Blockstream Proposes SHRINCS Post-Quantum Signature Scheme for Bitcoin

Martian Finance reports that Blockstream has released a Bitcoin Improvement Proposal (BIP) introducing the SHRINCS post-quantum signature scheme. SHRINCS is a hash-based post-quantum signature scheme with a minimum size of 548 bytes—approximately nine times larger than current Schnorr signatures, but significantly smaller than NIST-approved ML-DSA and SPHINCS+ schemes. The scheme has undergone production testing on the Liquid sidechain and includes a copy of the Bitcoin whitepaper. However, the scheme has limitations: the BIP warns that security proofs are still pending, and the scheme is stateful, requiring storage of used keys; each signature increases in size by 16 bytes with usage, and recovering from device loss requires a stateless fallback transaction of approximately 5,777 bytes. Blockstream estimates that adopting SHRINCS would allow Bitcoin to maintain a throughput of about 3 TPS, potentially increasing to 6.7 TPS when combined with ZK-proof aggregation.