OAuth stands for Open Authorization. It is an open standard for token-based authentication and authorization. OAuth provides client applications a “secure delegated access” to server resources on behalf of a resource owner.Examples: While creating MVC applications we might see the click here to login with external links such as Google, Facebook, twitter, Linkedin etc.,. So in order to use these external links, we need to authencate the users with third party resources, as they provide access in order to use their websites to login with our applications.
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O Auth is security feature of Asp.net mvc , like asp.net has three Authentication feature as Form Authentication, window Authentication, passport Authentication, instead of passprot Authentication we use O Auth Authentication in MVC .
what is OAuth. Suppose user has some resources stored on the server and there is a third party application which wants’ to access these user resources. This is a scenario which we might have encountered many times in our lives. Let’s say we have got some really cool pictures in Flickr that we want to share with our Facebook friends. We go to Facebook which redirects us to Flickr where we provide our authentication details and we are done. The great thing in all this is that we never need to share our Flickr details with Facebook.This is an example of OAuth authorization.
More detail to implement OAuth in MVC 5:OAuth 2 Implementation(MSDN)
REST Web API is Access by the user (client) which should be checked the authenticated or not again which resources autherized to that user should check.In .net Web api use OAuth security via a specific username/password with the combination of a secret key