title: time-based encryption algorithm?
body: <p>I've an idea in my mind but I've no idea what the magic words are to use in Google - I'm hoping to describe the idea here and maybe someone will know what I'm looking for.</p>
  
  <p>Imagine you have a database.  Lots of data.  It's encrypted.  What I'm looking for is an encryption whereby to decrypt, a variable N must at a given time hold the value M (obtained from a third party, like a hardware token) or it failed to decrypt.</p>
  
  <p>So imagine AES - well, AES is just a single key.  If you have the key, you're in.  Now imagine AES modified in such a way that the algorithm itself requires an extra fact, above and beyond the key - this extra datum from an external source, and where that datum varies over time.</p>
  
  <p>Does this exist?  does it have a name?</p>