What other projects are attempting Hybrid Rollup besides Fraxchain?

OP+ZK Rollup technology may seem distant, but in fact, many encryption protocols and projects have tried in this area.

By 0xFacai and Jaleel, Edited by Jack

On June 16th, Frax Finance, a hybrid algorithm stablecoin protocol, announced the launch of an Ethereum Layer2 network called Fraxchain, where all Frax assets will be natively supported on Fraxchain. It is reported that Fraxchain will adopt a hybrid rollup expansion plan that combines OP Rollups and ZK Rollups, with the aim of achieving better scalability, faster finality, and enhanced security against competitors, which has attracted a lot of attention from the encryption community.

In fact, this fusion Rollup idea is not new. As early as July 2022, Kelvin Fichter, the core developer of Optimism, developed the Optimism Bedrock architecture and proposed this hybrid approach, stating that “Optimistic Rollup currently has advantages over ZK similar products, and Bedrock aims to make seamless transitions between Optimistic and ZK possible”, and discussed this topic with Ethereum co-founder Vitalik Buterin.

Following Kelvin Fichter’s approach, Ethereum co-founder Vitalik Buterin also expressed his own views: combining Optimistic Rollups with ZK Rollups, with governance only used to solve bugs between the two, the process is: 1) publish blocks, 2) wait for a 24-hour fraud challenge period; 3(a) if there is no challenge, publish ZK SNARK for final confirmation; 3(b) if there is a challenge, make a decision based on 2 out of 3 options (challenge game, ZK SNARK, and governance).

As more and more teams join the second-layer expansion competition, this Rollup fusion expansion approach is increasingly being applied. In addition to Optimism, there are multiple teams on the market that are researching and trying Hybrid Rollups. What is a Hybrid Rollup? Why choose a Hybrid Rollup? Which teams are doing development in this field? BlockBeats has analyzed and organized this in this article.

What is a Hybrid Rollup?

A Hybrid Rollup is a Rollup with a hybrid approach, which combines Optimistic Rollups and ZK Rollup technologies to optimize product features to the greatest extent possible, thereby finding the best product-market fit (PMF), that is, achieving product-market matching.

Translated text:

To understand what Hybrid Rollup is, you first need to understand what Optimistic Rollup and ZK Rollup are.

First, Rollup is one of the Ethereum scaling solutions that transfers the computation and storage of transactions on the Ethereum mainnet (Layer1) to Layer2 for compression and then uploads the compressed data to the Ethereum mainnet to expand Ethereum’s performance.

Essentially, users do not send transactions to miners on L1, but rather send them to Rollup servers to execute transactions outside of the Ethereum main chain. However, these Rollups will publish transaction data to the Ethereum mainnet. When transactions are published to the main chain, they inherit L1 security attributes. Rollups are typically programmable, so they need to support smart contract (SC) languages for developers to develop DApps. However, because Privacy is incompatible with the EVM (Ethereum Virtual Machine), Solidity cannot be used directly to develop privacy-type DApps. This requires a customized smart contract language to support the writing of public and private contracts. Of course, this requires VM module adaptation to maintain different types of state trees.

Rollups usually run on the second layer and have the flexibility to run on the third layer.

According to different schemes to ensure the validity of compressed data (i.e. data correctness), Rollup can be divided into ZK Rollup and Optimistic Rollup.

ZK Rollup is a Rollup scheme that uses zero-knowledge proofs. The difference from other Rollups is that ZK Rollup uses zkSNARK algorithm (a zero-knowledge proof algorithm) to compress data and ensure security.

Optimistic Rollup literally means optimistic Rollup, which optimistically assumes that the data synchronized to Layer1 is correct. If anyone thinks that the synchronized data is incorrect (i.e. fraudulent behavior), they can issue a challenge during the challenge period. Therefore, this proof method is also called fraudulent proof, which ensures that the data finally synchronized to Layer1 is valid.

Hybrid is a mixed Rollup scheme that combines the two Rollup schemes and supports more contract types, including public, private, and mixed contract types. For developers, they can freely choose the contract type based on their own needs. For users, it also has a high degree of freedom to freely choose the transaction type in the mixed contract.

