Offline Signing and Broadcasting

c8ntinuum EVM transactions use standard Ethereum transaction construction, signing, raw-transaction encoding, broadcasting, and receipt polling. Use eth_sendRawTransaction to submit an offline-signed withdrawal. The JavaScript and Java examples below are excerpted from compile-tested samples. See Tested Exchange Examples.

Online and offline split

Step	Environment	Data
Fetch nonce and fees	Online	`eth_getTransactionCount`, `eth_gasPrice`, `eth_maxPriorityFeePerGas`, `eth_estimateGas`
Build unsigned transaction	Online or signing coordinator	`chainId`, `nonce`, `to`, `value`, gas fields, optional `data`
Sign transaction	Offline signer or HSM	Private key, unsigned transaction
Analyze signed transaction	Offline and online	Hash, sender, chain ID, nonce, recipient, value, data selector
Broadcast	Online	Raw signed transaction bytes via `eth_sendRawTransaction`
Confirm	Online	`eth_getTransactionReceipt` with `status == 1`

Never move private keys to the online broadcasting host. Move only unsigned transaction data into the signer and signed raw transactions out of it. The examples below run the online and offline steps in one process for brevity. In production, run only the signing step, with the private key, on the isolated signer or HSM, and pass it the unsigned transaction fields that the online host prepared.

Transaction fields

Field	Native CTM transfer	ERC-20 transfer
`to`	Recipient account	Token contract
`value`	Amount in wei	`0`
`data`	Empty	ABI-encoded `transfer(address,uint256)`
`gasLimit`	Usually `21000`	Use `eth_estimateGas` plus policy buffer
`chainId`	`8` mainnet, `8888` testnet	Same
`type`	Dynamic fee transaction is recommended	Dynamic fee transaction is recommended

JavaScript

import {
  Interface,
  JsonRpcProvider,
  Transaction,
  Wallet,
  parseEther,
  parseUnits,
} from "ethers";

const provider = new JsonRpcProvider("https://public-evm-rpc.c8ntinuum.com");
const chainId = 8;
const privateKey = process.env.PRIVATE_KEY;
const signer = new Wallet(privateKey);

async function feeFields() {
  const feeData = await provider.getFeeData();
  return {
    maxFeePerGas: feeData.maxFeePerGas ?? feeData.gasPrice,
    maxPriorityFeePerGas: feeData.maxPriorityFeePerGas ?? 0n,
  };
}

async function signNativeTransfer(to) {
  const from = await signer.getAddress();
  const nonce = await provider.getTransactionCount(from, "pending");
  const fees = await feeFields();

  const unsignedTx = {
    type: 2,
    chainId,
    nonce,
    to,
    value: parseEther("1.0"),
    gasLimit: 21000n,
    ...fees,
  };

  const raw = await signer.signTransaction(unsignedTx);
  const decoded = Transaction.from(raw);
  console.log({
    hash: decoded.hash,
    from: decoded.from,
    chainId: decoded.chainId?.toString(),
    nonce: decoded.nonce,
    to: decoded.to,
    value: decoded.value.toString(),
  });

  return raw;
}

async function signErc20Transfer(token, recipient) {
  const from = await signer.getAddress();
  const nonce = await provider.getTransactionCount(from, "pending");
  const erc20 = new Interface(["function transfer(address to,uint256 amount) returns (bool)"]);
  const data = erc20.encodeFunctionData("transfer", [
    recipient,
    parseUnits("12.5", 18),
  ]);

  const fees = await feeFields();
  const gasLimit = await provider.estimateGas({
    from,
    to: token,
    value: 0,
    data,
  });

  return signer.signTransaction({
    type: 2,
    chainId,
    nonce,
    to: token,
    value: 0,
    data,
    gasLimit: (gasLimit * 120n) / 100n,
    ...fees,
  });
}

async function broadcast(raw) {
  const sent = await provider.broadcastTransaction(raw);
  const receipt = await sent.wait();
  if (receipt.status !== 1) throw new Error(`transaction failed: ${sent.hash}`);
  return receipt;
}

