcoin's earliest years, and any standard address reveals its public key the first time it spends. Outputs whose public keys have not yet been published are harder targets, because an attacker would also need to break the address's hash. No quantum computer today is believed capable of running Shor's algorithm against a 256-bit elliptic-curve key; researchers estimate that this would require fault-tolerant machines on a scale far beyond current hardware.

StarkWare researcher Avihu Levy developed Quantum Safe Bitcoin to address this vulnerability using Bitcoin's existing scripting capabilities. The method adds a hash-based security layer that does not depend on elliptic-curve assumptions.

QSB uses a technique called signature grinding. Instead of producing a conventional private-key signature, the sender performs substantial computation to find a transaction hash that satisfies the required conditions. The resulting security depends on hash preimage resistance rather than on the secrecy of an elliptic-curve private key.

A Costly Lifeboat, Not a Permanent Fix

The breakthrough comes with significant limitations. Producing a QSB transaction requires expensive off-chain computation, with the original research estimating costs of several hundred dollars under its assumptions.

The transaction also uses a nonstandard format. Ordinary Bitcoin nodes do not relay it through the normal mempool process, so StarkWare relied on MARA's Slipstream service to deliver the transaction directly to a miner. MARA, one of the largest publicly traded Bitcoin mining companies, launched Slipstream in March 2024 as a direct-submission channel for large or nonstandard transactions that the standard relay network will not carry.

StarkWare continues to favor a Bitcoin protocol upgrade as the long-term solution. The company has also participated in BIP 360, a proposal aimed at enabling quantum-resistant Bitcoin script functionality through a soft fork. Such a fork would need broad support from node operators and miners, a consensus process that in Bitcoin's history has tended to unfold over extended public debate rather than quick decisions.

The mainnet demonstration therefore represents a practical emergency option rather than a complete network upgrade. Still, it moves quantum-resistant Bitcoin protection from research into real-world execution and gives holders another potential path before quantum computers become capable of threatening existing cryptographic protections. The open questions ahead are concrete: how far grinding costs can fall as the technique is refined, and how BIP 360 advances through Bitcoin's open review process.