Bridge
Asset transfer

Asset transfer#

This section describes how asset transfers are enabled by three smart contracts.

Bridge smart contract#

The zkEVM bridge smart contract (ZkEVMBridge.sol) has two main functions:

  • Bridge (or deposit) function
  • Claim (or withdrawal) function

With these functions, we can bridge assets from one network to another, as well as claim assets received in the destination network.

  1. User Deposit transfers assets L2 -> L1 or L1 -> L2. The bridge smart contract SC adds an exit leaf to the L2 exit tree, which holds the relevant transfer data.
  2. Subsequently, the root of the L2 Exit Tree is updated and becomes accessible for the next smart contract in the workflow, known as the Global Exit Root Manager SC.
  3. User Claim assets on opposite chain bridge SC.
Note
There are two bridge smart contracts: ‌one deployed in L1, while the other is deployed in L2, both using the same ZkEVMBridge.sol code file, but using different parameters for the interaction.

Global exit root manager contract#

The Global Exit Root Manager SC, implemented as ZkEVMGlobalExitRoot.sol, serves as the overseer of the Global Exit Tree Root. Its primary responsibilities include updating the Global Exit Tree Root and serving as a storage hub for the historical data of the Global Exit Tree.

This contract has a main function:

  • updateExitRoot This separation of logic between the zkEVM bridge SC and the Global Exit Root Manager SC has been strategically implemented to enhance interoperability, which uses the function updateExitRoot.

This means that the zkEVM bridge SC, in conjunction with the Global Exit Root Manager SC, can be installed on any network to achieve the same results.

Note
There are two Global Exit Root manager SCs: one deployed in L1, while the other is deployed in L2. Their Solidity code files are; ZkEVMGlobalExitRoot.sol, and ZkEVMGlobalExitRootL2.sol, respectively.

Consensus contract#

The ZkEVM.sol smart contract is the consensus algorithm used in X Layer. It is located in L1 for batch verification purposes. The consensus smart contract (ZkEVM.sol) has two main functions:

  • sequenceBatchesValidium
  • verifyBatchesTrustedAggregator

With these functions, L2 tx data can be rolled up and verified.

  1. Transactions are grouped into batches by sequenceBatchesValidium function.
  2. A request is made to an aggregator to prove the validity of these sequenced batches by verifyBatchesTrustedAggregator. This proof is zk-proof and is verified using the Verifier.sol contract.
  3. Following the verification of the zk-proof, the consensus contract sends the zkEVM Exit Tree Root to the Global Exit Root Manager SC ZkEVMGlobalExitRoot.sol to update the Global Exit Root.

Smart contract interactions#

The following diagram illustrates the interactions among the three bridge-related smart contracts when assets are bridged. It describes the overall process of transferring assets from L1 to L2 and from L2 to L1 through contracts.

Smart contract interactions

L1 → L2 transfer#

  1. When the user commits to transferring assets, the bridge SC ZkEVMBridge.sol uses the Bridge method to append the corresponding exit leaf to the L1 Exit Tree.
  2. The updated root of the L1 Exit Tree is sent to the Global Exit Root Manager ZkEVMGlobalExitRoot.sol for an update of the Global Exit Root.
  3. The consensus contract ZkEVM.sol retrieves the updated Global Exit Root from the Global Exit Root Manager, which is used for syncing with the zkEVM as the destination network for bridged assets.

The transfer from L1 to zkEVM is completed with the Claim method of the bridge SC, but this time in the zkEVM side.

L2 → L1 transfer#

Now the reverse process, while focusing only at the L1 smart contracts.

  1. The consensus contract uses the sequenceBatchesValidium function to sequence batches, which includes, among other transactions, asset transfer information.
  2. A special smart contract called Verify.sol calls the VerifyBatches function and takes batch info as input. As part of the consensus process, but not shown in the above figure, the aggregator produces a validity proof for the sequenced batches. And this proof is automatically verified in this step.
  3. The zkEVM Exit Tree gets updated only after a successful verification of sequenced batches (again, for the sake of simplicity, this is not reflected in the above figure). The consensus contract (ZkEVM.sol) sends the updated zkEVM Exit Root to the Global Exit Root Manager, which subsequently updates the Global Exit Root.
  4. The zkEVM bridge SC (ZkEVMBridge.sol) then retrieves the updated Global Exit Root and uses the Claim function to complete the transfer.

Specific interaction in L2#

The focus in this subsection is on the bridge-related smart contracts used in L2, especially in the zkEVM.

  1. The bridge SC (ZkEVMBridge.sol) in L2 is the same smart contract deployed on L1.
  2. The L2 Global Exit Root Manager SC is different, and it appears in code as ZkEVMGlobalExitRootL2.sol. This is a special contract that allows synchronization of L2 Exit Tree Root and the Global Exit Root.
  3. The L2 Global Exit Root Manager has storage slots to store the Global Exit Root and the L2 Exit Tree Root. In order to correctly ensure validity of the synchronization between the Global Exit Tree and L2 Exit Tree, these storage slots are directly accessible to the low-level ZK-proof-generating circuit.

Here is the step-by-step process:

L2 interactions with ZK-circuit

  1. When a batch of transactions is processed, the zkEVM bridge SC ZkEVMBridge.sol appends an exit leaf with the batch information to the L2 Exit Tree and updates the L2 Exit Tree Root.
  2. The bridge SC communicates the L2 Exit Tree Root to the L2 Global Exit Root Manager. The L2 Global Exit Root Manager, however, does not update the Global Exit Tree at this stage.
  3. For proof and verification, the circuit for generating zk-proofs obtains the L2 exit tree root from the L2 Global Exit Root Manager.
  4. Only after the batch has been successfully proved and verified does the L2 Global Exit Root Manager append the L2 Exit Tree Root to the Global Exit Tree. As a result, the Global Exit Root is updated.

The circuit for generating zk-proofs also writes the L2 Exit Tree Root to the mainnet. The zkEVM bridge SC deployed on L1 can then finalize the transfer by using the Claim function.