Bitcoin Faces Quantum-Security Choice: Bigger Blocks or STARK Proofs

Bitcoin relies on elliptic-curve signatures to authorize spending, leaving coins potentially exposed if a sufficiently powerful quantum computer uses Shor’s algorithm to derive private keys. The risk is greatest for addresses whose public keys are already visible and during the confirmation window after a holder broadcasts a transaction. Coinbase’s quantum advisory council estimated in June 2026 that roughly 7 million BTC could be vulnerable because of exposed public keys and address reuse, sharpening calls for a network-wide migration.

On Aug. 26, StarkWare said its Quantum-Safe Bitcoin (QSB) transaction was confirmed in mainnet block 964,199, spending a 10,000-satoshi output. The test took hours of computation and cost about $150 to $200; because the format is nonstandard, it was submitted directly to MARA Pool through Slipstream. QSB, proposed by StarkWare researcher Avihu Levy in April, combines hash-based one-time signatures with “signature grinding.” StarkWare said the method protects individual transactions but not already-exposed public keys, and still favors a protocol-level soft fork for comprehensive protection.

A sufficiently powerful quantum computer could eventually break the elliptic-curve signatures protecting Bitcoin, putting coins at risk once their public keys are exposed. That prospect has pushed developers to seek an upgrade well before such machines become practical. Blockstream’s SHRINCS design uses post-quantum signatures while seeking to prevent their much larger data footprint from displacing ordinary transactions, a central obstacle facing earlier proposals.

Blockstream published the SHRINCS Bitcoin Improvement Proposal in August 2026 after testing the mechanism on its Liquid sidechain. The proposal separates bulky post-quantum signature data in a way intended to preserve Bitcoin’s regular transaction capacity. The trade-off is higher data and verification overhead, along with added implementation complexity. SHRINCS must still undergo technical review and win broad developer and community support before any activation path can be considered.

Quantum computers could eventually use Shor’s algorithm to break the elliptic-curve cryptography used by most blockchains, threatening wallet private keys and transaction verification. The proof layer of Ethereum Layer 2 network Starknet uses hash-based STARKs, giving it a foundation for quantum resistance. Legacy contracts and components connecting Starknet to Ethereum still require upgrades, however, making the work critical to the long-term security of assets.

On June 30, 2026, StarkWare unveiled a three-phase roadmap. Phase one will replace Pedersen with BLAKE2, with the operating system configuration hash expected to reach mainnet in early July and the remaining work taking about two months. Phase two will take roughly another month to provide migration tools for legacy contracts. Phase three will address bridging and data availability, with its timetable depending on Ethereum.

Bitcoin transactions rely on elliptic-curve digital signatures. A sufficiently powerful quantum computer running Shor’s algorithm could derive private keys from exposed public keys and steal assets. Replacing the signature mechanism would require network-wide consensus and asset migration, drawing attention to StarkWare’s upgrade-free approach as a fallback until longer-term changes such as BIP-360 are completed.

On April 9, 2026, StarkWare Chief Product Officer and BIP-360 co-author Avihu Levy published the QSB paper and open-source code. The system uses Bitcoin’s existing Script constraints and hash-based proofs to create quantum-resistant transactions without a soft fork or miner activation. Each transaction requires substantial offline GPU computation at an estimated cost of $75–$200, and the tool is currently intended for emergency recovery.