Axelar cross-chain gateway protocol solidity implementation

Protocol overview

Axelar is a decentralized interoperability network connecting all blockchains, assets and apps through a universal set of protocols and APIs. It is built on top off the Cosmos SDK. Users/Applications can use Axelar network to send tokens between any Cosmos and EVM chains. They can also send arbitrary messages between EVM chains.

Axelar network's decentralized validators confirm events emitted on EVM chains (such as deposit confirmation and message send), and sign off on commands submitted (by automated services) to the gateway smart contracts (such as minting token, and approving message on the destination).

Build

node version

We recommend using the current Node.js LTS version for satisfying the hardhat compiler

Run in your terminal

npm ci

npm run build

npm run test  # Test with hardhat

Example flows

Token transfer

1. Setup: A wrapped version of Token A is deployed (AxelarGateway.deployToken()) on each non-native EVM chain as an ERC-20 token (BurnableMintableCappedERC20.sol).
2. Given the destination chain and address, Axelar network generates a deposit address (the address where DepositHandler.sol is deployed, BurnableMintableCappedERC20.depositAddress()) on source EVM chain.
3. User sends their token A at that address, and the deposit contract locks the token at the gateway (or burns them for wrapped tokens).
4. Axelar network validators confirm the deposit Transfer event using their RPC nodes for the source chain (using majority voting).
5. Axelar network prepares a mint command, and validators sign off on it.
6. Signed command is now submitted (via any external relayer) to the gateway contract on destination chain AxelarGateway.execute().
7. Gateway contract authenticates the command, and mint's the specified amount of the wrapped Token A to the destination address.

Token transfer via AxelarDepositService

1. User wants to send wrapped token like WETH from chain A back to the chain B and to be received in native currency like Ether.
2. The un-wrap deposit address is generated by calling AxelarDepositService.addressForNativeUnwrap().
3. The token transfer deposit address for specific transfer is generated by calling AxelarDepositService.addressForTokenDeposit() with using the un-wrap address as a destination.
4. User sends the wrapped token to that address on the source chain A.
5. Axelar microservice detects the token transfer to that address and calls AxelarDepositService.sendTokenDeposit().
6. AxelarDepositService deploys DepositReceiver to that generated address which will call AxelarGateway.sendToken().
7. Axelar network prepares a mint command, and it gets executed on the destination chain gateway.
8. Wrapped token gets minted to the un-wrap address on the destination chain B.
9. Axelar microservice detects the token transfer to the un-wrap address and calls AxelarDepositService.nativeUnwrap().
10. AxelarDepositService deploys DepositReceiver which will call IWETH9.withdraw() and transfer native currency to the recipient address.

Cross-chain smart contract call

1. Setup:
   1. Destination contract implements the IAxelarExecutable.sol interface to receive the message.
   2. If sending a token, source contract needs to call ERC20.approve() beforehand to allow the gateway contract to transfer the specified amount on behalf of the sender/source contract.
2. Smart contract on source chain calls AxelarGateway.callContractWithToken() with the destination chain/address, payload and token.
3. An external service stores payload in a regular database, keyed by the hash(payload), that anyone can query by.
4. Similar to above, Axelar validators confirm the ContractCallWithToken event.
5. Axelar network prepares an AxelarGateway.approveContractCallWithMint() command, signed by the validators.
6. This is submitted to the gateway contract on the destination chain, which records the approval of the payload hash and emits the event ContractCallApprovedWithMint.
7. Any external relayer service listens to this event on the gateway contract, and calls the IAxelarExecutable.executeWithToken() on the destination contract, with the payload and other data as params.
8. executeWithToken of the destination contract verifies that the contract call was indeed approved by calling AxelarGateway.validateContractCallAndMint() on the gateway contract.
9. As part of this, the gateway contract records that the destination address has validated the approval, to not allow a replay.
10. The destination contract uses the payload for it's own application.

Cross-chain NFT transfer/minter

See this example cross-chain NFT application.

