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){ .... });