Interpreting the Beacon Layer: The Key to Rollup Network Security and Cross-Chain Transfers
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Interpreting the Beacon Layer: The Key to Rollup Network Security and Cross-Chain Transfers
This article discusses the Enshrined cross-chain bridge in Ethereum Rollup networks and AltLayer's Beacon Layer architecture.
Navigating Web3 tides with focused insightsAuthor: AltLayer
Translation: TechFlow
Introduction: This article explores the concept of Enshrined bridges within Ethereum's Rollup networks and AltLayer’s Beacon Layer architecture. The Enshrined bridge enables users to transfer assets between Ethereum and Rollups, while the Beacon Layer acts as an intermediate layer providing critical services such as shared sequencing, validation, staking, and interoperability. Understanding these concepts helps clarify the security model and asset transfer mechanisms in Rollup networks.
Preface
Recently, the Ethereum Rollup community has engaged in intense debate over what truly defines classic Rollups such as Arbitrum, Optimism, and ZKSync.
Today’s Rollups operate as off-chain transaction executors and provide a “native bridge” between the Rollup and Ethereum. We refer to this native bridge as the Enshrined bridge. This bridge allows users to lock an asset on Ethereum and mint a wrapped equivalent of equal value on the Rollup; conversely, when the Enshrined bridge confirms that the wrapped asset has been burned on the Rollup, it unlocks the original asset on Ethereum. Since the Enshrined bridge relies on fraud proofs or validity proofs to ensure the correctness of the Rollup state, it is minimally trusted.
The crux of the debate lies in whether the canonical state of a classic Rollup is defined by the Enshrined bridge contract on Ethereum—which tracks the Rollup state, verifies fraud proofs (or validity proofs for ZK Rollups), and enables withdrawals from the Rollup back to Ethereum—or whether it is defined by the Rollup full nodes themselves, which observe transaction data posted by the Rollup operator on Ethereum, re-execute them via the state transition function (or verify validity proofs in ZK Rollups), and validate whether the computed state matches the state proposed by the Rollup operator.
This distinction is significant because if Rollup validator nodes themselves determine the canonical Rollup state, then the Enshrined bridge contract is merely another observer of the Rollup state—just like any other Rollup validator node. In this case, the canonical state of the Rollup is socially determined rather than being dictated by the Enshrined bridge contract on Ethereum.
Therefore, this difference in the "source of truth" implies that non-Enshrined bridges can be built to run Rollup validator nodes in the background, enabling instant withdrawals from the Rollup to Ethereum without waiting for the typical 7-day withdrawal period. They can allow instant withdrawals simply because the Rollup validator nodes operated by the non-Enshrined bridge can be confident that any future challenges to the withdrawal would fail.
AltLayer has been thinking about this topic for a year and a half, although our focus has largely been around Rollups provided through our Rollup-as-a-Service (RaaS) offering, placing us in a more closed ecosystem.
This article explains our position in this debate through the network design supporting our RaaS products.
For standard RaaS offerings, you can use an SDK or dashboard to launch a Rollup—for example, using Ethereum as the data availability layer. In this case, the modular stack looks like:
Rollup (execution) → Ethereum (data availability) → Ethereum (settlement)
AltLayer’s RaaS network features 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 via AltLayer are Enshrined into the Beacon Layer. The term “Enshrined” means that the state of any Rollup launched via AltLayer can be directly verified by the Beacon Layer, and each Rollup hosts an Enshrined bridge on the Beacon Layer. Furthermore, as an intermediary between the execution and data availability layers, the Beacon Layer provides the following key services:
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Shared sequencing layer;
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Validation layer;
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Staking/slashing layer;
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Interoperability layer;
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Upgradability layer;
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Social consensus layer.
We elaborate on these services below.
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Shared Sequencing Layer: The Beacon Layer is a network where nodes interested in serving as Rollup sequencers can register their interest. When an end user requests a Rollup via AltLayer’s RaaS platform, they specify the minimum and maximum number of sequencers required to operate the Rollup, along with the minimum economic stake amount per sequencer and the list of tokens acceptable as collateral. These stakes are then deposited on the Beacon Layer and used to slash any misbehaving sequencers. Once the required number of sequencers have deposited the minimum stake, they can begin ordering transactions for the Rollup.
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Validation Layer: As discussed in the preface, Rollup validator nodes themselves can determine the canonical Rollup state. However, this implies that every client must run its own full Rollup validator node to avoid waiting for the 7-day withdrawal window of the Enshrined bridge—a requirement that may not be ideal for many lighter clients, especially for Optimistic Rollups. Therefore, the Beacon Layer serves as the Enshrined validation layer for all Rollups. Since the Beacon Layer knows all Rollups and their respective state transition functions, it has the capability to fully validate newly submitted Rollup states. To achieve this, the Beacon Layer consists of a set of validators that regularly verify all new proposed states across all Rollups. Unlike pre-confirmations provided by Rollup sequencers, confirmations from the Beacon Layer offer stronger finality, enabling immediate asset withdrawals.
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Staking/Slashing Layer: Since both shared sequencing and validation assume a network of nodes, it is essential to prevent Sybil attacks. Hence, the Beacon Layer includes a staking mechanism requiring each participant to deposit sufficient economic collateral, which can be slashed upon detection and proof of malicious behavior.
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Interoperability Layer: If a RaaS provider has many instantiated Rollups, achieving interoperability between them typically involves building an Enshrined bridge on the DA layer—one per Rollup—or establishing pairwise bridges between every two Rollups sharing the same DA layer.
AltLayer adopts a third design, leveraging the Beacon Layer as a central hub to coordinate interactions among all Rollups. In this design, each Rollup incorporates an Enshrined bridge into the Beacon Layer, which then operates as a shared intermediate “settlement layer” atop the base DA layer. By Enshrining bridges into this shared intermediate layer, Rollups gain direct trust-minimized bridging and can also offer indirect trust-minimized bridging to other Rollups that similarly Enshrine their bridges into the Beacon Layer. Effectively, as a universal settlement layer, the Beacon Layer acts as a bridge hub facilitating asset transfers between Rollups, as well as general message passing. It can also help detect invalid state transitions across Rollups and even provide dispute resolution for fraud proofs.
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Upgradability Layer: As mentioned in the preface, most Rollups today feature an Enshrined bridge connected to Ethereum. This bridge tracks the Rollup state, validates fraud proofs (or validity proofs for ZK Rollups), and enables withdrawals from the Rollup to Ethereum. However, these Enshrined bridges are mostly controlled by multi-signature wallets, meaning the bridge could potentially steal user funds through a malicious upgrade.
The Beacon Layer acts as a social layer for upgrading all Rollups it encompasses. Sequencers and Rollup validators registered on the Beacon Layer can socially fork the Rollup, regardless of whether the Enshrined bridge contract on Ethereum undergoes an upgrade. Recall that the Enshrined bridge is merely an observer of the Rollup, just like other Rollup validators.
Sequencers and Rollup validators on the Beacon Layer who disagree with the fork may choose not to support the new chain.
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Social Consensus Layer: The Beacon Layer can also serve as a governance layer, where Beacon Layer nodes can run on-chain governance mechanisms to approve upgrades to Rollup logic, enabling them to act on behalf of the community to upgrade any bridge contracts on the Beacon Layer or the DA layer.
Conclusion
This article introduced a key component of AltLayer’s network architecture—the Beacon Layer. It serves as a universal intermediate layer between various Rollups launched through AltLayer’s RaaS platform and the underlying data availability layer, such as Ethereum.
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