Java

import java.math.BigInteger;
import java.util.Arrays;
import org.web3j.abi.FunctionEncoder;
import org.web3j.abi.TypeReference;
import org.web3j.abi.datatypes.Address;
import org.web3j.abi.datatypes.Bool;
import org.web3j.abi.datatypes.Function;
import org.web3j.abi.datatypes.generated.Uint256;
import org.web3j.crypto.Credentials;
import org.web3j.crypto.RawTransaction;
import org.web3j.crypto.TransactionDecoder;
import org.web3j.crypto.TransactionEncoder;
import org.web3j.protocol.Web3j;
import org.web3j.protocol.core.DefaultBlockParameterName;
import org.web3j.protocol.core.methods.request.Transaction;
import org.web3j.protocol.core.methods.response.EthSendTransaction;
import org.web3j.protocol.http.HttpService;
import org.web3j.utils.Convert;
import org.web3j.utils.Numeric;

Web3j web3j = Web3j.build(new HttpService("https://public-evm-rpc.c8ntinuum.com"));
Credentials credentials = Credentials.create(System.getenv("PRIVATE_KEY"));
long chainId = 8L;

BigInteger nonce = web3j
    .ethGetTransactionCount(credentials.getAddress(), DefaultBlockParameterName.PENDING)
    .send()
    .getTransactionCount();
BigInteger maxPriorityFeePerGas = web3j.ethMaxPriorityFeePerGas().send().getMaxPriorityFeePerGas();
BigInteger maxFeePerGas = web3j.ethGasPrice().send().getGasPrice().multiply(BigInteger.valueOf(2));

RawTransaction nativeTx = RawTransaction.createEtherTransaction(
    chainId,
    nonce,
    BigInteger.valueOf(21_000),
    "0x1111111111111111111111111111111111111111",
    Convert.toWei("1.0", Convert.Unit.ETHER).toBigInteger(),
    maxPriorityFeePerGas,
    maxFeePerGas);

byte[] signed = TransactionEncoder.signMessage(nativeTx, chainId, credentials);
String raw = Numeric.toHexString(signed);
var decoded = TransactionDecoder.decode(raw);

System.out.println("raw=" + raw);
System.out.println("hash=" + org.web3j.crypto.Hash.sha3(raw));
System.out.println("from=" + credentials.getAddress());
System.out.println("chainId=" + chainId);
System.out.println("nonce=" + decoded.getNonce());
System.out.println("to=" + decoded.getTo());
System.out.println("value=" + decoded.getValue());
System.out.println("gasLimit=" + decoded.getGasLimit());
System.out.println("maxPriorityFeePerGas=" + maxPriorityFeePerGas);
System.out.println("maxFeePerGas=" + maxFeePerGas);

EthSendTransaction sent = web3j.ethSendRawTransaction(raw).send();
String txHash = sent.getTransactionHash();
System.out.println("broadcastHash=" + txHash);

Function transfer = new Function(
    "transfer",
    Arrays.asList(new Address("0x1111111111111111111111111111111111111111"), new Uint256(new BigInteger("12500000000000000000"))),
    Arrays.asList(new TypeReference<Bool>() {}));
String data = FunctionEncoder.encode(transfer);
String token = "0xc8Fb80fCc03f699C70ff0CC08C09106288888888";
BigInteger estimatedGas = web3j
    .ethEstimateGas(Transaction.createEthCallTransaction(credentials.getAddress(), token, data))
    .send()
    .getAmountUsed();
BigInteger erc20GasLimit = estimatedGas.multiply(BigInteger.valueOf(120)).divide(BigInteger.valueOf(100));

RawTransaction erc20Tx = RawTransaction.createTransaction(
    chainId,
    nonce.add(BigInteger.ONE),
    erc20GasLimit,
    token,
    BigInteger.ZERO,
    data,
    maxPriorityFeePerGas,
    maxFeePerGas);

