Wireless Part 6

4-Way handshake

So this week i thought i would continue on with handshakes, and more importantly the most important handshake in wireless you would ever learn which is the 4-Way handshake. Now the 4-way handshake is merely the connection between the wireless client and the wireless AP, but its incredibly important when we get into WPA (which will be done in more detail than WEP as it is seen more, making it more important to understand, it also gives the basis of WPA-Enterprise).

There are 5 parts of the handshake, that's 5 important bit sequences

• ANounce
• Snounce
• MIC
• GTK
• Ack

These are absolutely key for our understanding, they are the main blocks of the handshake.
The handshake itself is just 4 EAPOL packets. The structure is as follows

1. AP sends client ANounce
2. Client uses this to construct the PTK (pairwise transient key), replying with a SNounce + MIC
3. AP uses this to construct a PTK, replying with GTK + MIC
4. Client replies with Ack
5. Connection Made!

So what happened?
The client attempts to connect with the access point, and has a pre-shared key to use to connect.
The client can now send the pre-shared key it has been told. To do this, the client applys PBKDF2 to create a 256-bit pre-shared key. PDKDF2, is a hashing algorithm, using 4 inputs, the pre-shared key, SSID (and SSID length), then 2 numbers, 4096 + 256. The 4096 is important as the passphrase is hashed 4096 times, making it completely one way, and 256 is the size of the output. The AP already has its own version.

But this key cannot be sent yet, so first of all, ANounce, all the ANounce is, is the authenticator (AP) Nounce (Nounce being a large random value). This is sent by the AP to the client. The client, or supplicant, can then create a SNounce, which is just a Supplicant Nounce. These are both used to create a PTK (pair-wise transient key). This is a dynamic key (making this handshake more secure as it is not guessable).

Now the PTK must be derived. It is just a function using the 256-bit pre-shared key from before, the ANounce, the SNounce, the Authenticator MAC and the Supplicant MAC. This is kept for later.

Next is message 2, this is just the SNounce and MIC, being sent to the AP. The MIC is the message integrity check, which is basically a checksum on the PTK.

At this stage, both the client and the AP both know the 256-bit pre-shared key, the SNounce, the ANounce, the Authenticator (AP) MAC address and the Supplicant (Client) MAC address. As a result, both can create the exact same PTK. As a result, when the authenticator creates its own MIC (based on the Authenticators PTK), the resulting MIC should be the same if the connection is legitimate. Thus, the pre-shared key should be the same.

If the connection is legitimate, the key installation is done, and finally, the key install is acknowledged with a ACK.

I don't think that until now i have properly explained, that was the connection in a WPA environment. Its nothing complex. I personally like to imagine it like 2 spies talking to eachother, checking if the other has the same information without saying what that information is (but i'm abit strange like that).

Anyway, I think that that is enough for this week, so i shall see you all next week for another installment.
-Spectr3