title: Android Fingerprint invalidation
body: <p>I'm playing around with the <a href="https://developer.android.com/samples/FingerprintDialog/index.html" rel="nofollow noreferrer">Fingerprint demo for Android</a>, in particular the invalidation scenarios, but need a little help to translate this into production worthy logic. </p>
  
  <p>I've tested the app and get the initCipher to fail due to invalidation after adding a fingerprint, but the app has to be running and the key generated while you change the settings. This is because the demo generates a new key each time the app starts. In reality you wouldn't want to do this, but instead generate the key if doesn't exist and reuse it if it does to enforce proper invalidation whether the app is running or not. </p>
  
  <p>How can you modify the app so that the key isn't generated each time, but instead a check to see if one exists first is performed, then that key loaded subsequently? Can you then remove the key once invalidated so the previous logic and enrolment cycle applies?</p>