Transfer Near tokens on behalf of a controlled account

In this part of the tutorial, we'll dig into how transaction arguments are built, how to request a signature according to the arguments from the smart contract, and how to broadcast the signed transaction into the network.

Building transaction arguments

Now that we've set up the account and derived the public key, it's time to build the transaction arguments.

For a transaction to be valid, we must attach both a nonce and a recent_block_hash - these values ensure that the transaction is unique and prevent replay attacks.

Let's fetch nonce first:

const accessKey = await near.connection.provider.query({
  request_type: "view_access_key",
  account_id: controllableAccountId, // "controllable.testnet"
  public_key: derivedPublicKey, // derived in the previous chapter
  finality: "optimistic",
});

const nonce = accessKey.nonce;

And recent_block_hash, now it's your turn:

const block = await near.connection.provider.block({
  finality: "final",
});

const blockHash = block.header.hash;

Now that we have everything needed, let's build transaction arguments:

const transactionArgs = {
  signer_id: controllableAccountId, // "controllable.testnet"
  signer_pk: derivedPublicKey, // derived in the previous chapter
  nonce: (nonce + 1).toString(),
  block_hash: blockHash,
};

Requesting signature from Smart Contract

After building transaction arguments, the next step is to request a signature from the smart contract.

const adminAccount = await near.account("admin.testnet");

const outcome = await adminAccount.functionCall({
  contractId: contractId, // "broken-rock.testnet"
  methodName: "transfer_on_behalf_of",
  args: {
    args: transactionArgs,
  },
  gas: "300000000000000",
  attachedDeposit: "100000000000000000000000", // 0.1 NEAR is enough in most cases to pay MPC fee
});

Broadcasting the transaction

With the signature in hand, we are now ready to send it to the network!

// Get the signed transaction from the outcome
result = providers.getTransactionLastResult(outcome);
const signedTx = new Uint8Array(result);

// Send the signed transaction
const transferOutcome = await near.connection.provider.sendJsonRpc(
  "broadcast_tx_commit",
  [Buffer.from(signedTx).toString("base64")]
);

console.log(
  `https://nearblocks.io/txns/${transferOutcome.transaction_outcome.id}`
);

Final Thoughts

With that, we've successfully completed the full journey of acting on behalf of another Near account securely and transferred to yourself 0.1 NEAR from a controllable account. By following this process, you now have a solid understanding of how to manage external accounts on Near using Multi-Party Computation (MPC).