Quantum security applied to Bitcoin: first test of post-quantum signatures on Blockstream’s sidechain.
Blockstream Research has successfully implemented the verification of cryptographic signatures resistant to quantum computers on Liquid Network.
Currently, funds on the Liquid network are protected by classical digital signatures based on ECDSA and Schnorr—algorithms that a sufficiently powerful quantum computer could potentially compromise.
To implement post-quantum signatures, Blockstream used Simplicity, the smart contract language developed in-house for Liquid. Thanks to Simplicity’s ability to express custom spending conditions, the team was able to build and deploy a complete verifier for post-quantum signatures without requiring changes to the network’s consensus rules.
The verifier implements a variant of SHRINCS, a compact post-quantum signature scheme based on hash functions, developed specifically by Blockstream Research.
SHRINCS offers two complementary operating modes:
- stateful mode for normal use, producing compact and efficient signatures;
- stateless fallback mode for recovery scenarios, ensuring that users never lose access to their funds even if the state is lost.
The scheme has been further optimized for the execution model of Simplicity, making it practical for on-chain verification in a real production environment.
Real transactions on mainnet
Blockstream has already broadcast actual transactions signed with post-quantum cryptography on the Liquid mainnet, demonstrating that the technology is fully operational. The transactions include examples in both stateful mode and stateless fallback mode.
Although the code still requires thorough audits and finalization of the specifications, Liquid users who wish to experiment with post-quantum protection can already begin using this verifier. The Simplicity script resides in the transaction output and carries no cost until it is spent.
This opt-in approach means that those who want quantum protection can move their funds into post-quantum contracts, while those who do not yet need it can continue using classical signatures.
Current limitations
It is important to note that this verifier does not make Liquid fully resistant to quantum computers. Several critical components are still protected with classical cryptography, including:
- the Bitcoin peg mechanism;
- Confidential Assets commitments;
- Liquid’s blocksigning consensus protocol.
Blockstream is working on quantum-resistant solutions for these components as well.
