First of all, I didn't manage to complete the Authentication-Authorization key exchange yet. I'm close, but at the moment the resultI get underneath contains invalid values (e.g. an AuthKey contains several types of unknown values while I expect mostly 0).
Although this VB.Net script closely resembles the AuthKey sequence for Telegram API, it somehow won't reach its completion and result in a valid AuthKey. Other discovered code does the same. I assume that either we're missing something critical or we need a more sophisticated implementation. Also during the process, I discovered that the library used to make the secure connection to Telegram Servers is missing this important method to cleanly close the connection.

Public Sub Close(ByVal asWait As Boolean)
     If _stream IsNot Nothing Then
        If asWait Then _stream.Close()
        If _are IsNot Nothing Then _are.WaitOne()
        _stream = Nothing
     End If
End Sub

I'll keep you updated about any further progress
Best