The Four-Way Handshake

 

The authentication process leaves two considerations: the access point (AP) still needs to authenticate itself to the client station (STA), and keys to encrypt the traffic need to be derived. The earlier EAP exchange has provided the shared secret key PMK (Pairwise Master Key). This key is, however, designed to last the entire session and should be exposed as little as possible. Therefore the four-way handshake is used to establish another key called the PTK (Pairwise Transient Key). The PTK is generated by concatenating the following attributes: PMK, AP nonce (ANonce), STA nonce (SNonce), AP MAC address, and STA MAC address. The product is then put through a cryptographic hash function.

The handshake also yields the GTK (Group Temporal Key), used to decrypt multicast and broadcast traffic. The actual messages exchanged during the handshake are depicted in the figure and explained below:

1. The AP sends a nonce-value to the STA (ANonce). The client now has all the attributes to construct the PTK.
2. The STA sends its own nonce-value (SNonce) to the AP together with a MIC, including authentication, which is really a Message Authentication and Integrity Code: (MAIC).
3. The AP sends the GTK and a sequence number together with another MIC. This sequence number will be used in the next multicast or broadcast frame, so that the receiving STA can perform basic replay detection.
4. The STA sends a confirmation to the AP.

All the above messages are sent as EAPOL-Key frames.

As soon as the PTK is obtained it is divided into five separate keys:

PTK (Pairwise Transient Key – 64 bytes)

1. 16 bytes of EAPOL-Key Confirmation Key (KCK)– Used to compute MIC on WPA EAPOL Key message
2. 16 bytes of EAPOL-Key Encryption Key (KEK) - AP uses this key to encrypt additional data sent (in the 'Key Data' field) to the client (for example, the RSN IE or the GTK)
3. 16 bytes of Temporal Key (TK) – Used to encrypt/decrypt Unicast data packets
4. 8 bytes of Michael MIC Authenticator Tx Key – Used to compute MIC on unicast data packets transmitted by the AP
5. 8 bytes of Michael MIC Authenticator Rx Key – Used to compute MIC on unicast data packets transmitted by the station

The Michael MIC Authenticator Tx/Rx Keys provided in the handshake are only used if the network is using TKIP to encrypt the data.

 

 

http://www.stanford.edu/class/cs259/WWW04/projects/project04/04%20-%20Slides.pdf