ECDiffieHellmanCng missing from unity?

I believe I found a bug. I tried to use ECDiffieHellmanCng which is found in System.Security.Cryptography on .Net 3.5 and newer.
Doesnt seem to exist in the System.core.dll Unity it using.
I then tried to replace the system.core.dll with a version from the .net/3.5 install. Visual studio then sees ECDiffieHellmanCng, but Unity does not.

Without ECDiffieHellmanCng a key exchange with the server will be impossible.

Unity doesn’t use .Net 3.5. It uses Mono, and those classes are not present in Mono: xamarin.ios - Are ECDSA and ECDH available for mono? - Stack Overflow