title: What does "Never use a proprietary cipher" mean?
body: <p>I'm taking cryptography course on cousera.
  When I'm reading the lecture slides, it says</p>
  
  <hr>
  
  <p>Encryption algorithm is publicly known</p>
  
  <h2>• Never use a proprietary cipher</h2>
  
  <p>What does "Never use a proprietary cipher" mean?</p>
  
  <p>Thanks</p>