String erc20Raw = Numeric.toHexString(
    TransactionEncoder.signMessage(erc20Tx, chainId, credentials));
var decodedErc20 = TransactionDecoder.decode(erc20Raw);
// web3j 4.14 returns decoded data without the 0x prefix.
System.out.println("erc20DataSelector=" + Numeric.prependHexPrefix(decodedErc20.getData()).substring(0, 10));

Go

package withdrawals

import (
	"context"
	"crypto/ecdsa"
	"math/big"
	"strings"

	"github.com/ethereum/go-ethereum"
	"github.com/ethereum/go-ethereum/accounts/abi"
	"github.com/ethereum/go-ethereum/common"
	"github.com/ethereum/go-ethereum/core/types"
	"github.com/ethereum/go-ethereum/crypto"
	"github.com/ethereum/go-ethereum/ethclient"
)

var chainID = big.NewInt(8)

func SignNativeTransfer(ctx context.Context, client *ethclient.Client, key *ecdsa.PrivateKey, to common.Address, amount *big.Int) ([]byte, common.Hash, error) {
	from := crypto.PubkeyToAddress(key.PublicKey)
	nonce, err := client.PendingNonceAt(ctx, from)
	if err != nil {
		return nil, common.Hash{}, err
	}
	tip, err := client.SuggestGasTipCap(ctx)
	if err != nil {
		return nil, common.Hash{}, err
	}
	feeCap, err := client.SuggestGasPrice(ctx)
	if err != nil {
		return nil, common.Hash{}, err
	}

	tx := types.NewTx(&types.DynamicFeeTx{
		ChainID:   chainID,
		Nonce:     nonce,
		GasTipCap: tip,
		GasFeeCap: new(big.Int).Mul(feeCap, big.NewInt(2)),
		Gas:       21_000,
		To:        &to,
		Value:     amount,
	})

	signed, err := types.SignTx(tx, types.LatestSignerForChainID(chainID), key)
	if err != nil {
		return nil, common.Hash{}, err
	}

	raw, err := signed.MarshalBinary()
	if err != nil {
		return nil, common.Hash{}, err
	}
	return raw, signed.Hash(), nil
}

func SignERC20Transfer(ctx context.Context, client *ethclient.Client, key *ecdsa.PrivateKey, token common.Address, recipient common.Address, amount *big.Int) (*types.Transaction, error) {
	erc20ABI, err := abi.JSON(strings.NewReader(`[{"name":"transfer","type":"function","inputs":[{"name":"to","type":"address"},{"name":"amount","type":"uint256"}],"outputs":[{"type":"bool"}]}]`))
	if err != nil {
		return nil, err
	}
	data, err := erc20ABI.Pack("transfer", recipient, amount)
	if err != nil {
		return nil, err
	}

	from := crypto.PubkeyToAddress(key.PublicKey)
	nonce, err := client.PendingNonceAt(ctx, from)
	if err != nil {
		return nil, err
	}
	tip, err := client.SuggestGasTipCap(ctx)
	if err != nil {
		return nil, err
	}
	feeCap, err := client.SuggestGasPrice(ctx)
	if err != nil {
		return nil, err
	}
	gas, err := client.EstimateGas(ctx, ethereum.CallMsg{From: from, To: &token, Data: data})
	if err != nil {
		return nil, err
	}

	tx := types.NewTx(&types.DynamicFeeTx{
		ChainID:   chainID,
		Nonce:     nonce,
		GasTipCap: tip,
		GasFeeCap: new(big.Int).Mul(feeCap, big.NewInt(2)),
		Gas:       gas * 120 / 100,
		To:        &token,
		Value:     big.NewInt(0),
		Data:      data,
	})
	return types.SignTx(tx, types.LatestSignerForChainID(chainID), key)
}

func Broadcast(ctx context.Context, client *ethclient.Client, signed *types.Transaction) error {
	return client.SendTransaction(ctx, signed)
}

Rust

Dependencies: alloy 2.x with default features, tokio with the macros and rt-multi-thread features, and eyre. Set FROM_ADDRESS to the sender address and PRIVATE_KEY to its key.

