David Kimura PRO said almost 4 years ago on Adaptive Bitrate Streaming with Active Storage :
  this will add a layer of complexity as the video is divided into small segments, but the end user browser still gets an unencrypted stream. You cannot really prevent downloading since the end user will need to play the video. However, if you were to have the video segments encrypted on the server side and provide a key to the end user to have their browser decrypt the video, this would add a level of complexity that most would just assume that they cannot access the videos.