Generate HMAC in NodeJS

gen_hmac.js

var crypto = require('crypto');
var SHARED_SECRET = "sup3rs3cr3t!!";

function signString(string_to_sign, shared_secret) {
	var hmac = crypto.createHmac('sha512', shared_secret);
	hmac.write(string_to_sign);
	hmac.end()
	return hmac.read();
}

var payload = {
	'name': 'joe smith',
	'category': 'people',
	'action': 'transport',
	'where': 'pluto',
	'timestamp': Math.round(new Date().getTime()/1000)
};

var json_payload = JSON.stringify(payload);
var signature = signString(json_payload, SHARED_SECRET);

var encoded_json = new Buffer(json_payload).toString('base64');
var encoded_signature = new Buffer(signature).toString('base64');

console.log("/?data="+encoded_json+"&signature="+encoded_signature);