Android App with Reward system.

Android App with Reward system. Back end Technologies - PHP MySQL REST service. I need a very basic, Android App which how has a reward system of its own, the rewards will be given to the users in form of coins. This project also includes a Simple PHP based back end application (RESTful web services), which creates a user account when the user registers, and maintains user's coin balance, when the coins are added or deducted from user's account. 1. Users should be able to Register and Sign in using their Google accounts. 2. When a new user account is created, a corresponding account will be created on the server with let's say 50 coins. 3. When the user is logged in the app shows a single screen, with the current coin balance and two buttons. 4. First button rewards coins to the user and second button deducts coins from user's account. 5. When any of the button is pressed, the App shows a (Waiting/Spinning image), makes a web service call to the back end, adds/deducts coins from user's account on the server, then the current balance is updated on the screen. 6. The webservice calls for Adding/Removing coins, Obtaining Coin Balance, need to be protected by user authentication. 7. Need a secure authentication mechanism, between the App and the Server preferable using OAuth2. I am open for discussion in terms of how Authentication can be implemented, Please suggest your ideas and past experience.