StarkWare researcher Avihu Levy has carried out what the company describes as the first quantum-safe Bitcoin transaction on mainnet, demonstrating that certain BTC transfers can be shielded from a future quantum attacker without any change to Bitcoin's consensus rules.

The mainnet transaction applied Levy's Quantum-Safe Bitcoin (QSB) construction, moving coins into an output protected by hash-based security rather than relying entirely on elliptic-curve cryptography. StarkWare engineer Tomer Giladi helped translate the April research into a working transaction.

The method does not make Bitcoin itself quantum-resistant. Instead, it gives holders whose public keys have not already been exposed a way to move BTC into a protected output before a sufficiently capable quantum computer could exploit the existing signature system, which makes the work relevant as Bitcoin users and infrastructure operators weigh longer-term upgrade paths without waiting for a network-wide change.

Signature Grinding Adds a Second Lock

Bitcoin signatures rest on elliptic-curve cryptography, which could eventually become vulnerable to Shor's algorithm running on a sufficiently powerful quantum computer.

Most Bitcoin addresses initially hide the public key behind a hash. Spending reveals that key, opening a window between broadcast and confirmation in which a future quantum attacker could theoretically derive the private key and race the legitimate transaction.

Levy's QSB method avoids that exposure through signature grinding. Computing power is spent offchain searching through candidate transactions until the transaction hash itself has the structure required to function as a valid signature. The resulting security depends on reversing a cryptographic hash rather than recovering an elliptic-curve private key. StarkWare currently estimates the computation required for one QSB transaction at several hundred dollars.

The construction also has an important limitation: QSB cannot protect coins whose public key was already exposed before the protection process begins. About 6.04 million BTC already had visible public keys in May, putting roughly 30% of the issued supply inside the longer-term quantum-risk set.

MARA Mines the Nonstandard Transaction

QSB transactions do not fit Bitcoin's standard relay policy, meaning ordinary nodes will not propagate them through the normal mempool. The transaction instead reached a miner directly through MARA Slipstream, which accepts nonstandard Bitcoin transactions that still comply with consensus rules. MARA then included it in a mainnet block.

The demonstration follows the addition of BIP 360 to Bitcoin's official BIPs repository earlier this year. The proposal introduces Pay-to-Merkle-Root outputs designed to remove Taproot's quantum-vulnerable key-path spend, but publication as a BIP does not mean the change has been activated on Bitcoin.

Levy continues to favor a protocol-level upgrade as Bitcoin's long-term answer to quantum computing, while QSB provides a transaction-level mechanism that works under the network's existing consensus rules and could be used before any formal soft fork.

Bitcoin Security Work Accelerates Across the Stack

The mainnet test arrives during an unusually active period for Bitcoin security research. Core Lightning operators were told this week to take affected nodes offline while developers prepare signed binaries for a separate group of undisclosed vulnerabilities. Hardware-wallet developers have also been dealing with security failures and emergency patches, and AI-assisted review recently identified thousands of potential issues across hundreds of Bitcoin projects.

The quantum question also has practical relevance because BTC is becoming more deeply embedded in conventional financial products. Coinbase and Better this week expanded Bitcoin-backed mortgages that let eligible U.S. borrowers pledge BTC toward home down payments without selling their holdings, increasing the number of places where Bitcoin security assumptions matter beyond self-custody alone.

StarkWare's QSB transaction was mined on August 26, 2026, with Bitcoin's consensus rules unchanged and MARA Slipstream providing the direct mining route required by the nonstandard transaction.