Today marks a significant milestone in preparing Bitcoin infrastructure for a post-quantum future. Blockstream Research has successfully deployed post-quantum signature verification on the Liquid Network using Simplicity, enabling users to protect their funds against future quantum computer attacks.
Blockstream has broadcast what are, to the best of our knowledge, the first transactions on a production Bitcoin sidechain signed with a post-quantum signature scheme. Real transactions securing real value on Liquid mainnet. This works not only with bitcoin, but also with any asset issued on Liquid.
The Challenge: Post-Quantum Signature Readiness
Today, funds on Liquid are protected by classical ECDSA/Schnorr signatures that a cryptographically relevant quantum computer could break. While such computers don't exist yet, preparing Bitcoin-like systems for this eventual threat is critical infrastructure work that needs to happen now, not in a crisis.
The traditional approach to adding post-quantum signatures would require consensus changes across the network—a slow, careful process involving all stakeholders. But Simplicity, Blockstream's smart contract language on Liquid, offers a different path.
The Solution: Simplicity-Powered Post-Quantum Verification
Because Simplicity allows users to express custom spending conditions, Blockstream was able to build and deploy a complete post-quantum signature verifier without any changes to Liquid's consensus rules. Users who want quantum protection can opt into the verifier by locking their Liquid assets, including LBTC, stablecoins and tokenized securities to a Simplicity contract that requires post-quantum signatures to spend.
This project demonstrates the expressiveness and power of Simplicity as a programming language. A complete cryptographic signature verifier is a non-trivial program. The fact that Simplicity can express this efficiently enough to run in production shows the language's capabilities for advanced blockchain applications.
SHRINCS: Optimized Hash-Based Signatures
The verifier implements a variant of SHRINCS, a compact hash-based post-quantum signature scheme developed by Blockstream Research specifically for blockchain use cases. This builds on the team's ongoing work of optimizing post-quantum cryptography for Bitcoin's unique constraints.
SHRINCS offers two modes:
- Stateful mode for normal use, producing compact signatures
- Stateless fallback for recovery scenarios, ensuring users never lose access to their funds even if they lose state
The scheme has been further optimized for Simplicity's execution model, making it practical for on-chain verification. For technical details on SHRINCS itself, see the write-up on Delving Bitcoin.
Real Transactions, Real Protection
Blockstream has broadcast actual post-quantum-signed transactions on Liquid mainnet:
- Stateful signature transaction, demonstrating normal operation
- Stateless fallback transaction, showing recovery mode
An interesting technical note: Liquid requires transaction sizes to be proportional to the computational budget consumed. Rather than padding these transactions with zeros, Blockstream filled the extra space with the Bitcoin whitepaper—a nod to the cypherpunk roots of this work.
What This Means for Users
While the code still needs thorough auditing and specification finalization, Liquid users who want to try post-quantum protection for their funds can begin using this verifier. The Simplicity script sits in the transaction output and costs nothing until it's spent. There's no wallet integration yet, but the verifier library is available on GitHub for wallet developers to build on.
This is opt-in protection. Users who want it can move their funds to post-quantum-secured contracts. Users who don't need it yet can continue using classical signatures. The beauty of implementing this in Simplicity is that no one needs permission or consensus changes. It's available now for anyone who wants it.
Important Limitations
This verifier does not make Liquid fully quantum-resistant. Several critical components remain classically secured:
- The Bitcoin peg mechanism
- Confidential Assets commitments
- Liquid's blocksigning consensus protocol
Blockstream is actively working on quantum-resistant solutions for these components as well. This verifier is a first building block, not a complete solution. But it's an important building block that users can start benefiting from immediately.
Why This Matters for Bitcoin
While this implementation is on Liquid, the implications extend to Bitcoin itself. Simplicity is designed for Bitcoin-like blockchains, and this work demonstrates that complex post-quantum cryptography can be efficiently verified in environments with Bitcoin's constraints.
Quantum computers capable of breaking Bitcoin's cryptography don't exist today and may not for years or decades. But when they do arrive, the transition needs to be smooth and well-tested. What we’ve done on Liquid—building, testing, and deploying post-quantum solutions on production systems—is how we prepare Bitcoin infrastructure for the future.
Building Blocks for a Quantum-Resistant Future
This release represents the intersection of three major Blockstream Research initiatives:
- Post-quantum cryptography research, developing signature schemes optimized for blockchain constraints
- Simplicity language development, creating expressive smart contract capabilities for Bitcoin-like systems
- Liquid Network infrastructure, providing a production environment to test and deploy advanced Bitcoin technology
Each piece enables the others. Simplicity's expressiveness makes deployment possible without consensus changes. Liquid's production environment proves the approach works with real value and creates a venue for building community consensus around quantum-proof technologies that Bitcoin could eventually adopt itself. This cryptographic research ensures the solutions are optimized for blockchain reality, not just theoretical security.
Get Involved
The SHRINCS verifier library and corresponding signing code are open source and available now. Wallet developers interested in implementing post-quantum protection can start integrating it today.
For developers interested in Simplicity itself, join the weekly Office Hours every Tuesday at 8AM PST to discuss smart contract development, ask technical questions, and see what's being built.
For researchers interested in our post-quantum cryptography work, see our Delving Bitcoin post for full technical details on SHRINCS.
What's Next
This is just the beginning. Blockstream Research continues work on:
- Quantum-resistant peg mechanisms
- Post-quantum Confidential Assets
- Quantum-resistant consensus protocols
Follow us on X to stay in the loop. We will share updates as this work progresses. In the meantime, Liquid users who want to protect their funds against future quantum threats can start using post-quantum signatures today.
The future is quantum-resistant. Blockstream is building it one transaction at a time.
For technical questions about this implementation, join the discussion on our GitHub repository or visit Delving Bitcoin.
To learn more about Simplicity, visit simplicity-lang.org.