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Description of wireless authentication.

There are two kinds of wireless authentication: open-system and shared-key authentication. Wireless authentication is only one way street. Station has to authenticate to the access point, but the access point has no obligation to authenticate to station.

Open-System Authentication - access point accepts every request for authentication. Without encryption, every device that knows SSID, can access the network. With enabled WEP , on access point, WEP key becomes an access control. If station does not have correct WEP key, device cannot transfer data, even though authentication was successful.

Open-system authentication exchange has two frames:

First frame (Sequence number - 1) is from the station.
It is authentication request with two elements.

Direction
Client->AP
Algorithm Identification
0
Transaction Sequence
1

Authentication Algorithm Identification set to 0 – which means this is an Open-System.
Authentication Transaction Sequence is set to 1 – this means it is the first frame in the sequence.

After that, access point returns its response with 3 elements:

Direction
AP->Client
Algorithm Identification
0
Transaction Sequence
2
Status code
0

Authentication Algorithm Identification is set to 0 – Open-System.
Sequence number is 2.
Status Code - result of the authentication request.



Shared-Key Authentication - use WEP. This means that authentication itself does not guarantee you high level of the security.

Access point need to transfer the shared-key to stations, before authentication. Shared-key authentication exchange four frames:



Client sends an authentication request to the access point. This frame is almost identical to the first frame in the Open-System.

Direction
Client->AP
Algorithm Identification
1
Transaction Sequence
1
Algorithm Identification set to 1 – which means this is Shared-Key Authentication. Transaction Sequence is set to 1 – this means it is the fist frame in the sequence.

Access point responds with the frame that serves as the challenge.

Direction
AP->Client
Algorithm Identification
1
Transaction Sequence
2
Status Code
0
Challenge text
128 B text
To proceed, Status Code should be 0 (successful). Access Point may deny the authentication request.
If Status Code is 0, then frame includes Challenge text – 128 bytes generated using WEP key.

Third frame is stations response to challenge.

Direction
Client->AP
Algorithm Identification
1
Transaction Sequence
3
Status Code
0
Challenge text
128 B
Station use WEP key to encrypt the Challenge text. Its response is the subsequent authentication request. Request is again hidden with WEP key as shared-key.

If access point can decrypt the authentication request, then it responds with the authentication response that grants access to the client.

Direction
AP->Client
Algorithm Identification
1
Transaction Sequence
4
Status code
0
If Status code is 0 (successful), then network can grant access to station. If any problem occurs, AP returns status code 1 - unsuccessful.




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