SHRINCS BIP published: Quantum-secure Bitcoin comes with a catch

Blockstream has released a Bitcoin Improvement Proposal (BIP) introducing the SHRINCS hash‑based signature scheme as a potential post‑quantum upgrade for Bitcoin. The proposal follows successful testing of SHRINCS on Blockstream’s Liquid sidechain earlier this year.

Why a quantum‑secure signature matters

Researchers agree that sufficiently advanced quantum computers could derive private keys from public keys, threatening Bitcoin’s core security. Upgrading the signature algorithm is therefore a priority for the ecosystem.

SHRINCS compared to other post‑quantum options

Current NIST‑approved post‑quantum signatures are 38–123 times larger than Bitcoin’s existing Schnorr signatures, which would drastically reduce transaction throughput. SHRINCS, at a minimum of 548 bytes (plus a 48‑byte public key) and up to 4,619 bytes, is considerably smaller—about nine times the size of Schnorr signatures—but still larger than today’s signatures.

Because signatures reside in the SegWit witness, their impact on block space is reduced. Estimates suggest Bitcoin could process roughly 3 transactions per second (TPS) with SHRINCS, comparable to current performance, whereas other schemes would drop TPS to below 1.

Trade‑offs and challenges

SHRINCS is a stateful scheme: each signature uses a one‑time key stored on the device. Reusing a key enlarges signatures by 16 bytes per use, and loss of the device requires a large “stateless fallback” transaction (~5,777 bytes). The BIP notes that a formal security proof is still pending, and incompatibilities between implementations could lead to lost funds.

Blockstream has also introduced a companion scheme, formerly called SHRIMPS, now integrated into the same BIP to provide a stateless backup path, reducing size by about 26 %.

Future directions

Blockstream is exploring additional optimizations, including lattice‑based signatures and zero‑knowledge proof aggregation, which could double throughput to around 6.7 TPS if combined with SHRINCS. However, any consensus change will require broad community support, and governance remains the primary hurdle.