Interpretation of the beacon layer: the key to Rollup network security and cross-chain transfers

This article discusses the Enshrined cross-chain bridge and AltLayer's beacon layer architecture in the Ethereum Rollup network.

Written by: AltLayer

Compilation: Deep Tide TechFlow

Introduction: This article discusses the Enshrined cross-chain bridge in the Ethereum Rollup network and the beacon layer architecture of AltLayer. The Enshrined cross-chain bridge provides users with the ability to transfer assets between Ethereum and Rollup, while the beacon layer serves as an intermediate layer to provide key services such as shared ordering, verification, staking, and interoperability. Understanding these concepts will help us understand the security and asset transfer mechanisms of the Rollup network.

Preface

Recently, the Ethereum Rollup community has had a heated debate around what really defines classic Rollups such as Arbitrum, Optimism, and ZKSync.

Today's Rollups serve as off-chain transaction executors and provide a "native cross-chain bridge" between Rollup and Ethereum. We call this native cross-chain bridge the Enshrined cross-chain bridge. The cross-chain bridge allows users to lock an asset on Ethereum and mint an encapsulated asset of the same value on Rollup; on the contrary, when the Enshrined cross-chain bridge is sure that the encapsulated asset has been burned on Rollup, it can unlock the encapsulation asset from Ethereum. assets. Since the Enshrined cross-chain bridge relies on fraud proofs or validity proofs to ensure the validity of the Rollup state, it is trust-minimum.

At issue is whether the canonical state of a classic Rollup is defined by an Enshrined cross-chain bridge contract that tracks Rollup state on Ethereum, verifies fraud proofs (or ZK Rollup proofs of validity) and allows assets to be withdrawn from Rollups to Ethereum to define. Or, if defined by Rollup full nodes themselves, they observe the transaction data published by Rollup operators on Ethereum, then re-execute them by applying state transition functions (or check validity proofs in ZK Rollup), and finally verify the calculated Does the state of the match the state proposed by the Rollup operator.

This is an important distinction because if the Rollup validators themselves determine the canonical Rollup state, the Enshrined cross-chain bridge contract is just another observer of the Rollup state, just like other Rollup validators. In this case, the canonical state of Rollup is determined by society, not by the Enshrined cross-chain bridge contract on Ethereum.

Therefore, this "source of truth" distinction means that it is possible to build a non-Enshrined cross-chain bridge, run Rollup validator nodes in the background, and allow instant withdrawals from Rollup to Ethereum without waiting for Rollup's 7-day withdrawal period. They were able to allow instant withdrawals simply because Rollup validators running on non-Enshrined cross-chain bridges could be confident that even if someone later challenged the withdrawal, it would not succeed.

AltLayer has been thinking about this topic for a year and a half, and although it's more around our Rollup-as-a-service (RaaS) delivery of Rollup, we're in a more closed loop.

This article explains our position on this debate by supporting the network design of our RaaS offering.

For normal RaaS offerings, you can use the SDK or dashboard to start a Rollup, for example using Ethereum as the data availability layer. Also, a modular stack looks like:

Rollup (Execution) -> Ethereum (Data Availability) -> Ethereum (Settlement)

AltLayer's RaaS network has a slightly different architecture:

Rollup (Execution) -> Beacon Layer (Enshrined Interlayer) -> Ethereum (Data Availability) -> Ethereum (Settlement)

The beacon layer is an intermediate layer between the execution layer and the data availability layer, and all Rollups instantiated through AltLayer are Enshrined beacon layers. The term "Enshrined" refers to the fact that the state of any Rollup initiated by AltLayer can be directly verified by the beacon layer, and each Rollup has an Enshrined cross-chain bridge on the beacon layer. Furthermore, as an intermediate layer between the execution and data availability stacks, the beacon layer provides the following main services:

1. Shared sorting layer;
2. Verification layer;
3. Staking/slashing layer;
4. Interoperability layer;
5. Upgradability layer;
6. Social consensus layer.

