You are designing a photo-sharing mobile app. The application will store all pictures in a single Amazon S3 bucket. Users will upload pictures from their mobile...

You are designing a photo-sharing mobile app. The application will store all pictures in a single Amazon S3 bucket.
Users will upload pictures from their mobile device directly to Amazon S3 and will be able to view and download their own pictures directly from Amazon S3.
You want to configure security to handle potentially millions of users in the most secure manner possible.
What should your server-side application do when a new user registers on the photo-sharing mobile application?