¿Cómo declarar claves privadas al usar node.js y ethereum para ejecutar un contrato?

   var infuraApiKey =process.env.INFURA_API_KEY;
  // var privateKey = process.env.PRIVATE_KEY;

   var web3js = new web3(new web3.providers.HttpProvider("https://kovan.infura.io/v3/"+infuraApiKey));

   web3js.eth.defaultAccount = myAddress;
   var privateKey=new Buffer(process.env.PRIVATE_KEY, 'hex');

//   var toAddress = 'ADRESS_TO_SEND_TRANSACTION';

   //contract abi is the array that you can get from the ethereum wallet or etherscan
   var contractABI =bonusABI;
   var contractAddress =bonusAddress;
   //creating contract object
   var contract =  web3js.eth.contract(contractABI).at(contractAddress);
   var count;
   var nounce;
   var errcode="";
   web3js.eth.getTransactionCount(myAddress, function(err, result) {
        nounce=result;
        var nounceHex = web3js.toHex(nounce);

        var rawTransaction = {"from":myAddress,
        "gasPrice":web3js.toHex(2*1e9),
        "gasLimit":web3js.toHex(920000),
        "to":contractAddress,
        "data":contract.addBonus.getData(bonusType, target, year, month, day, token, bonus, bonusName, ineq),
        "nonce":nounceHex}

        var transaction = new Tx(rawTransaction);
        transaction.sign(privateKey);

        var serializedTx = transaction.serialize();
        web3js.eth.sendRawTransaction('0x'+serializedTx.toString('hex'), function(err1, hash) {
           if (!err1) {
               errcode=hash;
          }
           else
               errcode=err1;
        });
  })

puede agregarlo a su archivo .env si solo está utilizando una cuenta para transferir fondos, o puede almacenarlo en forma cifrada en una base de datos https://www.npmjs.com/package/bcrypt .

Existe una forma mejor y más sencilla de firmar y ejecutar la función de contrato inteligente. Aquí su función es addBonus.

En primer lugar, crearemos la instancia de contrato inteligente:

 const createInstance = () => {
  const bscProvider = new Web3(
    new Web3.providers.HttpProvider(config.get('bscRpcURL')),
  );
  const web3BSC = new Web3(bscProvider);
  const transactionContractInstance = new web3BSC.eth.Contract(
    transactionSmartContractABI,
    transactionSmartContractAddress,
  );
  return { web3BSC, transactionContractInstance };
};

Ahora crearemos una nueva función para iniciar sesión y ejecutar la función addBonus

   const updateSmartContract = async (//parameters you need) => {
     try {
    const contractInstance = createInstance();
// need to calculate gas fees for the addBonus
    const gasFees =
      await contractInstance.transactionContractInstance.methods
        .addBonus(
         // all the parameters
        )
        .estimateGas({ from: publicAddress_of_your_desired_wallet });
   const tx = {
      // this is the address responsible for this transaction
      from: chainpalsPlatformAddress,
      // target address, this could be a smart contract address
      to: transactionSmartContractAddress,
      // gas fees for the transaction
      gas: gasFees,
      // this encodes the ABI of the method and the arguments
      data: await contractInstance.transactionContractInstance.methods
        .addBonus(
       // all the parameters
        )
        .encodeABI(),
    };
  // sign the transaction with a private key. It'll return messageHash, v, r, s, rawTransaction, transactionHash
    const signPromise =
       await contractInstance.web3BSC.eth.accounts.signTransaction(
        tx,
        config.get('WALLET_PRIVATE_KEY'),
      );
    // the rawTransaction here is already serialized so you don't need to serialize it again
    // Send the signed txn
    const sendTxn =
      await contractInstance.web3BSC.eth.sendSignedTransaction(
        signPromise.rawTransaction,
      );
    return Promise.resolve(sendTxn);
} catch(error) {
  throw error;
}
    }

Feliz codificación :)