How to decode JWT in PCL that's compatible with Xamarin Android

As you can see from http://developer.xamarin.com/guides/android/under_the_hood/assemblies/ the .NET framework in Mono for Android doesn't seem to contain the assembly System.Web.Extensions

Edit: this means that you are referencing a library that was created for desktop use, and has not been recompiled with the Android reference assemblies. This is not supported.