Section 2: Bridge Feature

The zkEVM bridge uses the same logic as the Ethereum 2.0 deposit contract, with some changes added on this basis.

Relevant differences

• It uses a specially designed append-only Merkle tree and introduces exit trees and global exit trees

• The deposit contract uses SHA256, while zkEVM uses the Keccak hash function. This is all because, in addition to being EVM compatible, Keccak hash functions are also cheaper in terms of Ethereum gas fees.

• The first time a bridge adds a new token to the zkEVM network, the bridge generates a wrapped token. Additionally, the ERC20 token’s metadata, such as name, decimal or symbol, is added to the information contained in the leaves.

Therefore, as stated in this document, every transfer is protected by a smart contract.

Global exit tree

The main feature of the zkEVM bridge smart contract is the use of exit trees and global exit trees, where the global exit tree root serves as the main source of state facts.

The use of two different global exit root managers for L1 and L2, as well as separate logical bridges and each global exit root manager, allows for broad network interoperability.

At the same time, all asset transfers can be verified by any L1 and L2 node due to data availability.