title: Most secure way to combine two keys
body: <p>I am developing a application where the idea is to combine two separate keys into a single key. From my research there exists two common ways to do this:</p>
  
  <p>1) Use concatenation of hashes and perhaps hash again
  2) Use HMAC</p>
  
  <p>Which would be the more secure approach, and if possible is there any source that could be used to back a specific approach?</p>