Many people think that ZK-Rollup is the endgame of Ethereum layer2, but as research deepens, I found that what can be called the endgame is not the ZK-Rollup form, but the ZK technology itself. Because theoretically, OP-Rollup can also use ZK technology to reduce the 7-day challenge period and even eliminate the MPC multi-signature governance defects. How should we understand this? This article will use @MetisL2 and its hardcore project @ProjectZKM as examples to discuss:

In a previous article, I also explained that the "ZK technology" for layer2 expansion has only explored limited potential, and there is still a lot of room for development in the area of trust-free interoperability across the entire chain. In the context of layer2 application scenarios, it can directly bring about two major impacts:

1) The cross-chain of funds between layer2 and layer1 can directly achieve secure asset determinism based on ZK underlying technology, using ZK technology, OP-Rollup can greatly shorten the 7-day waiting challenge period required for fund withdrawal;

2) The Prover verification system of layer2 and the Rollup verification contract of layer1 can achieve a trusted environment construction across different chains, theoretically, it is possible to no longer rely on centralized or semi-centralized MPC multi-signature governance to ensure security.

In fact, @MetisL2, as a layer2 chain based on the OP-Rollup technology framework, has already achieved a paradigm shift in the underlying architecture of layer2 in these two dimensions through the underlying ZK technology:

On the one hand, by optimizing the user experience of Withdrawal by using a hybrid Rollup framework of OP-Rollup + ZK-Rollup, it can release liquidity more quickly;

On the other hand, by experimenting with the "trust-free" framework for cross-chain interoperability proposed by @ProjectZKM, Metis layer2 uses ZK as the underlying infrastructure to build a decentralized coprocessor as a trusted entity to generate Proof, verify Proof, and manage communication and trust mechanisms between cross-chain blockchain nodes. How is this done?

-Hybrid Rollup:

Normally, a layer2 is either an OP-Rollup structure based on optimistic 7-day challenge period proof, with fast batch transaction rates and low fees, but the drawback is that when users withdraw funds from layer2 to layer1, they need to wait for a 7-day challenge period, because 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 ZK-SNARKs proof transmission structure can achieve asset security confirmation in a single block, but in addition to the cost of batch transactions, it will also incur additional Prover system verification costs, and the development threshold for the layer2 ecosystem for the development team is relatively high, and the ecosystem development speed is slow.

So, how can we achieve the ease of use of the overall OP-Rollup architecture while avoiding its 7-day challenge period (locking liquidity)? The answer is: to process the transactions from layer2 back to the main network in a separate "channel", with common transactions going through the OP-Rollup channel, while special Withdrawal transactions go through the ZK-Rollup channel, thus adopting the strengths of both OP and ZK to become a compromise and optimal solution.

Metis has already implemented the hybrid Rollup solution into its own chain operation, and the "brand new" hardcore ZK technology underlying project @ProjectZKM incubated and created by the Metis technical team provides this hybrid Rollup capability. The Metis team has discovered some inherent normal problems in the operation of layer2, in addition to taking the lead in solving the problem of decentralized Sequencer, they also aim to solve a series of problems in the development of layer2 through ZKM, such as "MPC semi-centralization" and "locking liquidity";

At first glance, everyone may find it abstract. The reason is that, as I explained in the previous article on ZK technology, such services are currently at the upstream of the layer2 technology service supply chain, although hardcore, they have not been widely adopted, so they still sound unfamiliar. Similar to RISC Zero, the goal of ZKM is to provide more upstream security consolidation services for layer2 projects based on the ZK underlying technology framework.

Currently, ZKM provides the ZK-Rollup channel service required for the hybrid Rollup of layer2, and there is also an Entangled Rollup protocol at the underlying level to provide unified liquidity management services.

