using Chilkat RSA with xamarin.ios

I am trying to use Chilkat RSA with my xamarin.ios project but can't seem to find the correct dependency

I have tried all the available chilkat packages on nuget :

• ChilkatNativeLib installs but I can't seem to import the Chilkat.RSA class in to my c# classes.

These packages fail with a nuget error saying target is not compatible with Xamarin.iOS,Version=v1.0:

• Chilkat.Net class library (x86)
  
• Chilkat.Net class library (x64)
• Chilkat runtime components for universal apps
• ChilkatDNCore

I am using Visual Studio for Mac version 7.1.5.

I would like clarification on which packages I should be using, and where i might be going wrong.