Once the connection is setup, the client can authenticate with the AP. For this, it sends an
ClientResponseEncrypted message, using packet type 0xab.
A few different authentication methods are available, athough only one has really been tried, the traditional user / password based authentication.
The AP will then reply with either a APWelcome message using packet type 0xac if authentication
is successful, or an APLoginFailed with packet type 0xad otherwise.
TODO: investigate other authentication methods (Zeroconf, facebook, stored credentials)