In fact, @ProjectZKM, @RiscZero, and @SuccinctLabs, and other upstream ZK technology service providers are all promoting similar technical solutions, thereby helping to further reduce the differences between OP-Rollup and ZK-Rollup, and at the same time reducing the impact of the 7-day challenge waiting period on liquidity without any Battle-Tested challenges. Incomplete statistics show that projects such as Metis, Fraxchain, Aztec, and Ola are already exploring the implementation of such hybrid Rollup solutions.

-Trust-free cross-chain bridge mechanism based on ZK:

Currently, most layer2 projects are criticized for the problem of the mainnet Rollup contract being controlled by a semi-centralized MPC security governance committee, which has led to the security of most layer2 projects remaining at what Vitalik called Stage 0.

Upstream ZK technology service providers such as ZKM and RISC Zero can theoretically enable every chain with smart contract functionality to achieve a decentralized and trusted environment for cross-chain bridges without the need for MPC multi-signature governance.

The principle is simple, ZK-SNARKs can allow nodes maintaining consensus on two chains to establish direct communication. When the nodes of chain A receive the transaction submitted by the nodes of chain B and the ZKP proof, they can independently verify the validity of the ZKP proof, and then accept the validity of the Proofs submitted by chain B. The entire process is completely based on zero-knowledge proof algorithms and does not require a third-party entity to control it.

As for the coprocessor that schedules communication between nodes, it can be implemented using a decentralized open-source architecture, thereby completely solving the "centralization" problem in Rollup cross-chain.

Since Ethereum has complete smart contract functionality, theoretically, layer2 of Ethereum has the basic conditions for cross-chain based on ZK technology. In an ideal situation, all layer2 should use the same ZK technology framework to deploy cross-chain solutions, and other layer1 chains and even layer3 chains should also be included in this network communication architecture, making the future Ethereum public chain a universal settlement layer in a full-chain environment, right?

The reason why most layer2 projects are unwilling to give up MPC multi-signature governance is mainly due to the security and control required for security. In my opinion, the fundamental reason is that the popularization of decentralized Sequencer, Prover, and zkBridge, and other basic component services at the layer2 level is not in place. In this situation, retaining centralized or semi-centralized security governance committees has become a safer governance method.

However, in the long run, as layer2 becomes more decentralized, the security governance of layer2 will inevitably be criticized, and a cross-chain settlement solution based on ZK underlying technology will become necessary. The potential for the subsequent development of the ZK technology infrastructure race is imaginable.

In conclusion, it is not an exaggeration to say that the current Ethereum layer2 has fallen into a development dilemma: either relying on a phenomenal project in the layer2 or layer3 application ecosystem to boost everyone's confidence in the layer2 market, or relying on further decentralization of key components such as Sequencer, Prover, and zkBridge to continue to enrich the technical narrative. The latter will inevitably require a bottom-up framework based on ZK technology, and only then will the ZK endgame characteristics truly emerge.

Only by following this path of development can the development direction of layer2 not be refuted, and it can also extend to the development of ZK hardware devices acceleration; ZK+DePIN PC, mobile terminals, IOT and other hardware device support; ZK tamper-resistant cloud computing data protection; decentralized Prover system construction and algorithm resource optimization, and many other narrative directions.

In fact, at the intersection of the AI, ZK, and DePIN race, there are already many similar projects emerging day by day.

The development concept of Metis in layer2 and the original intention of the hardcore ZKM project are also the same, to make ZK technology more universally applicable to every aspect of the global layer2 market; in addition, I personally have noticed some projects, such as @cysicxyz, which are continuously working on ZK hardware and chip acceleration, and @olazkzkvm, which is gradually implementing decentralized Prover computing power systems.

In short, I want to reiterate my point of view that the application of ZK technology in the layer2 field is just the tip of the iceberg. When ZK technology becomes more widely used in areas such as hybrid Rollup, ZK cross-chain bridges, and ZK hardware acceleration, I believe it will bring new vitality to the layer2 market.

After all, it will take a long time for the entire layer2 market to adopt this ZK bottom-up upgrade paradigm.

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