ZK-Rollup

Rollup is a widely adopted Layer 2 solution in blockchain scaling, known for batching multiple off-chain transactions into a single transaction which will be executed on the host chain. A key feature of Rollup is its ability to inherit the security guarantees of the host chain, maintaining equivalent security properties. There are two primary types of Rollups: Optimistic Rollups and Zero-Knowledge Rollups. They employ different transaction verification mechanisms, and therefore offer different trade-offs between latency, cost-effectiveness, and security.

The Rollup technology is implemented by the Rollup Nodes and Endpoints. The Rollup Node plays a critical role in processing transactions, primarily responsible for sequencing and batching transactions. These transactions are then relayed to an Endpoint contract deployed on the Layer 1 blockchain. A key feature of this framework is the incorporation of Merkle proofs, which are essential during the withdrawal processes to ensure the integrity of transactions.

Rollup State Update

This figure illustrates a typical state transition in the Rollup verification procedure. When a transaction is sent to Layer 2, it updates the blockchain’s virtual machine (VM) state. Afterwards, the transaction details and the updated Layer 2 VM state are synchronized with Layer 1 via a smart contract call. Subsequently, Layer 2 generates a Zero-Knowledge (ZK) proof , enabling the Layer 1 contract to verify that the old VM state in Layer 2 correctly transitions to the new VM state after executing the transaction. This process demonstrates how transactions occurring on Layer 2 are validated on Layer 1, explaining why the security guarantees of Layer 2 are underpinned by the Layer 1.

Another critical concept in ZK-Rollup is data availability, referring to the assurance that all necessary data for validating and verifying Layer 2 transactions is publicly accessible to all stakeholders. In the context of blockchain, data availability ensures that all nodes in the network can retrieve and validate the data required to maintain the integrity and security of the blockchain.