title: Character frequency scoring operation
body: <p>The cryptography challenge as stated at <a href="https://cryptopals.com/sets/1/challenges/3" rel="nofollow">https://cryptopals.com/sets/1/challenges/3</a>
  is : </p>
  
  <pre><code>Single-byte XOR cipher
  The hex encoded string:
  
  1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736
  ... has been XOR'd against a single character. Find the key, decrypt the message.
  
  You can do this by hand. But don't: write code to do it for you.
  
  How? Devise some method for "scoring" a piece of English plaintext. Character frequency is a good metric. Evaluate each output and choose the one with the best score.
  </code></pre>
  
  <p>Is the solution to this ? : </p>
  
  <p>As the inverse of XOR is XOR should I XOR the string 1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736 with each character in range <code>a-z</code> but how should I score this string ? How is character frequency related to score ?</p>