Blockstream has published a Bitcoin Improvement Proposal that introduces SHRINCS, a hash-based post-quantum signature scheme. The document notes that SHRINCS has been tested in production on the Liquid sidechain and includes a copy of the Bitcoin white paper.

According to the BIP, the minimum signature size is 548 bytes, roughly nine times larger than a Schnorr signature, though smaller than the NIST-approved ML-DSA and SPHINCS+ schemes. The proposal also states that the security proof remains unfinished and that SHRINCS is stateful, requiring storage of used keys for each signature.

Blockstream notes that the signature grows by 16 bytes with each use, and that a stateless fallback transaction of about 5,777 bytes would be needed to recover funds if a device is lost.

The company estimates Bitcoin could sustain roughly 3 TPS with SHRINCS, or 6.7 TPS if combined with ZK proof aggregation.