System.Security.dll
is not an assembly that Xamarin.iOS ships. The version number is not correct either. This generally happens when adding a reference to code that was not built with Xamarin.iOS assemblies (or PCL).
IOW your application must be using something else (beside monotouch, System, System.Core and System.Xml_ or one of your references is not the assemblies that Xamarin.iOS ships (e.g. an assembly that comes from MS .NET).
How do I solve this?
Review all your assemblies reference and make sure they are all coming from Xamarin.iOS shipped assemblies.