SslStream Class Review
new byte[0x1f80]


I think using decimal would be more readable here. And I don't understand why is your buffer 8096 bytes, usual sizes are powers of two, like 8192.

IpAddress = "192.168.1.65"


This looks like a perfect example of something that should be configurable, not set in stone. You should accept the IP address in the constructor, like you already do with port number.

public abstract void Receive(byte[] buffer);
public abstract void Error(Exception e);


I'm not sure I like the use of inheritance here. I think using events, or something like that would make more sense here.

_socket.BeginConnect(…)


I don't like that you're using the old Asynchronous Programming Model when you could (and do in other parts of your code) use async-await. Unfortunately, Socket doesn't expose awaitable methods, but there are ways around that.

private async void Authenticate(IAsyncResult ar)


I think Authenticate is not a very good name for this method, it does much more than just authentication.

throw new Exception("not^authenticated")


You shouldn't throw Exception, you should instead create a custom class that derives from Exception and use that. And the exception message should be human-readable, not some error code like it seems in your case. If you need to differentiate different errors, either have different exception types for them, or add a property for that to your custom exception.

if (size == 0) throw new Exception("invalid^size");


When ReadAsync() returns 0, it's not invalid size, it means the end of the stream was reached, so you should act accordingly.

Native.MemoryCopy(Buffer, buffer, (uint)buffer.Length);


Why are using some custom memory copying method here? Do you have some reason for not using Buffer.BlockCopy()? Also, why are you even doing the copying here? It seems you class is intended only for limited use (since you have a constant IP address), so couldn't you make sure the buffer isn't used after Receive() returns, so you didn't have to copy at all?

return true;


I'm no expert on security, but I think you should perform at least some checks of the certificate here. Otherwise, your class isn't really a “secure stream”.

MessageBox.Show(…);


You shouldn't mix code like this with GUI code, they should be separated.