For blockchain to scale and achieve widespread adoption, embracing a multi-chain future is unavoidable. However, this approach introduces significant hurdles, notably degrading the user experience by fragmenting liquidity across different chains.
Polygon's Aggregation Layer aggregates ZKPs from various chains, facilitating secure and seamless cross-chain transactions. It enables users to engage with numerous networks simultaneously, providing a unified experience akin to operating within a single network.
On January 24, 2024, Polygon introduced a new concept called the Aggregation Layer (AggLayer). AggLayer aims to provide a seamless cross-chain infrastructure within the blockchain ecosystem, similar to the role of TCP/IP for the web, by aggregating Zero-Knowledge Proofs (ZKPs) from various connected chains to enable safe and atomic cross-chain transactions. This innovation is designed to give users the experience of interacting with a single network, even as they engage with multiple networks, aligning with the vision of unified liquidity presented in the previously unveiled Polygon 2.0 roadmap.
Efforts to improve blockchain scalability have been diverse, ranging from increasing block sizes and reducing block times to introducing unique consensus algorithms and building Layer 2 solutions on Ethereum. These endeavors aim to create an environment where numerous users can use blockchain networks swiftly and affordably.
However, as public blockchains are operated by many nodes, they inherently offer lower scalability than centralized systems. This limitation has been manageable for many blockchains due to the currently low level of user and activity in the blockchain ecosystem. Yet, as seen with the recent surge in inscription activity causing disruptions across many networks, a single public blockchain will not be able to handle the vast number of users if mass adoption of blockchain is achieved. Therefore, the future of blockchain mass adoption inevitably involves multi-chain architectures.
Another reason supporting the inevitability of multi-chains is the demand for app chains. From the perspective of projects requiring rapid scalability, like games and order book exchanges or isolated ecosystems, app chains offer an attractive solution. Given that the activities on one service could impact another on general-purpose blockchains, the demand for app chains will continue to grow.
One significant disadvantage of the multi-chain ecosystem compared to a single network is the compromised user experience (UX). Users can seamlessly interact with all services using one wallet in a single network environment. In contrast, within a multi-chain ecosystem, users may need to adjust their wallet settings for different networks or even require different types of wallets. Additionally, using services across different networks often involves the inconvenience of transferring funds via bridges, and atomic transactions are nearly impossible without shared validators or sequencers.
Today's most crucial need in the blockchain industry is chain abstraction, a concept recently discussed by Illia Polosukhin, co-founder of Near Protocol. As the blockchain tech stack evolves towards a more modular approach, community and liquidity fragmentation worsen, making it essential to abstract the blockchain from users to lower the barrier to entry and provide a seamless experience.
Improvements in chain abstraction require enhancements in various areas. Front-end improvements are crucial, such as wallets and user interfaces mediating blockchain interactions. Near Protocol has been working on decentralizing the front end with BOS and mapping NEAR addresses to specific network addresses for EVM, Bitcoin, etc., through account aggregation.
On the back end, infrastructure that facilitates rapid interactions between multi-chains is essential. Although there are various cross-chain messaging protocols like LayerZero, Wormhole, and Axelar, they have limitations in providing a seamless cross-chain transaction environment because they rely on third-party transaction verification and require confirmation from one chain before proceeding on another.
To offer an experience akin to using a single chain while utilizing multi-chains, it is necessary to either share block producers (validators for L1, sequencers for L2) or generate ZKPs for transactions in real time. Sharing block producers enables the use of atomic transactions, and if transactions on each chain can be proved in real-time, other chains can quickly verify these results. This is the ultimate direction the multi-chain ecosystem must pursue.
The Ethereum ecosystem is home to numerous Layer 2 networks. However, providing a seamless cross-chain experience to users is challenging because each network has its own sequencer set and separate bridge contracts. To address this issue, Polygon has introduced the concept of the AggLayer. The AggLayer v1 mainnet is scheduled to launch in February.
AggLayer allows any network capable of generating ZKPs for computations to connect, regardless of whether it's L1 or L2. Proofs submitted by each network are recursively compressed within AggLayer, with only the final proof being submitted to the Ethereum network. Thanks to its unique structure, AggLayer offers several advantages:
Validity: Cross-chain communication is often cumbersome because verifying the validity of computations across different networks is complex. Networks connected to AggLayer submit proofs of computation to AggLayer, facilitating easy verification of transactions and enabling secure cross-chain transactions.
Sovereignty on Sequencing: Networks using a shared sequencing layer often sacrifice some sequencing and MEV revenues because they outsource sequencing. Networks connected to AggLayer can maintain their preferred sequencing mode while benefiting from atomic transactions.
UX Improvement: AggLayer can process transactions optimistically, offering users a significantly faster cross-chain experience if desired.
By leveraging AggLayer, for example, a user of Polygon zkEVM could purchase an NFT on another network using their ETH without needing to bridge, significantly enhancing the user experience. Let's explore how cross-chain messaging is facilitated through the use of AggLayer.