Design Notes

• AxelarGateway.execute() takes a signed batched of commands. Each command has a corresponding commandID. This is guaranteed to be unique from the Axelar network. execute intentionally allows retrying a commandID if the command failed to be processed; this is because commands are state dependent, and someone might submit command 2 before command 1 causing it to fail.
• Axelar network supports sending any Cosmos/ERC-20 token to any other Cosmos/EVM chain.
• Supported tokens have 3 different types:
  • External: An external ERC-20 token on it's native chain is registered as external, e.g. USDC on Ethereum.
  • InternalBurnableFrom: Axelar wrapped tokens that are minted by the Axelar network when transferring over the original token, e.g. axlATOM, axlUSDC on Avalanche.
  • InternalBurnable: v1.0.0 version of Axelar wrapped tokens that used a different deposit address contract, e.g. UST (native to Terra) on Avalanche. New tokens cannot be of this type, and this is only present for legacy support.
• Deploying gateway contract:
  • Deploy the AxelarAuthWeighted contract.
  • Deploy the TokenDeployer contract.
  • Deploy the AxelarGateway contract with the token deployer address.
  • Deploy the AxelarGatewayProxy contract with the implementation contract address (from above) and setup params obtained from the current network state.

Smart Contracts

Interfaces

IAxelarGateway.sol

IERC20.sol

IERC20BurnFrom.sol

IAxelarExecutable.sol

This interface needs to be implemented by the application contract to receive cross-chain messages. See the token swapper example for an example.

Contracts

AxelarGatewayProxy.sol

Our gateway contracts implement the proxy pattern to allow upgrades. Calls are delegated to the implementation contract while using the proxy's storage. setup is overidden to be an empty method on the proxy contract to prevent anyone besides the proxy contract from calling the implementation's setup on the proxy storage.

AxelarGateway.sol

The implementation contract that accepts commands signed by Axelar network's validators (see execute). The signature proof verification is performed by AxelarAuthWeighted contract.

AxelarAuthWeighted.sol

Weighted multisig authentication contract that is used by the gateway. It accepts a set of operators with corresponding weights. To verify the message weights of provided signatures are summed and need to meet the specified threshold

AdminMultisigBase.sol

Multisig governance contract. Upgrading the implementation is done via voting on the new implementation address from admin accounts.

ERC20.sol

Base ERC20 contract used to deploy wrapped version of tokens on other chains.

ERC20Permit.sol

Allow an account to issue a spending permit to another account.

MintableCappedERC20.sol

Mintable ERC20 token contract with an optional capped total supply (when capacity != 0). It also allows us the owner of the ERC20 contract to burn tokens for an account (IERC20BurnFrom).

BurnableMintableCappedERC20.sol

The main token contract that's deployed for Axelar wrapped version of tokens on non-native chains. This contract allows burning tokens from deposit addresses generated (depositAddress) by the Axelar network, where users send their deposits. salt needed to generate the address is provided in a signed burn command from the Axelar network validators.

TokenDeployer.sol

When the Axelar network submits a signed command to deploy a token, the token deployer contract is called to deploy the BurnableMintableCappedERC20 token. This is done to reduce the bytecode size of the gateway contract to allow deploying on EVM chains with more restrictive gas limits.

DepositHandler.sol

The contract deployed at the deposit addresses that allows burning/locking of the tokens sent by the user. It prevents re-entrancy, and while it's methods are permisionless, the gateway deploys the deposit handler and burns/locks in the same call (see _burnToken).

Ownable.sol

Define ownership of a contract and modifiers for permissioned methods.

EternalStorage.sol

Storage contract for the proxy.

ECDSA.sol

Modified version of OpenZeppelin ECDSA signature authentication check.

AxelarDepositService.sol

This service is used to generate deposit addresses for an ERC20 token or native currency transfer. The third type of deposit address is for unwrapping native currency from a wrapped ECR20 token.

AxelarGasService.sol

This contract is used for cross-chain gas payment. It accepts payments for covering gas cost on the destination chain. Gas payment should happen with the same params right before calling callContract or callContractWithToken on the gateway.

References

Network resources: https://docs.axelar.dev/resources

Token transfer app: https://satellite.money/

General Message Passing Usage: https://docs.axelar.dev/dev/gmp

Example token transfer flow: https://docs.axelar.dev/dev/cli/axl-to-evm

Deployed contracts: https://docs.axelar.dev/resources/mainnet

EVM module of the Axelar network that prepares commands for the gateway: https://github.com/axelarnetwork/axelar-core/blob/main/x/evm/keeper/msg_server.go