I am searching a good way to encrypt and decrypt user authentication data between my Android app and my ASP.NET Webservice built in C#. I want to do a user registration on my Android app and send the authentication to the back-end server that is a ASP.NET Webservice built in C# and then every time the app calls the webservice it should send a hashed string with authentication information so webservice now what user it is. What is the best practice for this and have i missed something, give me some ideas please!