Skip to main content

Chromia bridge client setup

This topic will walk you through setting up the Chromia Bridge Client to facilitate token transfers between Chromia and Ethereum Virtual Machine (EVM) networks.

Prerequisites​

Before you begin, ensure you have the following:

  • TokenBridge contract: You need the address of a TokenBridge contract deployed on an EVM network.
  • ERC20 token: The address of an ERC20 token on the same EVM network.
  • Chromia blockchain: The blockchain RID of a Chromia blockchain using the hbridge Rell library.

Configuration checklist​

Approve ERC20 token​

The ERC20 token must be approved by the TokenBridge contract owner using the allowToken function. You can accomplish this with the @chromia/bridge-client using the allowToken method:

const contractTransactionResponse = await bcl.allowToken(token);

Check spending allowance​

Verify the spending allowance with the checkAllowance method:

const allowance: bigint = await bcl.checkAllowance();

Set blockchain RID​

Use the setBlockchainRid function in the TokenBridge contract to set the blockchain RID. You can use the built-in method setBlockchainRid with @chromia/bridge-client:

const contractTransactionResponse = await bcl.setBlockchainRid(rid);

Validator configuration​

Convert the public keys of the validating nodes in the Chromia network to EVM addresses and configure them in the Validator contract, either during deployment or by calling the updateValidators function.

Work with the client​

To handle all Chromia-related queries and operations, the bridge client requires an active Session. However, you don't need to instantiate it with a session immediately.

Set up your application with @chromia/ft4:​

Set up a connection to Postchain​

codeconst pcl = await createClient({
  nodeUrlPool: 'YOUR_NODE_URL_POOL',
  blockchainRid: 'YOUR_BLOCKCHAIN_RID',
});

Create an account​

const evmKeyStore = await createWeb3ProviderEvmKeyStore(window.ethereum);
const ad = createSingleSigAuthDescriptorRegistration([AuthFlag.Account, AuthFlag.Transfer], evmKeyStore.id);
const response = await registerAccount(pcl, evmKeyStore, registrationStrategy.open(ad));

User login​

const evmKeyStoreInteractor = createKeyStoreInteractor(pcl, evmKeyStore);
const accounts = await evmKeyStoreInteractor.getAccounts();
const session = await evmKeyStoreInteractor.getSession(accounts[0].id);
note

Ensure that you save your evmKeyStore in your application state, as some methods in @chromia/bridge-client will require it.

With your Session object ready, you can initialize the bridge client. You will also need to provide an EVM Provider, either a BrowserProvider or a JsonRpcProvider:

const provider = new BrowserProvider(window.ethereum);
const bcl = await bridgeClient(
  { bridgeAddress: "YOUR_BRIDGE_ADDRESS", tokenAddress: "YOUR_TOKEN_ADDRESS" },
  provider,
  session
);

If your application setup prevents having an active Session when instantiating the bridge client, you can still set up the client without it:

const provider = new BrowserProvider(window.ethereum);
const bcl = await bridgeClient({ bridgeAddress: "YOUR_BRIDGE_ADDRESS", tokenAddress: "YOUR_TOKEN_ADDRESS" }, provider);

Later, when you have access to the Session, you can set it on the client with setSession(session: Session):

bcl.setSession(session);

Example usage: Bridge from EVM to Chromia and vice versa​

Bridge from EVM to Chromia​

Approve token spending​

Ensure the user approves token spending by the token bridge:

const approvalResponse = await bcl.approveDepositAmount(BigInt(10));

Deposit to Chromia EVM bridge​

Use the depositToEvmBridgeContract method, specifying the number of tokens to bridge:

const contractTransactionResponse = await bcl.depositToEvmBridgeContract(BigInt(100));

After a deposit, link the EVM account with the corresponding FT4 account created during the deposit process. Provide the evmKeyStore you created earlier:

const accountLinkingResponse = await bcl.linkEvmEoaAccount(evmKeyStore);
note

If the EVM account is linked, it will be returned to the accountLinkingResponse. An EVM account can have multiple FT4 accounts linked to it.

Bridge from Chromia to EVM​

Bridge from Chromia​

Call the bridgeFromChromia method with the amount and the asset ID:

// Network ID does not need to be provided as it will be fetched from the provider
const transactionResponse = await bcl.bridgeFromChromia(BigInt(10), Buffer.from("YOUR_ASSET_ID", "hex"));

Request withdrawal from EVM bridge​

Create a pending withdrawal request​

This needs to be accepted by the user to start the withdrawal process using the requestEvmWithdraw method:

const erc20WithdrawalInfo = await bcl.getErc20WithdrawalByTransactionRid(
  transactionResponse.receipt.transactionRid,
  opIndex
);
// Get event proof for withdrawal
const eventProof = await bcl.getWithdrawRequestEventProof(erc20WithdrawalInfo.event_hash);
// Request withdrawal
const requestedWithdraw = await bcl.requestEvmWithdraw(eventProof);

Check withdrawal status​

Depending on the bridge contract configuration, the user must wait a certain number of blocks on EVM to complete their withdrawal. This can be done using the getPendingWithdrawFromProof method:

const { block_number } = await getPendingWithdrawFromProof(eventProof);

Once the block_number has been reached on the target EVM chain, the user can withdraw their tokens:

const withdrawal = await bcl.evmWithdraw(eventProof.leaf as Buffer);

Additional methods​

  • getErc20Deposits(filter?: DepositFilter, pageSize?: number, pageCursor?: string): Returns all deposits specified by the filter.
  • getErc20Withdrawals(filter?: WithdrawFilter, pageSize?: number, pageCursor?: string): Returns all withdrawals from the EVM bridge specified by the filter.
  • setBlockchainRid(blockchainRid: Buffer): Sets the blockchain RID.