Blockstream releases SHRINCS quantum-safe signature scheme BIP

Blockstream has released a Bitcoin Improvement Proposal (BIP) proposing the SHRINCS post-quantum signature scheme. SHRINCS is a hash-based post-quantum signature scheme with a minimum size of 548 bytes, approximately 9 times larger than the existing Schnorr signatures, but much smaller than the NIST-approved ML-DSA and SPHINCS+ schemes. The scheme has completed production environment testing on the Liquid sidechain and includes a copy of the Bitcoin white paper.

The scheme also has limitations. The BIP warns that security proofs are still pending, and the scheme is stateful, requiring the storage of used keys for each signature, which increases the signature size by 16 bytes with each use. In the event of device loss, approximately 5777 bytes are needed for a stateless fallback transaction recovery. Blockstream estimates that Bitcoin can maintain a speed of about 3 TPS after adopting SHRINCS, which can be increased to 6.7 TPS when combined with ZK proof aggregation.