Mob Crypto Challenge

gistfile1.js

function decrypt(cipherText, key) {
  var keyInt = key.codePointAt(0),
      plainText = '';

  for (var char = 0; char < cipherText.length; char += 2)
    plainText += String.fromCharCode(parseInt(cipherText.substr(char, 2), 16) ^ keyInt);

  return plainText;
}

function getKey(cipherText) {
  var commonChars = 'eotha sinrd luymw fgcbp kvjqxz',
      mostLikely = 0,
      key = '';

  for (var possibleKey = 0; possibleKey <= 127; possibleKey++) {
    var possibleMatch = '',
        chanceOfMatch = 0;

    for (var plainChar = 0; plainChar <= cipherText.length; plainChar += 2)
      possibleMatch += String.fromCharCode(parseInt(cipherText.substr(plainChar, 2), 16) ^ possibleKey);

    for (var possibleChar = 0; possibleChar < possibleMatch.length; possibleChar++)
      for (var commonChar = 0; commonChar < commonChars.length; commonChar++)
        if (possibleMatch[possibleChar].toLowerCase() == commonChars[commonChar]) {
          chanceOfMatch++;
          break;
        }

    if (chanceOfMatch > mostLikely) {
      key = String.fromCharCode(possibleKey);
      mostLikely = chanceOfMatch;
    }
  }

  return key; 
}

var cipherText = '6B4A52055C4A500257400548444E4C4B4205484A474C49400548445151405704', 
    key = getKey(cipherText);

console.log('key:', key);
console.log('decrypt:', decrypt(cipherText, key));