title: Protect associated files in app
body: <p>I am making an app which shows learning videos for kids. The videos used in this app are stored in an external SD card .
  I want to know how can I encrypt these files so that no one can access these video files.
  My idea is to encrypt the video files and decrypt these video files dynamically, i.e., when one is accessing a particular file, then only that video will decrypt and played but not stored.
  How can I achieve this? Has anyone did this ?
  Any better ideas ?</p>