Many people believe that ZK-Rollup is the endgame for Ethereum Layer 2, but as my research deepened, I discovered that the endgame is not actually the ZK-Rollup form, but rather the ZK technology itself. Because theoretically, OP-Rollup can also use ZK technology to reduce the 7-day challenge period and even eliminate the flaws of MPC multi-signature governance. How should we understand this? This article will discuss this using @Metis L2 and its underlying hardcore project @ProjectZKM as examples.
In a previous article, I also explained that the potential of “ZK technology” for layer 2 scalability has only been partially explored, and there is still a lot of room for development in terms of trustless interoperability across the entire chain. In the context of layer 2 application scenarios, it can directly bring two major impacts:
1) Cross-chain transactions between layer 2 and layer 1 can be directly based on the underlying ZK technology for secure asset settlement. By using ZK technology, OP-Rollup can significantly reduce the 7-day waiting period for funds withdrawal challenges.
2) The Prover verification system of layer 2 and the Rollup verification contract of layer 1 can establish a trusted environment for cross-chain communication between different chains. In theory, it is possible to eliminate centralized or semi-centralized MPC multi-signature governance models that are vulnerable to criticism and ensure security.
In fact, @Metis L2, as a layer 2 chain based on the OP-Rollup framework, has already achieved a paradigm shift in the underlying architecture of layer 2 in these two dimensions using ZK technology:
On the one hand, by optimizing the user experience of fund withdrawal through the hybrid Rollup framework of OP-Rollup + ZK-Rollup, @Metis L2 can shorten the 7-day waiting period for fund withdrawal, allowing for faster liquidity release.
On the other hand, @Metis L2 aims to gradually implement the “trustless” framework for cross-chain interoperability based on @ProjectZKM, a hardcore ZK technology underlying project incubated and developed by the @Metis technical team. It uses ZK as the underlying infrastructure to build a decentralized co-processor as a trusted entity to generate proofs, verify proofs, and manage communication and trust mechanisms between cross-chain blockchain nodes. How is this done exactly?
Hybrid Rollup
In normal circumstances, a layer 2 is either an OP-Rollup structure based on optimistic 7-day challenge period proofs, or a ZK-Rollup structure based on ZK-SNARKs with single-block finality proofs.
The former has fast batch transaction rates and low fees, but the drawback is that when users withdraw funds from layer 2 to layer 1, they need to wait for the 7-day challenge period. According to the optimistic batch essence of OP-Rollup, the withdrawal of funds can only be considered safe if it is not challenged within 7 days.
The latter uses ZK-SNARKs proofs transmission structure to achieve asset security confirmation in a single block. The disadvantage is that besides the DA cost of batch transactions, there is also additional cost for the Prover system computation and verification. Moreover, the development threshold for the layer 2 ecosystem is high, resulting in slower ecosystem development.
So, how can we achieve both the usability of the OP-Rollup overall architecture and avoid the 7-day challenge period (liquidity lock) restriction? The answer is: divide the transactions from layer 2 back to the mainnet into different “channels”. Common transactions will go through the OP-Rollup pipeline, while special withdrawal transactions will go through the ZK-Rollup pipeline. This way, the best of both OP and ZK can be adopted, creating an optimal compromise solution.
Metis has already implemented the hybrid Rollup solution in its chain operation process, and it is the “brand new” hardcore ZK technology underlying project @ProjectZKM developed by the Metis technical team that provides this hybrid Rollup capability. The Metis team has identified some inherent issues in the operation of layer 2, and in addition to tackling the decentralized Sequencer challenge, they also aim to solve a series of issues in the development of layer 2, such as “MPC semi-centralization” and “liquidity lock” through ZKM.
At first glance, this may seem abstract. As I explained in my previous article on ZK technology, these services are currently in the upstream of the layer 2 technology service supply chain. Although they are hardcore, they have not been widely adopted, so they still seem unfamiliar. Similar to ZKM and RISC Zero, the goal is to provide upper-level security consolidation services for layer 2 projects based on the ZK underlying technology framework.
Currently, ZKM provides the ZK-Rollup pipeline service needed for the hybrid Rollup of layer 2, and there is also an Entangled Rollup protocol underlying it, which provides unified liquidity management services.
In fact, service providers in the ZK technology upstream, such as @ProjectZKM, @RiscZero, and @SuccinctLabs, are all promoting similar technical solutions to help further reduce the differences between OP-Rollup and ZK-Rollup, and reduce the liquidity loss caused by the 7-day challenge waiting period, which is not battle-tested. Notably, projects such as Metis, Fraxchain, Aztec, and Ola are already exploring and implementing such hybrid Rollup solutions.
Based on ZK Trustless Cross-chain Bridge Mechanism
Currently, most layer 2 projects are criticized for having mainnet Rollup contracts controlled by a semi-centralized MPC governance committee, which means that the security of most layer 2 is still at Vitalik’s Stage 0.
ZKM, RISC Zero, and other upstream ZK technology service providers theoretically enable decentralized trustless cross-chain bridge construction between chains with smart contract capabilities.
The principle is simple: ZK-SNARKs allows nodes maintaining consensus on two chains to establish direct communication. When a node on Chain A receives a transaction and ZKP proof submitted by a node on Chain B, it can independently verify the validity of the ZKP proof and accept the validity of the Proofs submitted by Chain B. The entire process is implemented solely based on zero-knowledge proof algorithms and does not require a third-party entity to control it.
As for the co-processor that schedules communication between nodes, it can be realized using a decentralized open-source architecture, thereby completely solving the “centralization” issue in Rollup cross-chain communication.
Since Ethereum has smart contract capabilities, theoretically, all layer 2 projects on Ethereum have the basic conditions for cross-chain based on ZK technology. In an ideal situation, all layer 2 would adopt the same ZK technology framework to deploy cross-chain solutions, and other layer 1 and even layer 3 chains would also be included in this network communication architecture. In this way, the future Ethereum mainnet will become a universal settlement layer in a global chain environment.
The reason why most layer 2 projects currently do not want to give up MPC multi-signature governance is mainly due to the need for security control. In my opinion, the fundamental reason is that the popularization of decentralized Sequencer, decentralized Prover, decentralized zkBridge, and other basic component services at the layer 2 level is not sufficient. In this situation, retaining a centralized or semi-centralized governance committee has become a relatively secure governance method.
However, in the long run, as decentralized components become more popular in layer 2, the governance methods of layer 2 will undoubtedly be criticized, and a cross-chain settlement solution based on the ZK underlying framework will become necessary. The potential for the further development of the ZK technology infrastructure track can be imagined.
To reiterate my point, the application of ZK technology in the layer 2 field is currently just the tip of the iceberg. When ZK technology becomes more widely used in areas such as hybrid Rollup, ZK cross-chain bridge, and ZK hardware acceleration, it will undoubtedly bring new vitality to the layer 2 market.
After all, it will take a long time for the entire layer 2 market to fully embrace this ZK bottom-up upgrade paradigm.
