I am building a project for Windows Phone (8.1) in Unity. I have installed Windows SDK 8.1, MS VS 2013 and I have required .NET frameworks installed in my Windows 8.1 Laptop. When I build the project in Unity, it gives me VS .sln project. I opened it in VS 2013 and tried to build it. But, I am getting a error i.e. Cannot find type System.Security.Cryptography.SymmetricAlgorithm in module mscorlib.dll I have read posts about this kind of error and those didn't serve my purpose because I didn't import the mentioned namespace in the error in anywhere of the Unity Project. I searched in the plugins too. I also changed the target framework in MonoDevelop to 4.5. I am using Unity 5.3 and MonoDevelop as IDE. Any Help is highly appreciated.