app.js

//
// Copyright (c) Microsoft. All rights reserved.
// Licensed under the MIT license.
//
const crypto = require('crypto');
const sharedSecret = "<Security token generated by Microsoft Teams>"; // e.g. "+ZaRRMC8+mpnfGaGsBOmkIFt98bttL5YQRq3p2tXgcE="
const bufSecret = Buffer(sharedSecret, "base64");

var http = require('http');
var PORT = process.env.port || process.env.PORT || 8080;

http.createServer(function(request, response) { 
	var payload = '';
	// Process the request
	request.on('data', function (data) {
		payload += data;
	});
	
	// Respond to the request
	request.on('end', function() {
		try {
			// Retrieve authorization HMAC information
			var auth = this.headers['authorization'];
			// Calculate HMAC on the message we've received using the shared secret			
			var msgBuf = Buffer.from(payload, 'utf8');
			var msgHash = "HMAC " + crypto.createHmac('sha256', bufSecret).update(msgBuf).digest("base64");
			// console.log("Computed HMAC: " + msgHash);
			// console.log("Received HMAC: " + auth);
			
			response.writeHead(200);
			if (msgHash === auth) {
				var receivedMsg = JSON.parse(payload);
				var responseMsg = '{ "type": "message", "text": "You typed: ' + receivedMsg.text + '" }';	
			} else {
				var responseMsg = '{ "type": "message", "text": "Error: message sender cannot be authenticated." }';
			}
			response.write(responseMsg);
			response.end();
		}
		catch (err) {
			response.writeHead(400);
			return response.end("Error: " + err + "\n" + err.stack);
		}
	});
		
}).listen(PORT);

console.log('Listening on port %s', PORT);