Blockstream Solves Quantum Threat for Bitcoin

Blockstream Research has implemented post-quantum signature verification on the Liquid Network using its Simplicity smart contract language, according to an announcement from the company.

The deployment includes transactions signed with a post-quantum signature scheme that are broadcast on Liquid mainnet. 

Blockstream states that the transactions represent the first use of post-quantum signatures on a production Bitcoin sidechain. 

The transactions are used to secure assets on the network, including Bitcoin and other digital assets issued on Liquid.

Preparing for Potential Quantum Risks

Assets on Liquid are currently secured using classical cryptographic signatures such as ECDSA and Schnorr. 

These signature schemes are widely used across Bitcoin infrastructure but could theoretically be vulnerable to sufficiently advanced quantum computers.

Quantum computers capable of breaking Bitcoin’s current cryptography do not exist today. 

However, researchers and developers have discussed the need to explore potential long-term solutions before such technology becomes practical.

In many blockchain systems, introducing a new signature scheme would require changes to the network’s consensus rules, a process that typically involves coordination across developers, node operators, and other stakeholders.

Implementation Using Simplicity

The verification system was implemented using Simplicity, a smart contract language used on the Liquid Network and designed for Bitcoin-like blockchains.

Because Simplicity allows developers to define custom spending conditions, Blockstream created a verifier that lets users lock assets into contracts requiring post-quantum signatures in order to spend them. 

According to the company, this allows the feature to be deployed without modifying Liquid’s existing consensus rules.

The feature is optional, allowing users to move assets into contracts requiring post-quantum signatures while others continue using the network’s existing cryptographic methods.

Signature Scheme and Demonstration Transactions

The implementation uses a hash-based post-quantum signature scheme developed by Blockstream Research. 

The verifier supports two operating modes: a standard mode designed for routine use and a fallback mode intended for recovery scenarios if certain data is lost.

Blockstream reported broadcasting two transactions on Liquid mainnet to demonstrate the system. One used the standard signature mode, while the other demonstrated the fallback option.

• Stateful signature transaction, demonstrating normal operation.
• Stateless fallback transaction, showing recovery mode.

Liquid requires transaction sizes to correspond to the computational resources consumed. 

According to Blockstream, unused space in the demonstration transactions was filled with text from the Bitcoin Whitepaper.

Availability for Developers

The verifier library and signing tools have been released as open-source software. 

While the system is available for developers to experiment with, Blockstream said the implementation still requires additional auditing and specification work before broader deployment.

There is currently no wallet integration, but the company said the open-source library could allow wallet developers to add support in the future.

Remaining Limitations

Blockstream noted that the verifier does not make the Liquid Network fully resistant to potential quantum attacks. 

Several parts of the system remain secured using classical cryptography, including the Bitcoin peg mechanism, Confidential Assets commitments, and Liquid’s block-signing consensus protocol.

The company said it is continuing research into quantum-resistant alternatives for these components.

Broader Implications

Although the deployment took place on Liquid, Blockstream said the work could inform future research related to the Bitcoin network.

While quantum computers capable of breaking Bitcoin’s cryptography have not yet been demonstrated, some researchers and developers view testing potential solutions on production systems as a way to better understand how quantum-resistant cryptography could eventually be implemented if needed.