title: iOS: encrypt / decrypt video file
body: <p>We are in the process of talking about how to set up an online video rental store. Besides streaming we would love it if the user would be able to download a movie first, which is then stored for a certain amount of time on the device itself, in this scenario an iOS-device.</p>
  
  <p>But how can we achieve this and keep it save? Obviously we don't want the user to be able to simply copy the file from the filesystem which is a pretty easy task once the device is jailbroken.</p>
  
  <p>So there is the need to store the file encrypted or with some sort of DRM. But even then: How to make sure that the user can't decrypt it himself? And once playback is started how can we achieve that we don't decrypt the whole file (which would be too big to store it in the memory) so that the user doesn't get access it.</p>
  
  <p>Any ideas / solutions?</p>