use alloy::consensus::{SignableTransaction, TxEip1559, TxEnvelope};
use alloy::eips::eip2718::Encodable2718;
use alloy::network::TxSignerSync;
use alloy::primitives::{address, Address, Bytes, TxKind, U256};
use alloy::providers::{Provider, ProviderBuilder};
use alloy::signers::local::PrivateKeySigner;

fn build_native_tx(
    nonce: u64,
    to: alloy::primitives::Address,
    value: U256,
    max_fee_per_gas: u128,
    max_priority_fee_per_gas: u128,
) -> TxEip1559 {
    TxEip1559 {
        chain_id: 8,
        nonce,
        gas_limit: 21_000,
        max_fee_per_gas,
        max_priority_fee_per_gas,
        to: TxKind::Call(to),
        value,
        input: Bytes::new(),
        access_list: Default::default(),
    }
}

fn sign_offline(tx: &mut TxEip1559, private_key: &str) -> eyre::Result<Vec<u8>> {
    let signer: PrivateKeySigner = private_key.parse()?;
    let signature = signer.sign_transaction_sync(tx)?;
    let signed = tx.clone().into_signed(signature);
    let envelope: TxEnvelope = signed.into();
    Ok(envelope.encoded_2718())
}

#[tokio::main]
async fn main() -> eyre::Result<()> {
    let provider = ProviderBuilder::new().connect_http(
        "https://public-evm-rpc.c8ntinuum.com".parse()?,
    );

    // Online: read the sender's nonce and current fees. Leave headroom over the base fee.
    let from: Address = std::env::var("FROM_ADDRESS")?.parse()?;
    let nonce = provider.get_transaction_count(from).pending().await?;
    let max_priority_fee_per_gas = provider.get_max_priority_fee_per_gas().await?;
    let max_fee_per_gas = provider.get_gas_price().await? * 2;

    let mut tx = build_native_tx(
        nonce,
        address!("1111111111111111111111111111111111111111"),
        U256::from(1_000_000_000_000_000_000u128),
        max_fee_per_gas,
        max_priority_fee_per_gas,
    );

    let raw = sign_offline(&mut tx, &std::env::var("PRIVATE_KEY")?)?;
    let pending = provider.send_raw_transaction(&raw).await?;
    let receipt = pending.get_receipt().await?;

    if !receipt.status() {
        eyre::bail!("transaction failed");
    }

    Ok(())
}

Signed data analysis

Before broadcasting, decode the raw signed transaction and verify:

chainId is 8 for mainnet or 8888 for testnet.
from matches the expected hot wallet, withdrawal wallet, HSM key, or custody account.
nonce matches the reserved nonce in your withdrawal database.
to, value, and data match the approved withdrawal instruction.
ERC-20 withdrawals call transfer(address,uint256) on the intended token contract.
Gas fields satisfy your policy and are not extreme.
The raw transaction hash is stored before broadcast.

Broadcasting and polling

Broadcast with eth_sendRawTransaction, then poll eth_getTransactionReceipt by hash until one of these happens:

Result	Action
Receipt `status == 1`	Mark withdrawal confirmed.
Receipt `status == 0`	Mark failed and investigate before retrying.
`null` receipt	Continue polling until timeout.
Nonce too low	Check if the transaction was already mined or replaced.
Transaction underpriced	Rebuild with a replacement transaction using the same nonce and higher fee, if policy allows.
Insufficient funds	Stop retries until the sender has native CTM for value plus gas.

Store every raw transaction, decoded transaction, broadcast hash, receipt, and replacement attempt for auditability.

Introduction

Developer Essentials

Tooling and Infrastructure

Reference

Guides

Resources

Offline Signing and Broadcasting

Online and offline split

Transaction fields

JavaScript

Java

Go

Rust

Signed data analysis

Broadcasting and polling

​Online and offline split

​Transaction fields

​JavaScript

​Java

​Go

​Rust

​Signed data analysis

​Broadcasting and polling

Online and offline split

Transaction fields

JavaScript

Java

Go

Rust

Signed data analysis

Broadcasting and polling