SHRINCS: 324-byte stateful post-quantum signatures with static backups

A Delving Bitcoin forum post introducing SHRINCS, a hash-based signature scheme designed to keep signatures small enough to be practical on bitcoin. It pairs a stateful tree of one-time signatures (which a wallet must track carefully) with a stateless fallback that still works if the state is lost or corrupted — trading some size for safety against the classic failure mode of stateful schemes.

The headline is size: in normal operation SHRINCS signatures are on the order of a few hundred bytes, far smaller than the standardized post-quantum signatures, at the cost of requiring reliable state management. The same construction is treated more formally in the academic paper Hash-based Signature Schemes for Bitcoin. It is a research proposal under discussion, not a deployed standard.