Introduction:

With Scroll’s recent token launch gaining attention, the spotlight is now on its technical architecture and the potential it holds for Ethereum scalability. As an EVM-equivalent zkRollup solution, Scroll not only ensures full compatibility with the Ethereum Virtual Machine (EVM) but also leverages zero-knowledge proofs to significantly improve transaction efficiency and security. In this article, we’ll explore the architecture and working principles of Scroll from a technical perspective, including the Sequencer, zkEVM, Rollup Contracts, and Bridge. Whether you're an investor curious about the technology or a developer looking to build on Scroll, this comprehensive analysis will help you understand what makes Scroll a critical player in the Layer 2 space.

The Overall Architecture of Scroll：

Scroll’s architecture consists of three primary layers:

Sequencer Layer: This layer handles transaction reception, block generation, and interaction with the Ethereum mainnet.

Prover Layer: The prover layer generates zero-knowledge proofs (zk-SNARKs) to verify the correctness of transactions on Layer 2.

Rollup and Bridge Contracts: These contracts ensure data availability and enable trustless asset transfers between Layer 1 (L1) and Layer 2 (L2).

These layers work in concert to provide a scalable, decentralized, and EVM-compatible environment for processing a high volume of transactions while leveraging Ethereum’s security guarantees.

The Role of the Sequencer

The Sequencer is one of the core components of Scroll, responsible for receiving, processing, and generating Layer 2 blocks. The Sequencer performs several key functions:

1. Transaction Reception and Execution

The Sequencer interacts with users via a JSON-RPC interface, gathering transactions for Layer 2. It periodically selects batches of transactions from the Layer 2 mempool, executing them to produce new L2 blocks.

2. State Updates and Block Generation

As transactions are executed, the Sequencer updates the Layer 2 state root in real time. Once a new block is generated, the Sequencer forwards the execution trace and updated state root to the coordinator, which then triggers the Roller network to generate zk-SNARKs proofs. Because the Sequencer is built on a modified version of Go-Ethereum (Geth), it inherits Ethereum’s security and stability while ensuring full EVM equivalence.

3. Interaction with Rollup Contracts

After generating a new L2 block, the Sequencer ensures data availability by submitting the transaction data to Ethereum’s Rollup contracts. This guarantees that if the Layer 2 network fails, users can still recover transaction data from Layer 1.

zkEVM: The Core of Scroll’s Technology

zkEVM is the heart of Scroll, providing the mechanism by which Layer 2 transactions are validated through zero-knowledge proofs, all while maintaining full compatibility with Ethereum's EVM.

1. Zero-Knowledge Proofs

zkEVM relies on zk-SNARKs (Zero-Knowledge Succinct Non-Interactive Arguments of Knowledge) to verify the correctness of transactions without revealing transaction details. Scroll leverages zk-SNARKs to ensure that all Layer 2 transactions are executed according to the rules of the EVM.

2. zkEVM Circuit Design

zkEVM functions through a series of sophisticated cryptographic circuits that simulate the workings of the EVM. These circuits handle:

Storage access: How data is read from and written to storage.

Computation: Simulating EVM’s bytecode execution.

State transitions: Encoding the state changes after transactions are executed.

These circuits then produce the cryptographic proofs necessary for zk-SNARKs, which serve as evidence that the transactions are valid and compliant with the EVM.

3. Proof Generation and Aggregation

After the circuits generate individual zk-SNARKs proofs for transactions, they are submitted to the Scroll coordinator. To minimize the data footprint on Ethereum, Scroll utilizes proof aggregation, which combines multiple zk-SNARKs into a single, smaller proof. This drastically reduces the cost and computational load of verifying multiple transactions on Layer 1.

The Role of Rollup Contracts and Bridge Contracts

Scroll interacts with Ethereum through Rollup contracts and Bridge contracts, both of which are crucial for ensuring data availability and enabling asset transfers.

1. Data Availability

To ensure the availability of transaction data, Scroll’s Rollup contract stores the data for each Layer 2 block as calldata on Ethereum. This guarantees that even if the Layer 2 network experiences downtime, all data can still be recovered from the Ethereum mainnet, preserving decentralization and data integrity.

2. Cross-Chain Bridge and Asset Transfer

The Bridge contracts facilitate trustless transfers of assets and messages between Layer 1 and Layer 2. This process involves:

Deposits: Users lock assets on Layer 1 (Ethereum) and receive equivalent tokens on Layer 2 (Scroll).

Withdrawals: Users burn Layer 2 tokens, triggering the release of the corresponding assets from the Layer 1 contract.

Bridge contracts ensure the security of these transfers, with Rollup contracts validating the state transitions to guarantee correctness.

The Complete Scroll zkRollup Workflow

The Scroll zkRollup process can be broken down into several stages:

1. Transaction Execution

Users submit transactions to Layer 2 through the Sequencer or via a Layer 1 bridge. The Sequencer processes these transactions, updates the Layer 2 state, and generates new blocks, forwarding the execution trace to the coordinator.

2. Data Submission and Proof Generation

The Sequencer submits the transaction data as calldata to the Rollup contract on Ethereum to guarantee data availability. Simultaneously, the Roller network generates zk-SNARKs proofs, which are relayed to Ethereum for validation.

3. Block Verification and Finalization
The Rollup contract on Ethereum verifies the zk-SNARKs proofs, ensuring that the Layer 2 blocks were executed correctly. Once verified, the blocks are finalized, and the transactions are considered immutable.

Scroll’s Advantages and Challenges

Scroll presents several key advantages:

1.EVM Equivalence:

Scroll offers full compatibility with Ethereum’s EVM, allowing developers to migrate existing Ethereum dApps to Scroll without code changes.

2. Efficiency through zk-SNARKs:

By using zk-SNARKs, Scroll minimizes the computational and storage costs of validating transactions, making it an efficient scaling solution for Ethereum.

3. Data Availability and Recovery:

Even if Scroll’s Layer 2 network encounters issues, all transaction data can be retrieved from Ethereum’s calldata, ensuring continuous access to transaction records.

However, Scroll also faces certain challenges, especially with decentralizing the Sequencer. While the current Sequencer is centralized, Scroll aims to decentralize this component in the future, distributing the task of transaction processing across multiple nodes.