Bitcoin's Quantum Defense Takes a Major Step Forward

Blockstream Files Formal BIP for SHRINCS Quantum-Resistant Signature Scheme

Adam Back's Blockstream has published a Bitcoin Improvement Proposal for SHRINCS, its experimental post-quantum signature scheme, marking one of the most concrete steps the industry has taken toward protecting Bitcoin against future quantum computer attacks.

SHRINCS was designed by Blockstream Research cryptographers Mikhail Kudinov and Jonas Nick and is a hash-based scheme built for Bitcoin's constraints rather than adapted to them. It leans only on the security of SHA-256, the same hash function Bitcoin already uses for proof-of-work, address derivation, and Merkle trees, meaning it adds no new cryptographic assumption to the system.

Blockstream Research's Jonas Nick has called it the first concrete proposal designed specifically for Bitcoin. However, Nick has been clear that SHRINCS is not intended to become Bitcoin's final signature scheme, framing it instead as a pragmatic near-term option as the broader community works toward a longer-term standard.

Tested in Production, But Trade-offs Remain

In March 2026, Blockstream deployed SHRINCS verification on the Liquid sidechain mainnet via Simplicity smart contracts and broadcast what it describes as the first post-quantum-signed transactions on a production Bitcoin sidechain. More recently, it demonstrated that SHRINCS and a range of other post-quantum signature schemes can effectively run on common hardware wallets.

SHRINCS gives a single public key two signing paths: a compact stateful path for everyday use, where signatures start at 324 bytes and grow by roughly 16 bytes with each additional signature, and a stateless fallback that still works if the signer loses its state, at the cost of a larger signature size.

The core challenge is one of scale. Quantum-safe signatures are much larger than current ones, and larger signatures take up more space in each block, meaning fewer transactions fit and fees rise. The compact SHRINCS configuration reaches up to 3 transactions per second with 580-byte signatures, compared with roughly 0.36 transactions per second for the NIST-standardized SLH-DSA.

Blockstream co-founder and CEO Adam Back has a reputation as a quantum computer skeptic, yet his company is among the leaders in researching practical solutions. Back told Cointelegraph earlier this year that "the safe thing" is to prepare for the threat well in advance.

Critics point to the added complexity. Yoon Auh, founder of BOLTS Technologies, notes that in reducing signature size, SHRINCS' designers added "statefulness, compact signing paths, fallbacks, assumptions about how many times a seed is initialized, and rules for when devices must switch to larger stateless signatures." Deployment on Bitcoin's base layer would require its own soft fork, with Blockstream noting the opcode could ship via Taproot, a proposed Taproot v2, or BIP-360's P2MR leaves.

Sources:
CoinTelegraph: SHRINCS BIP Published, Quantum-Secure Bitcoin Comes With A Catch
Blockstream Research: OP_CHECKSHRINCS, A Hash-Based Signature Opcode for Post-Quantum Bitcoin
Blockstream: Bitcoin's Quantum Defense