RESTfull web service using oAuth for mobile application

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http://stackoverflow.com – I am building application that consists of two main parts: Web application with rest API. Mobile application(android, iPhone). Web application has database that stores data about users (like posts, events and similar). Mobile application uses web application's rest api to access that data, but to do that I need some kind of authentication (user must authenticate itself in order to access/modify his data). I know that could be easily done in such way that mobile application provides username and password in each request towards web api, and web application authenticates that username an (HowTos)