Post-Quantum Signatures on Liquid
Blockstream deploys quantum-resistant spending via Simplicity — no consensus changes
WHAT HAPPENED
Blockstream Research demonstrated quantum-resistant transaction signing on the Liquid sidechain using Simplicity smart contracts. Users can protect their L-BTC, stablecoins, and tokenized securities with post-quantum signatures — without any changes to Liquid's consensus rules.
THE SIGNATURE SCHEME: SHRINCS
SHRINCS is a compact hash-based post-quantum signature scheme developed by Blockstream Research specifically for blockchain use cases. It operates in two modes:
Stateful mode — typical usage with compact 324-byte signatures. Requires tracking which one-time keys have been used.
Stateless fallback — recovery scenario mode. Larger signatures but no state tracking required.
Stateful mode — typical usage with compact 324-byte signatures. Requires tracking which one-time keys have been used.
Stateless fallback — recovery scenario mode. Larger signatures but no state tracking required.
WHY THIS MATTERS
This is the first live deployment of post-quantum signatures on a Bitcoin sidechain. While Bitcoin mainnet proposals like BIP-360 (which defines the P2MR output type, but not PQ signature algorithms) require a soft fork, Liquid's Simplicity scripting language allowed Blockstream to deploy actual PQ signature verification immediately — proving the cryptographic approach works in production.
How Simplicity Enables This
SIMPLICITY: PROGRAMMABLE SPENDING CONDITIONS
Simplicity is Blockstream's smart contract language for Liquid (and eventually Elements-based sidechains). Unlike Bitcoin Script, Simplicity allows users to express arbitrary spending conditions — including custom cryptographic signature verification. This means you can implement an entirely new signature scheme without touching consensus code.
1
User locks assets to a Simplicity contract
L-BTC, stablecoins, or tokenized securities are sent to an output with a Simplicity program that requires a SHRINCS post-quantum signature to spend.
2
Simplicity verifier checks the PQ signature
The Simplicity program contains a complete cryptographic signature verifier for SHRINCS. When spending, the user provides a post-quantum signature that the program validates.
3
No consensus changes needed
Liquid nodes don't need to know about SHRINCS. They execute the Simplicity program, which internally verifies the PQ signature. The spending condition is entirely user-defined.
THE WHITEPAPER EASTER EGG
Liquid requires transaction sizes to be proportional to the computational budget consumed. Rather than padding extra space with zeros, Blockstream filled it with the Bitcoin whitepaper — a nod to the community and a demonstration of the format's flexibility.
What This Means for Bitcoin
LIQUID AS TESTBED
Liquid serves as a proving ground for solutions that Bitcoin could eventually adopt. PQ signatures working on Liquid validates the approach before Bitcoin mainnet deployment.
SHRINCS vs NIST PQC
SHRINCS is purpose-built for blockchains with 324-byte stateful signatures. Compare to NIST's SPHINCS+ (~7-49KB) or ML-DSA/Dilithium (~2.5KB). Compact signatures matter on-chain.
CONNECTION TO BIP-360
BIP-360 (Pay to Merkle Root) proposes a new output type for Bitcoin mainnet that removes the key-path spend, protecting against long-exposure quantum attacks. It does not itself introduce PQ signature algorithms -- those would come via future companion BIPs. The Liquid SHRINCS deployment validates the PQ cryptographic primitives in production, providing real-world experience that can inform future PQ opcode design for Bitcoin. See the Quantum Resistance topic for BIP-360 details.
THE BIGGER PICTURE
This demonstrates the power of programmable spending conditions. With sufficiently expressive scripting (Simplicity on Liquid, or theoretically OP_CAT on Bitcoin), new cryptographic schemes can be deployed without protocol changes. This is the same pattern as:
• Binohash — transaction introspection without new opcodes
• PIPEs v2 — covenants without consensus changes
• BitVM — arbitrary computation without protocol modifications
• Binohash — transaction introspection without new opcodes
• PIPEs v2 — covenants without consensus changes
• BitVM — arbitrary computation without protocol modifications