Interpret the new idea of ZKP bridging: use the ZKP requester-prover separation model to create a more secure decentralized cross-chain bridge

PANews

2023-11-04 03:33:36

原文标题：ZKP Requester-Prover Separation model to support Full ZK and Optimistic ZK

Original author: 0x3d18, ZKPool

Compiler: Qianwen, ChainCatcher

There are many use cases for zero-knowledge proofs, including rollups, bridges, and oracles. This led to the development of projects such as ZK-Rollup, ZK-bridge, and ZK-oracle.

Hybrid and optimistic designs have recently been applied to ZKP technology. For example, Orbiter Finance has proposed the Optimistic ZK bridging protocol, while Taiko has proposed a progressive hybrid rollup solution.

Optimistic ZK assumes that all state transitions are correct and does not require immediate proof of validity. But it establishes a predetermined challenge window during which any participant can dispute fraudulent activity by submitting a proof of validity or proof of fraud.

This design reduces the total cost of proof for ZKP projects while ensuring security by incentivizing a decentralized challenger monitoring system and challenging fraud.

Optimistic ZK Bridging Protocol

Orbiter Finance is one of the more well-known cross-rollup projects. It proposes “Orbiter Cross-Rollup Protocol: Optimistic About the Compliant Majority, Tough Arbitration for the Malicious Minority”.

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Optimistic Rollup Cross Trading Process (from Orbiter Finance)

It defines a decentralized, secure, and cost-effective cross-rollup design, supported by ZKP technology.

Orbiter’s decentralized design

There are several important factors to consider for such a design:

First of all, the bridge project in the past has experienced multiple security issues, which have caused significant losses to users. Centralization also brings security risks. Therefore, decentralization is essential for bridging.

Second, there needs to be a mechanism to ensure the accuracy of the transaction flow between the source/rollup and the destination chain/rollup.

In addition, it is essential to find a cost-effective way to generate such proofs. Compared to on-chain Merkle trees, ZKP is a viable option with fewer gas fees.

In particular, for cross-rollup bridges, cost is a primary consideration, and the goal of the entire design is to minimize expenses. This means that it is critical to reduce on-chain transactions and minimize the amount of gas per on-chain transaction.

In Orbiter’s design, in addition to the bridging payment scheme, there is another scheme that requires ZKP. In this scenario, a role called a “submitter” aggregates cross-aggregated transaction information and sends it to L1 to ensure that the decentralized dealer receives accurate rewards.

Orbiter’s decentralized committer design

Orbiter’s protocol assumes that the majority of participants are infallible and optimistically handles cross-rollup events to ensure timely execution. If proofs were required for every cross-rollup transaction, then the execution of the entire bridging transaction would be slow. As a result, there is no need to generate proofs in the absence of malicious behavior, saving costs. However, if malicious behavior is detected by the maker or submitter, the challenger can generate a proof, and the challenged submitter should also submit the proof.

Orbiter Optimistic zk bridge design

ZKPool Requester Prover Separation Model

When it comes to using ZKP technology, there are different modes available:

Full zk: In this mode, one ZKP is required for each conversion. This can be achieved through projects like ZK-bridge (e.g. Polyhedra) or ZK-Rollup (e.g. Scroll).
Optimistic zk: In this mode, ZKP is only required when the conversion is challenged. Taiko and Orbiter are an example of this pattern.

Full zk and Optimistic zk

When defining abstract models, it is clear that ZK-bridge and ZK-Rollup share some similarities. Specifically, this difference is reflected in the relationship between the ZKP requester and the ZKP prover, as shown in the following diagram. Here, the ZKP requester references a module that has the need to generate a ZKP.

The scenario is as follows:

In the ZK-Rollup project:

In full zk mode, the sequencer works as a ZKP requestor.
In Optimistic zk mode, the challenger acts as the ZKP requester.

In the ZK-bridge project:

In all-zk mode, the maker acts as the ZKP requester.
In Optimistic zk mode, the challenger acts as a ZKP requester.

ZKP Requester and ZKP Prover

As mentioned earlier, in an optimistic zk, there may not always be a proof task. As a result, if ZKP requesters and ZKP prover are combined into the same module, the prover may be idle and its computing power may not be fully utilized.

If we design a requester-prover separation model and make the prover a shared pool, we can increase the utilization of the prover. When the optimistic scenario is not challenged, the prover can take on the proof task from other ZKP projects. This means that ZKPool plays an important role in the zk-bridge project, especially when combining Optimistic with others.

ZKPool shares the role of ZKP prover among ZKP requesters

The ZKP supplicant-prover separation model applies not only to rollups and bridges, but also to oracles and all other ZKP projects.

Summary

Based on the information provided, we can draw the following conclusions:

ZKP technology is essential to the ZKP project, including rollups, bridges, oracles, and other related projects.
ZKPool allows us to treat the creator/submitter of ZK-bridge and the sequencer of ZK-Rollup as the same role, collectively referred to as ZKP requester
By using ZKPool’s ZKP requester-prover separation model, you can improve the utilization rate of your prover. This model also promotes the decentralization of all ZKP projects.

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