We explain these services in more detail below.

Shared ordering layer: The beacon layer is a network where nodes interested in nodes acting as Rollup orderers can register their interest. When an end user requests a Rollup through AltLayer's RaaS platform, the user specifies the minimum and maximum number of sorters required to operate the Rollup, as well as the minimum economic guarantee amount required for each sorter and the list of tokens available for guarantee. These collaterals are then staked on the beacon layer, which is then used to slash any misbehaving orderers. Once the required number of coorters have staked the minimum collateral, those coorters can start queuing transactions for the Rollup.

Validation Layer: As mentioned in the preface, a Rollup's validation nodes themselves can determine the normalized Rollup state. However, this means that each client needs to run its own full node Rollup validator if they don't want to wait for the Enshrined cross-chain bridge's 7-day withdrawal period. This may not be ideal for many lighter clients, especially Optimistic Rollup. Therefore, the beacon layer acts as an Enshrined verification layer for all Rollups. Since the beacon layer knows about all Rollups and their respective state transition functions, it has the ability to fully validate the state of newly submitted Rollups. To this end, the beacon layer consists of a set of validators that periodically validate all new states across all Rollup proposals. Unlike the pre-confirmations provided by the Rollup orderer, the beacon layer provides stronger than usual confirmations, as this allows assets to be withdrawn immediately.

Pledge/slicing layer: Since shared sorting and verification assume a set of node networks, it is necessary to ensure that the network is free from Sybil attacks, so the beacon layer has a pledge mechanism that requires each network participant to deposit a sufficient amount of economic guarantee to When malicious behavior is detected and can be proven, it can be punished and reduced.

Interoperability layer: If the RaaS provider has many instantiated Rollups, in order to have interoperability between Rollups, an Enshrined cross-chain bridge is usually built on the DA layer, one for each Rollup. Another option is to build a cross-chain bridge between every two Rollups that use the same DA layer.

AltLayer adopts the third design, using the beacon chain as a bridging center to coordinate the interaction between all Rollups. In this design, each Rollup incorporates an Enshrined cross-chain bridge into the beacon layer, which then operates as a shared intermediate "settlement layer" on top of the base DA layer. By enshrining cross-chain bridges to this shared intermediate layer, Rollups can directly obtain trust-minimized bridges, and can also provide indirect trust-minimized bridges for other Rollups that also enshrined cross-chain bridges to the beacon layer. In effect, as a general-purpose settlement layer, the Beacon Layer acts as a bridging hub, facilitating asset transfers between Rollups, but also general messaging. It can also help detect any invalid state transitions across Rollups, and even provide dispute resolution for validating fraud proofs.

Upgradable Layer: As mentioned in the preface, most Rollups today have an Enshrined cross-chain bridge to Ethereum. This cross-chain bridge tracks Rollup state, verifies fraud proofs (or proofs of validity for ZK Rollups), and allows assets to be withdrawn from Rollups to Ethereum. However, most of these Enshrined cross-chain bridges are controlled by multi-signature wallets, so cross-chain bridges have the ability to arbitrarily steal user funds through malicious upgrades.

The beacon layer acts as a social layer for upgrading all summaries it contains. Register to operate Rollup's sequencer and beacon layer Rollup validators socially fork Rollup, regardless of whether the Enshrined cross-chain bridge contract on Ethereum has been upgraded. Remember, Enshrined bridges are just Rollup observers, just like other Rollup validators.

Sequencers and Rollup validators on the beacon layer who disagree with the fork may decide not to support the new fork.

Social Consensus Layer: The beacon layer can also act as a governance layer, where beacon layer nodes can run an on-chain governance mechanism to approve the upgrade of Rollup logic, so they can act on behalf of the community to upgrade any cross-chain on the beacon layer or DA layer bridge contract.

in conclusion

This article describes a key component of AltLayer's network architecture, known as the beacon layer. It acts as a common middle layer between different Rollups and underlying data availability layers such as Ethereum launched through AltLayer's RaaS platform.