title: Is "double hashing" a password less secure than just hashing it once?
body: <p>Is hashing a password twice before storage any more or less secure than just hashing it once?</p>
  
  <p>What I'm talking about is doing this:</p>
  
  <pre><code>$hashed_password = hash(hash($plaintext_password));
  </code></pre>
  
  <p>instead of just this:</p>
  
  <pre><code>$hashed_password = hash($plaintext_password);
  </code></pre>
  
  <p>If it is less secure, can you provide a good explanation (or a link to one)?</p>
  
  <p>Also, does the hash function used make a difference?  Does it make any difference if you mix md5 and sha1 (for example) instead of repeating the same hash function?</p>
  
  <p>Note 1:  When I say "double hashing" I'm talking about hashing a password twice in an attempt to make it more obscured.  I'm not talking about the <a href="http://en.wikipedia.org/wiki/Double_hashing" rel="noreferrer">technique for resolving collisions</a>.</p>
  
  <p><strong>Note 2:  I know I need to add a random salt to really make it secure.  The question is whether hashing twice with the same algorithm helps or hurts the hash.</strong></p>