title: 26 modulo in hill cipher encryption
body: <p>I am reading about <a href="http://en.wikipedia.org/wiki/Hill_cipher" rel="nofollow">Hill Cipher</a> encryption algorithm, from the wikipedia. I see that I have a key as a matrix that must multiply by the the matrix of values. But there are 2 things I don't understand. </p>
  
  <p>I don't know the mean of (mod 26). I know it is modulo 26, but I don't know for what it is applied?</p>
  
  <p>As for the second issue, I can't understand where the matrix [15 14 7] is coming from?</p>
  
  <p>Any good explanation will be highly appreciated .</p>