title: Best way in iOS and Swift to store a server-supplied credential between app launches
body: <p>I need to store a persistent remember token (a string) between app launches and device restarts. The token will be provided by a server once my user logs into the app and its back end service (which is already built). Specifically, I need to set up a persistent data placeholder for the remember token but I don't ever want the code to actually set the value of that placeholder.</p>
  
  <p>On the one hand, it seemed like NSUserDefaults (now called <a href="https://developer.apple.com/reference/foundation/userdefaults" rel="nofollow noreferrer">UserDefaults</a>) was a simple way to do this, but after reading the documentation, it doesn't seem like that was the intention of the feature. All the documentation I've see shows setting it up by assigning a value to a key. I definitely don't want to ever have the app assign a value to that key.</p>
  
  <p>What's the simplest way to do this?</p>