Iphone 5 app talking to Joomla 2.5 website

I have a Joomla 2.5 website which runs on a PHP5/Apache server with MySQL. I am creating an App for iphone5 that will perform following: 1. Login: username/password combination entered on the iphone 2. Display content for the logged in user in XML 3. Upload/Download: Upload file from iphone or download from webserver. a file from iPhone The idea is to have some code which can read user-specific data from a MySQL database and write data into such database. There shold not be a direct connection from app to the database, it has to use Joomla logic to talk to database. Rest API is preferred.