EDIT: See my own replies below.
My question's reason for existence:
I'm working on a Xamarin cross platform app (using PCL + forms), and I need to use System.Security.Cryptography and System.Net.Sockets. My PCL does not let me use either of these. My PCL is profile 78. I only intend to develop for mobile devices (iOS, Android) in particular. Windows phone would be a cherry on top, but is definitely not a requirement. I know that Android and iOS support both of these namespaces. I could simply use the DependencyService and put the same exact file in both my platform ios and platform droid folders, but then every time I edit one I would have to copy paste to the other.
How can I either:
A. Modify my PCL to let me use System.Net.Sockets and System.Security.Cryptography
B. Use the same C# file for both iOS and Android (i.e. my AES class or my Communications class that I currently believe I have to copy-paste in two places).
C. Some other alternative that you may notice while reading this post.