[рони]

Екатерина13,

<script>
function crypt(message, key, decrypt) {
    var a = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".split("");
    message = message.split("");
    key = ("" + key).split("");
    return message.reduce(function(message, current) {
        var i = a.indexOf(current),
            b = +key.shift();
        key.push(b);
        decrypt ? (i -= b, i < 0 && (i += a.length)) : (i += b, i %= a.length);
        return message + a[i]
    }, "")
};

  document.write(crypt("RAMMSTEIN", 2015)+"<br>") // encrypt
  document.write(crypt("TANRUTFNP", 2015, true))  // decrypt
  </script>