Bitcoin's Leading Post-Quantum Signature Proposal | CONDUITION

— Conduition co-authored a full specification for SHRINCS, a post-quantum signature scheme combining stateless (Sphinx-based) and stateful (hash-based) components for Bitcoin.

— Hash-based signatures offer conservative cryptographic assumptions compared to elliptic-curve cryptography, providing resilience against multiple threat vectors beyond quantum computers.

— SHRINCS uses WOTS+C (Winternitz one-time signatures with compression), FORS (Forest of Random Subsets), and XMSS (Extended Merkle Signature Scheme) as its core building blocks.

— The scheme introduces flexible tree structures (unbalanced and balanced XMSS) for different use cases, enabling address reuse for custodians and proof-of-reserve scenarios without sacrificing security.

— Stateful signatures are ~538 bytes versus stateless signatures at ~5,776 bytes, both roughly 10x larger than current Schnorr signatures, with faster verification offsetting size concerns.

— State management remains the critical risk: wallet developers must carefully track signature counters to avoid accidental key exposure through signature reuse.

— Bitcoin developers should prioritize implementing the stateless component of SHRINCS first, which requires only standard NIST libraries with tweaked parameters, before tackling complex stateful key management.

— Address reuse remains viable in a post-quantum Bitcoin via balanced XMSS (BXMSS), preserving functionality for custodians, proof-of-reserves, and lightning protocols without losing security guarantees.

— Witness discounts may become necessary to justify signature size, but this remains an open question requiring community discussion around verification costs versus blockchain storage constraints.