The following chart briefly depicts the positioning and functions of Hybrid mixed Rollup. Currently, there are multiple projects dedicated to building in this direction. Although the technical details of each project are different, they all have a common vision: to bring more possibilities, higher security performance, more real-world scenarios, and more daily users to the blockchain industry.

In the 2023 roadmap released by Metis Andromeda, Hybrid Rollup occupies a large part, which is enough to prove the importance of Metis to it.

Workflow of Hybrid Rollup in Metis network


BlockBeats previously reported that on May 4, Aztec announced that it will launch a public and privacy mixed zkRollup network. The network will continue to use the name of Aztec, and its feature is that it can execute public and private smart contracts in one environment, which will help expand developers’ design choices on the chain.

Aztec allows the execution of smart contract logic in a public and private manner, while still inheriting Ethereum’s security advantages. On Aztec, privacy is optional, not a default requirement. Its goal is to expand Ethereum, that is, to have the functions of privacy function calls, transaction privacy, and user anonymity while all data remains public. This allows users to protect sensitive information from being leaked, while still being able to share this information with relevant institutions when necessary to achieve compliance purposes. Aztec also stated that the team is currently completing the deployment of basic contracts, and is expected to establish a local test network before the third quarter of 2023. Users are expected to access a mature public test network before the beginning of 2024.


In May last year, Polygon established a partnership with global professional services and technology company Ernst & Young (EY) and released version 3.0 of Nightfall. Polygon Nightfall is essentially an Optimistic Rollup that uses zk encryption to protect privacy. Its most prominent feature is the effective combination of the backbone concept of Optimistic Rollup with the zero-knowledge (ZK) cryptography commonly used in ZK-Rollups, thereby achieving scalability and privacy.

In fact, as early as 2019, EY released the initial version of Nightfall. The most different point from other zk solutions is that Nightfall is a rollup focused on privacy, and EY positioned it as “one of the most prominent privacy solutions on Ethereum.” Specifically, on Nightfall, every transaction contains privacy, which means that if Alice sends an asset to Bob, others will not be able to see what the asset is, how much value it contains, or where it went.

Ernst & Young focuses on enterprise clients, which is why it places greater emphasis on transactional privacy. Initially, Nightfall attempted to build the first enterprise-level blockchain directly on Ethereum, but it ultimately found that privacy was too expensive on the Ethereum mainnet and turned to L2, ultimately partnering with Polygon.


The development team of the lightweight blockchain protocol Mina, O(1) Labs, has submitted a proposal to Optimism (now renamed OP Mainnet) to apply its MIPS zkVM-based zero-knowledge proof technology to OP Stack fraud proofing. The MIPS function not only allows the use of a general-purpose language to write applications in addition to the local high-performance snarkyJS, but also enables the seamless integration of Optimistic and ZK.

In addition, Optimism completed the Bedrock mainnet upgrade at 2:50 a.m. Beijing time on June 7, which not only upgraded transaction fees, reduced system latency, and improved node performance, but also made seamless integration between Optimistic and ZK possible.

Why choose Hybrid Rollups?

The use of a “hybrid” architecture such as “Optimistic + ZK Rollups” maximizes the advantages of both, which means that not only does it retain the Optimistic Rollup architecture, which is EVM-compatible, but also maximizes the generality of zero-knowledge proofs to enhance security, whose validity proof can clearly ensure the correctness of off-chain transactions and prevent operators from executing invalid state transitions.

ZK Rollup can increase throughput because zero-knowledge proofs eliminate the need for additional data and each transaction only retains the minimum amount required. This greatly increases throughput and reduces Ethereum congestion.

In addition, ZK Rollup brings additional benefits, namely, faster transaction speeds and faster time to finality.

By implementing zero-knowledge proofs on the Optimistic Rollup architecture, the time it takes to finalize transactions will be reduced from 7 days to about 4 hours. There are typically two types of time, confirmation time and finality time. Optimistic Rollup provides faster confirmation time, but the premise of Optimistic Rollup is that all transactions are valid from the beginning, so there is a 7-day buffer period, meaning that full transaction finality can only be achieved after 7 days of transaction confirmation, which is the window for validators to submit fraud proofs (if necessary). Combined with zero-knowledge proofs, transactions can be confirmed more quickly as long as the proofs are generated by the prover and approved by the verifier.

Users can withdraw funds from L2 without delay, which means that a merge-type Rollup will bring higher capital efficiency to users.

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