--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/progs/App4.js	Tue Sep 23 01:06:00 2014 +0100
@@ -0,0 +1,44 @@
+var express = require('express');
+var cookie  = require('cookie-parser')
+var crypto = require('crypto');
+
+var app = express();
+app.use(cookie());
+
+var salt = 'secret key' 
+
+function mk_hash(s) {
+    return crypto.createHash('sha1').update(s).digest('hex')
+}
+
+function mk_cookie(c) {
+    return c.toString() + '-' + mk_hash(c.toString() + salt)
+}
+
+function gt_cookie(s) {
+    var splits = s.split("-", 2);
+    var counter = parseInt(splits[0])
+    var hash = splits[1]
+    if (mk_hash(counter.toString() + salt) == hash) {
+	return counter
+    } else {
+	return 0
+    }
+}
+
+
+app.get('/', function(req, res){
+    var counter = gt_cookie(req.cookies.counter) || 0;
+    res.cookie('counter', mk_cookie(counter + 1));
+    if (counter >= 5) {
+	res.write('You are a valued customer ' + 
+		  'visting the site ' + counter + ' times.');
+    } else {
+	res.write('This is visit number '+ counter +'!');
+    }
+    res.end();
+});
+
+// starting the server
+app.listen(8000);
+console.log("Server running at http://127.0.0.1:8000/");