app.js

// npm install eth-crypto --save
const EthCrypto = require('eth-crypto');

const privateKey = '0x61ce8b95ca5fd6f55cd97ac60817777bdf64f1670e903758ce53efc32c3dffeb';

const encryptedMessage = {
	iv: '3c12aa4bdbb6219479eb0ebe62ee908f',
	ephemPublicKey: '04f70ca0c9e6741c8709de1cfec75f980823dd753b9c1a87cc4afbfdf44707b1830c0385b0fa808fa18f55a5f7551370242d95cd61546823fda2f73d26e1906a81',
	ciphertext: 'a21abfac3a44ca4c22bdfb23a97a115f',
	mac: 'b4d282d05422672689bd1027c948b2189dc52e7f9101e753be6fb3df844572c4'
};

(async() => {
	const decryptedMessage = await EthCrypto.decryptWithPrivateKey(privateKey, encryptedMessage);
	console.log(decryptedMessage);
})();

piyo! 😃