Introducing Hybrid Proving

Bridging the gap between Optimistic and ZK Rollups.

Introducing Hybrid Proving

At the end of 2020, Fuel V1 deployed as a Layer-2 (L2) scalability technology on Ethereum. It was the first optimistic rollup, trail-blazing fraud proofs on Ethereum mainnet. Since then, the Fuel project has grown significantly on a path toward Fuel V2, our next-generation rollup architecture, now boasting more than 40 ecosystem projects, all with the aim of building the next era of decentralized applications on Ethereum.

In this blog post, we introduce the FuelProver and hybrid proving - a key technique we believe offers untapped benefits never before utilized.

The Problem

The Layer-2 space has been dominated by two main approaches - optimistic rollups (ORU) and zero-knowledge (ZK) rollups. While both have shown promise, each comes with its own set of trade-offs. ORUs like Fuel V1, Arbitrum and Optimism, are significantly simpler in the optimistic case with no extra costs needed to verify proofs on-chain but are bound by long withdrawal periods and a complex interactive verification game (IVG) process, relying on fraud proofs to ensure security. On the other hand, ZK proofs offer instant execution verification and better data compression techniques, but require slow and costly proof generation and on-chain verification.

ZK will eventually be the future of blockchain verification, but proof generation efficiency is not there yet for multi-thousand TPS rollup architectures. Fuel aims to support many thousands of TPS, and so turning Fuel into a full ZK rollup today doesn’t align for us.

Introducing the FuelProver: A Hybrid Solution

Introducing the FuelProver - an optimistic / ZK hybrid. Recognizing the advantages of the single round fraud-proving model introduced with Fuel V1 and also acknowledging the potential of ZK proofs, we envision a middle ground that combines some of the best of both worlds. The FuelProver will be agnostic to ZK proving architecture, meaning it can support all the latest general-purpose provers. The FuelProver aims to ensure a flexible and adaptable system that can evolve with the latest advancements in ZK technology.

The hybrid approach of the FuelProver also enables the reduction of the withdrawal window and achieves high throughput without the downside of expensive proving for each block that gets passed onto users.

This is due to the fact that we have a more predictable period of time it takes to prove a single blocks validity. In the optimistic happy path, we can reduce the withdrawal time down from the typical 7 days in an optimistic rollup, to ~8 hours (note, this is still under active research, these numbers are approximations and are subject to change).

By blending the strengths of both proving paradigms, FuelProver serves as a stepping stone towards an inevitable ZK future while avoiding any lock-in, ensuring a flexible and optimal solution for developers.

Hybrid Proving and Finality

The concept of hybrid proving arises from the fusion of fraud proofs and ZK proving. It represents a new category of scalability solutions that straddle the line between optimistic rollups and ZK rollups. FuelProver is an implementation of this innovative idea.

Hybrid Proving can be further defined as a technique that utilizes not one, but potentially several ZK provers to verify correctness, thus not relying on a single ZK system to prove correctness but many. This is akin to other optimistic rollup fraud proving models, where they will aim to employ various types of fraud proving systems to reduce bug surface and ensure correctness. The FuelProver will use one prover initially, with the aim of evolving towards a multi-prover model for enhanced correctness. This stands as a key differentiator and improvement from other implementations of this idea (see ZKFPs and Hybrid Rollups). Much like Ethereum's multiple client implementations in different languages, this multi-prover model introduces diversity and robustness to Fuel's security model. That said, we believe the hybrid proving technique can be applied anywhere in the Ethereum rollup ecosystem.

With optimistic rollups, trust minimized actionable on-chain finality typically takes 7 days. This means that trust minimized bridging of digital assets back to the base layer can take a week or more to go through, depending on possible delays during fraud challenge games. Hybrid proving experiences faster on-chain actionable finality, thanks to the reduction in the number of rounds in the fraud proving game. ORU systems require a waiting period for parties to interact with enough time to avoid censorship. With only one round, the hybrid proving model doesn’t need to wait around for as long during multiple rounds of interaction. In theory, hybrid proving games should be able to beat multi-round fraud proving games since each round carries multiple hours of waiting to ensure censorship resistance.

An Agnostic Approach

Fuel is a blockchain operating system that takes an agnostic approach, meaning it aims to support many popular underlying architectures. This enables developers and projects to better leverage general-purpose architectures off the shelf while gaining the benefits of Fuel’s VM, tooling and parallel transaction execution. This flexibility also allows for a smoother transition to more advanced ZK-based systems in the future, without compromising on current scalability needs.

The FuelProver intends to support multiple proving mechanisms, including Risc Zero, zkSync, Powdr, and others. While hybrid proving has many promising benefits, the FuelProver will also support a fully optimistic option built out of our fully fraud provable virtual machine.

Looking forward, this presents a promising path for Fuel’s stack to eventually enable a full ZK rollup and achieve near-instant finality within minutes. With support already reaching out from multiple ZK teams, this new hybrid approach stands as a powerful near-term solution that offers some of the best of both worlds and sets a path forward to evolve alongside the ZK landscape as things become more efficient and battle-tested.

Venture into a new paradigm on Ethereum

The FuelProver marks an exciting milestone in the evolution of Fuel and Layer-2 scalability solutions for Ethereum. With its hybrid proving model, Fuel takes an agnostic and additive approach, unlocking new possibilities for developers and users in the blockchain space.

As Fuel ventures into this new paradigm of interconnected blockchains, the FuelProver serves as a powerful tool, enabling a seamless blend of efficiency, scalability, and security. Building upon the foundation of Fuel V1 and its introduction of the optimistic rollup, the FuelProver takes the next step towards unlocking the vast potential for decentralized applications.

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