Source: Polygon Forum; Author: Baishui, Golden Finance
Foreword
We invite the community to discuss a proposal to securely connect Polygon PoS to AggLayer using zero-knowledge (ZK) validators to verify the network's consensus mechanism, thereby establishing the finality of network settlement to AggLayer. This consensus proof is complementary to the pessimistic proof and ensures the security of all chains connected to AggLayer. For more information on pessimistic proofs, see here: Introducing Pessimistic Proof 2
The consensus proof will be secured by the Plonky3 proof system through Succinct Labs' SP1, a general-purpose zkVM that allows the execution of AggLayer proofs in standard Rust. For more information on the collaboration between Polygon Labs and Succinct Labs, see here: SP1 Built on Plonky3 Will Help Enable Unified Liquidity 1
This upgrade will have two major immediate benefits:
Greater network effects for users and developers by aggregating liquidity, users, and state through the AggLayer;
Upgraded bridge experience, including permissionless token transfers.
Background
A year ago, the community began discussing a proposal that would make Polygon PoS a zkEVM Validium, thereby upgrading the network to a ZK-powered L2. This remains the long-term goal for the final state of Polygon PoS. The challenge of achieving this goal largely depends on a commitment to maintaining the low-fee, Ethereum-for-all ethos of Polygon PoS.
The Type 1 zkEVM prover used to upgrade Polygon PoS to ZK L2 is already able to attest to existing Ethereum blocks at low cost. For the application of Polygon PoS, the question now is how to do this at scale.
For now, if this discussion is favorable and the subsequent PIP is accepted by the community, the upgrade will serve as an intermediate step to becoming a full ZK L2. Connecting Polygon PoS to the AggLayer will provide the benefits of a unified bridge while still being able to rely on the enhanced security guarantees provided by the validator set.
Technical Details
If community discussion develops around this proposal, a future PIP or PIPs will provide the appropriate technical specifications. However, at a high level, this upgrade requires at least the following:
Proof of Consensus:A new consensus validation contract will be deployed on Ethereum. This contract will be able to verify that Polygon PoS has reached consensus and that any withdrawals meet the criteria dictated by the pessimistic proof logic. A proposal will be created to add it to the list of available validation contracts for the Unified Bridge. (See Practical Questions below for additional considerations on this point.)
Bridge Implementation:The Unified Bridge contract will be deployed on Polygon PoS.
PoS Portal:Existing tokens in the PoS Portal Bridge will be upgraded to use the Unified Bridge and ownership will be destroyed, eliminating multi-signature control over those tokens.
Network Roles:Settling to the AggLayer using a consensus proof design will require some entities to submit the required information. (See Practical Issues below for additional considerations on this point.)
Client Changes
No significant client changes to Bor or Heimdall are required to complete this upgrade. While upgrading to Heimdall v2 may allow for future optimizations, this is not necessary for initial deployments.
Bridge Changes
No changes to the StateSync mechanism supporting FxPortal or Plasma Bridge are required to complete this upgrade. Additionally:
The Unified Bridge will be deployed as a new, standalone bridge.
Tokens mapped using the PoS Portal will be upgraded to use the Unified Bridge instead of the FxPortal, and the PoS Portal will be deprecated.
Timeline
If the results of this discussion are satisfactory, a formal PIP will be released. The main dependency is to complete a production-ready implementation of pessimistic proofs, which is expected to be completed in mid-summer. Once completed, a formal PIP will be released, most likely in the fall. Following this timeline, connecting Polygon PoS to AggLayer could be completed by the end of 2024, assuming community acceptance.
In the meantime, Polygon PoS core engineers expect the upcoming Ahmadabad hard fork to be deployed in July, with Heimdall v2 expected to be launched sometime after that. Currently, neither of these upgrades are dependencies for connecting Polygon PoS to AggLayer. However, there is another implementation of consensus proofs that requires deeper changes to Heimdall.
Real Issues
Signature-Based Proof of Consensus
The proposal outlines the use of Polygon Plonky3 to represent the consensus of Polygon PoS, with the SP1 zkVM providing the maintainability and developer velocity benefits of Rust. Broadly speaking, the goal of proof of consensus is to represent local chain state. This can also be achieved by modifying Heimdall to use different signing methods.
For example, if BLS or ICE-FROST threshold signatures were used during consensus, that single signature could represent the consensus of the AggLayer. One benefit of this approach is simplicity: communication between Polygon PoS and AggLayer can be added to Heimdall as a task, in the same process that currently submits checkpoints to L1.
A potential drawback is that this approach would require deeper changes to the network's consensus mechanism, which could extend development time. This design would eliminate the need for new network actors.
Network Roles Communicating with AggLayer
In order for Polygon PoS to connect to the AggLayer using Proof of Consensus, a new role is required. This role will be responsible for submitting the following information to the AggLayer:
How and who performs this function is an area of active investigation. At a high level, protocol researchers are considering one of the following:
Allow list addresses to perform this role first, with the goal of making it permissionless after the AggLayer stack has a chance to harden;
The role is immediately permissionless, in which case network participants will perform this function in exchange for some as-yet-unquantified reward. If this is the preferred option and there are no other security concerns, it will need to be determined by what mechanism this network participant is selected; one potential option is to use the existing leadership selection in Heimdall.
Summary
With 400 million+ unique addresses, $2 billion+ in assets bridged from Ethereum, and tens of thousands of applications, Polygon PoS remains one of the most widely used blockchains in the world. Users and developers love it because of its strong community, network effects, and low fees for everyone, which are typically orders of magnitude lower than the famous Ethereum L2.
The core developers at Polygon Labs have been focusing on the research and development of Ethereum scaling technology for many years; it started with Polygon PoS. Over the years, they have come to two conclusions: ZK technology is the only practical way to achieve verifiable off-chain computation, and aggregation is the only way to achieve multi-chain interoperability without sacrificing chain sovereignty.
The joint efforts of Succinct Labs and Polygon Labs are based on the shared belief that collaboration with open source technology can build the best public interest. The outcome of discussions on the future of Polygon PoS is ultimately in the hands of the community. We once again invite you to analyze, discuss, and hopefully reach a consensus to implement this proposal.
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