AggLayer offers two types of cross-chain messaging: 1) Optimistic confirmation and 2) Atomic cross-chain interaction. Before examining these methods, let's delve into how proof aggregation occurs in AggLayer.
3.2.1 Proof Aggregation
All Polygon chains connected to AggLayer manage a message queue, which lists messages to be sent to other Polygon chains. These messages contain information such as content, destination chain, destination address, and metadata.
When users initiate transactions on a specific chain, the sequencer organizes and executes these transactions, generating a ZKP that validates the chain's state and the message queue after computation. Instead of submitting these proofs directly to the Ethereum network, they are sent to AggLayer. AggLayer recursively compresses these proofs into a single proof, which is then submitted to the Ethereum network. Once the Ethereum network verifies the final proof, the connected Polygon chains can securely proceed with cross-chain messaging.
3.2.2 Optimistic Confirmation
A drawback of the above process is the time it takes for the final proof to be submitted and verified by the Ethereum network, which can significantly delay cross-chain messaging. To reduce latency, Polygon supports optimistic confirmation.
With optimistic confirmation, even if proof has not yet been generated, the mere submission of a Polygon chains' batch to AggLayer triggers an optimistic confirmation, allowing users to experience fast and seamless cross-chain messaging. This method relies on AggLayer's soft finality, similar to how optimistic rollups process transactions first and verify them later.
For example, if chain A sends a message to chain B, chain A initially submits only the batch and message queue to AggLayer without a ZKP. Although the transaction's validity cannot be guaranteed without proof, chain B proceeds with the transaction based on the submitted data and generates its proof. AggLayer later checks the consistency of proofs received from chains A and B. If AggLayer does not receive proof from chain A or if the proofs are contradictory, chain B will roll back the transaction, and chain A may be penalized.
3.2.3 Atomic Cross-Chain Interaction
While optimistic confirmation offers the advantage of processing transactions ahead of time and later verifying their validity through ZKPs, enhancing both UX and security, it inherently results in asynchronous cross-chain messaging. That is, the execution of a transaction on chain A does not guarantee the execution of a related transaction on chain B.
Polygon introduces atomic cross-chain interaction in AggLayer to address this, allowing users to initiate synchronous cross-chain transactions across multiple Polygon chains through atomic transactions. This means both transactions on chains A and B either succeed or fail, which is crucial for transactions like arbitrage between two chains.
The process follows: users submit an atomic bundle to AggLayer, and Aggregator Workers distribute the transactions within the bundle to the respective Polygon chains. These chains freeze the related states to prevent other transactions from altering the state until the atomic bundle is processed. Once the transactions in the bundle are correctly executed, the bundle is included in an AggLayer block, and each chain generates a ZKP for the block executing the bundle, submitting it to AggLayer. Upon receiving all proofs, AggLayer unlocks the previously frozen states.
This approach secures against malicious attacks aimed at asset theft by freezing states from the time of bundle submission until validation. However, it's vulnerable to attacks intended to disrupt the system, where an attacker submits a bundle affecting numerous states and delays processing by not generating proof. AggLayer counters such attacks by blacklisting malicious users and chains.
AggLayer distinguishes itself from other solutions in the cross-chain ecosystem in three main ways. First is the use of ZKP. With the maturation of ZK proving and verifying, many bridges are now utilizing ZKP for consensus verification, such as Polyhedra's zkBridge, Polymer Labs, and Electron Labs, with Wormhole also revealing a ZKP-focused roadmap. The adoption of ZK technology will become not just an option but a necessity in the future of cross-chain ecosystems.
Second, networks connected to AggLayer can maintain their own validator/sequencer set while executing atomic transactions. This is made possible by isolating an entity, referred to as the Aggregator Worker, solely for processing atomic transactions and freezing the related state until a validity proof is submitted. This autonomy in block production (sequencing and validating) is a significant benefit for networks, allowing them to retain revenue from these activities.
Third, AggLayer is not a siloed system. Unlike L1 and L2 protocols that support cross-chain messaging only within their ecosystems, AggLayer enables connections to networks outside the Polygon ecosystem, provided they can generate ZKP for executions. As more protocols adopt ZK technology, the number of networks connected to AggLayer is expected to increase.
While AggLayer introduces a new narrative, several questions arise regarding its implementation:
Aggregator Worker: How will the entity responsible for processing atomic bundles be selected and operated?
Proof Aggregation: How will the order of proofs be managed in the compression process using a Merkle tree?
Frontend: What kind of UI/UX will be provided for cross-chain messaging?
Polygon AggLayer leverages ZK technology to propose a fast and secure method of cross-chain messaging. It abstracts the backend process of cross-chain messaging, aiming to offer users an experience of interacting with a single network despite engaging with multiple networks. However, to truly revolutionize UX, innovation is required not only in the backend but also in the front end. For instance, if AggLayer enables users of Polygon zkEVM to purchase NFTs on network X1, frontend services like wallets should allow users to view NFTs held on X1 without switching networks. As we move towards an increasingly fragmented multi-chain era, it's worth looking forward to how AggLayer will innovate UX with its mainnet launch in February.
Thanks to Kate for designing the graphics for this article.
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