Authentication Series - Open System Authentication

October 16, 2019  

In this series of WLAN security blogs, we will understand different types of authentication methods and the steps required for a client station to connect to the 802.11 BSS. The first one is Open System Authentication. It is one of the legacy WLAN security but the only pre-RSNA security that has not been deprecated. RSNA stands for robust security network associations which indicates the better authentication algorithm with stronger encryption. It was defined in the 802.11i amendment ratified and published in 2004. Another legacy authentication methods WEP has been superseded by WPA.

Open System Authentication involves the exchange of above frames between the client station and the access point during the Authentication and Association process. However, it is considered to be a null authentication because there will not be any process to exchange or verify the client’s identity.

After the probe request/response, there will be two messages (Highlighted in Blue) involved in authentication frame exchange process and another two more messages (Highlighted in Green) for the association frame.

1.    Authentication Request
2.    Authentication Response
3.    Association Request
4.    Association Response

Authentication Request & Response (Frame 58 & 59)
For the first message, the client STA sends an 802.11 authentication management frame to an AP to request for authentication.

From the IEEE 802.11 wireless LAN part of the frame body, the authentication Algorithm 0 indicates it is Open System. The authentication is open and we can see the Authentication Sequence (SEQ) is set to be 1 (0x0001). Status code to be Successful (0).

Then for the second message, when the AP received the authentication management frame, it will respond to the client STA with the Authentication Sequence (SEQ) indicates 2 (0x0002).

Association Request & Response (Frame 61 & 62)
After authentication successful, client station and access points will exchange two more association frames.

The association request has included the station’s capabilities information

The AP response with status code to be Successful; Association ID is assigned to the associated client STA:

Once open system authentication and association occurs, the client station will join the BSS and is connected to the network.