On August 26, 2026, the Bitcoin network confirmed its first quantum-safe transaction, marking a milestone in cryptocurrency security. The transaction, developed by StarkWare researcher Avihu Levy, demonstrates that Bitcoin holdings can be protected against future quantum computer attacks without requiring a protocol-level soft fork.

What happened

StarkWare announced that a quantum-safe Bitcoin (QSB) transaction was successfully mined in block 964,199 on the Bitcoin mainnet. The transaction spent a 10,000-satoshi output using Levy's QSB scheme, with mining pool MARA confirming the block through its Slipstream service after receiving the transaction directly. This achievement moves Levy's April 2026 research paper from theory to on-chain reality, showing that Bitcoin's existing consensus rules can accommodate quantum-resistant spending without any network upgrade.

How QSB works

Bitcoin's current security relies on elliptic curve cryptography (ECDSA), which is vulnerable to Shor's algorithm running on sufficiently powerful quantum computers. When a Bitcoin transaction sits in the mempool awaiting confirmation, it reveals the public key, creating a window where a quantum adversary could derive the private key and steal the funds.

QSB closes this vulnerability window through a technique called signature grinding:

1. The sender performs intensive computational work off-chain, searching for a transaction whose hash happens to be a validly formatted Bitcoin signature.

2. This creates a second quantum-resistant lock using hash functions instead of elliptic curves.

3. Shor's algorithm cannot break hash functions, making the transaction secure even against quantum computers.

4. The process currently costs between $150–$200 and takes several hours of GPU computation.

Key limitations

While groundbreaking, the QSB transaction has important constraints:

1. Non-standard format: QSB transactions don't follow Bitcoin Core's default relay policies, so ordinary nodes won't propagate them through the mempool.

2. Direct miner submission required: The transaction must be sent directly to miners willing to accept it, as demonstrated by MARA's Slipstream service.

3. Doesn't protect exposed keys: QSB cannot help addresses whose public keys were already published before the transaction.

4. High cost: At $150–$200 per transaction, it's impractical for regular use.

5. Not a network upgrade: This is a specific construction that worked on mainnet, not a protocol-level quantum resistance solution.

Industry context and timing

The demonstration comes amid growing urgency around quantum threats to cryptocurrency. In March 2026, Google researchers estimated that a sufficiently capable quantum computer could derive a Bitcoin private key within 9 - 12 minutes after the public key becomes visible. Separately, Blockstream published a Bitcoin Improvement Proposal (BIP) for SHRINCS, a post-quantum signature scheme that reduces signature size by 13x but adds complexity. Blockstream has already tested SHRINCS on its Liquid sidechain, with CEO Adam Back emphasizing the importance of preparing for quantum threats well in advance.

What experts are saying

StarkWare CEO Eli Ben-Sasson framed the achievement as proof that "Bitcoin has no expiration date," while still advocating for a soft fork as the better long-term solution. He described Levy's work as providing "psychological reassurance" and lifeboats while the community builds permanent protections.

"A soft fork should happen, and I believe it will," Ben-Sasson said, noting that QSB serves as a safety net until protocol-level upgrades are implemented.

Levy himself developed QSB as a passion project after hours, building on years of work in the Bitcoin ecosystem. StarkWare engineer Tomer Giladi helped bring the implementation to completion.

What's next

StarkWare maintains that a soft fork remains the optimal path to comprehensive quantum resistance for Bitcoin. Developers are separately considering proposals including BIP-360, which would introduce a Pay-to-Merkle-Root output type while removing Taproot's quantum-vulnerable key-path spend. For now, QSB provides Bitcoin holders with an emergency option to move coins into quantum-resistant storage before a network upgrade arrives. The open-source implementation and research paper are available on GitHub for the community to review and build upon.