ROT-13 letter substitution cipher Constructor. Provides ROT-13 encoding and decoding.

Rot13.js

function Rot13() {
	var result = "";
	var decode = false;
	
	var lookup = function(char, i) {
		var lt1 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz ";
		var lt2 = "NOPQRSTUVWXYZABCDEFGHIJKLMnopqrstuvwxyzabcdefghijklm ";
		var orig = (decode) ? lt2: lt1;
		var enc = (decode) ? lt1: lt2;

		// base case
		if (i >= orig.length) {
			return;
		} else if (char === orig[i]) {
			result += enc[i];
		}
		lookup(char, i+1);
	};
	
	var cypher = function(name, i) {
		if (i > name.length) { return; }
		lookup(name[i], 0);
		cypher(name, i+1);
		return result;
	};
	
	this.encode = function(text) {
		result = "";
		return cypher(text, 0);
	};
	
	this.decode = function(text) {
		decode = true;
		result = "";
		return cypher(text, 0);
	};
}

var text = new Rot13();
console.log(text);

var name = text.encode("Brian Teachman"); // "Oevna Grnpuzna"
var token = text.decode(name); // "Brian Teachman"
console.log(name+" => "+token);

Here is my take on it so you do not have to figure out if decode or encode (figures it out) & added more characters so you can even encode JavaScript source code ;)

function cypher(r) {
  function t(r, e) {
    var a = "\n\x20\t`~_^*'\"<+,-.:;=>!?/{|}[\\]#$%&()@0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
    a.length > e && (r === a.split("").reverse().join("")[e] && (n += a[e]), t(r, e + 1));
  }
  function e(r, a) {
    return a > r.length ? void 0 : (t(r[a], 0), e(r, a + 1), n);
  }
  var n = "";
  return e(r, 0);
}

var name = cypher("Dave Browne modded this <script><\/script>");
var token = cypher(name);
console.log(name+" => "+token);