Q-day and blockchain: how to prepare for the quantum threat

Quantum technologies are advancing rapidly. As we know, they potentially pose a serious threat to blockchain projects. However, major players in the crypto market have long neglected this issue. Recently, amid news of quantum computer breakthroughs, the situation is changing. Corporations are initiating their own research and development to find ways to secure existing blockchain projects. But are the proposed options as good and reliable as they seem? And are there any alternative paths? Let's take a closer look.

Monolithic Architecture of Legacy Blockchains: Why a Quick Upgrade Is Impossible

The obvious solution for enhancing the quantum resistance of blockchains is the implementation of quantum-safe or post-quantum cryptography. However, this is only obvious in theory. In practice, things are not so simple, especially when it comes to first and second-generation blockchains with their monolithic architecture and single chain of blocks. Transitioning such projects to post-quantum cryptography quickly and seamlessly is virtually impossible, and here is why. These algorithms are significantly larger and heavier than traditional ones. Implementing them would lead to increased transaction sizes, which in turn would result in higher network load, lower transaction throughput, and greater costs for data storage on the blockchain.

Furthermore, post-quantum algorithms require substantial computational resources for signature generation and verification. For large-scale blockchain projects with high throughput demands, which are already grappling with scalability issues and high fees, such an upgrade could prove fatal and potentially lead to network paralysis.

Layer 2 Solutions: A Partial Fix

Acknowledging that an infrastructure-level network upgrade would be overly complex and disruptive, some developers are proposing gentler approaches. These involve transitioning to post-quantum security at the Layer 2 level.

Consider, for example, the work of the Zknox team. They have optimized the Number Theoretic Transform (NTT) mechanism used in the post-quantum FALCON algorithm and others. According to the authors, their implementation of NTT reduces the cost of processing a FALCON signature by a factor of 12. They propose integrating this compressed signature into Ethereum, not within the main chain, but as a Layer 2 solution.

This approach addresses the problem of reducing the load on the main network and lowering transaction costs: the very purpose of such L2 overlays. However, whether it truly solves the problem of quantum protection remains an open question. Why? Because a certain paradox arises here: yes, transactions within the L2 would be secured by post-quantum cryptography, but the data block ultimately still settles onto the L1 chain, which relies on classical cryptography. If the L1 layer is vulnerable to a quantum attack, then the L2 data remains at risk regardless.

Other Initiatives and Their Limitations

A similar issue applies to another quantum-resistant solution, the development of which is also funded by the Ethereum Foundation. This concerns the first-of-its-kind post-quantum aggregate signature called Chipmunk. It should be noted immediately that such a signature is a significant achievement and a major step forward in cryptography. Moreover, we are exploring the integration of this algorithm into the block consensus mechanism for Cellframe. However, its creators are currently only considering it within the context of L2 rollups.

On another front, the BIP-360 team, focused on Bitcoin's quantum security, is effectively proposing only an initial step: migrating users to quantum-safe addresses (using P2MR technology). While this would help protect user funds at the moment of a quantum attack, it does not resolve the broader challenges of fully integrating post-quantum cryptography into Bitcoin or addressing other "life after Q-day" issues like scalability, high fees, and low TPS.

What does all this indicate? It suggests that developers of these solutions are, in effect, acknowledging the technological limitations of first and second-generation blockchains in the face of the quantum threat. Post-quantum upgrades at the L2 level or quantum-safe addresses are merely temporary measures. While they might help secure user funds in the immediate term, they fail to address the problem at a fundamental level. Not only does the main network remain vulnerable to a potential breach, but the core issues of throughput and high fees also persist unaddressed.

The Blueprint for True Quantum Resistance

Quantum security is a comprehensive challenge that must be tackled at the blockchain's infrastructure and architectural level. There is no other way. Furthermore, it is crucial to understand that, concerning the quantum threat, it will not be sufficient to implement post-quantum signatures and a strictly defined consensus algorithm once and for all. Mechanisms must be in place that allow for quick and seamless adaptation to changing external conditions. It is evident that first and second-generation blockchains, with their monolithic architecture and single chain, where significant code changes are only possible through forks, are ill-suited for this purpose.

A quantum-resistant blockchain must be flexible. This implies a modular architecture, multiple independent chains interconnected at the infrastructure level, mechanisms for optimizing network load, combined signatures, and the capability for rapid updates to consensus and cryptography.

These features are, to varying degrees, already present in third-generation blockchains like Polkadot, Solana, and Cosmos. The remaining task is to integrate post-quantum cryptography. Yes, this will be challenging: even the most flexible and advanced blockchains will require significant changes at the infrastructure level. However, it is entirely feasible. And it needs to be done as soon as possible.

The Cellframe Approach: Building for the Quantum Future

Here at the Cellframe blockchain team, we incorporated quantum security considerations right from the architectural design phase of our platform.

• The platform's core code is written in the low-level C language, a deliberate choice. This ensures not only high performance but also maximum portability across different operating systems.

• To address performance and network load optimization, we designed a specialized two-level sharding mechanism. This will enable our network to efficiently process a high volume of transactions secured with post-quantum signatures, which are known to be large and computationally heavy.

• To make the blockchain maximally adaptive, we implemented functionality that allows for rapid updates to cryptography and consensus parameters without requiring hard forks. Furthermore, we have implemented so-called "multi-signatures" that utilize several algorithms simultaneously. This means that if one signature algorithm within the chain were to be compromised, the remaining ones would continue to guarantee cryptographic protection.

In light of news regarding the rapid advancement of quantum computers, such foresight no longer seems premature. We urge the crypto community to follow our lead and prepare for quantum risks proactively, while there is still time.