Pairwise Transient Key

The Pairwise Transient Key (PTK) is the set of session keys used to protect unicast traffic between a single client (supplicant) and an access point in WPA/WPA2/WPA3 networks. It is derived per-session during the 4-way handshake, so every association gets fresh keys.

The PTK is derived from the Pairwise Master Key (PMK) together with both MAC addresses and the two nonces exchanged in the handshake (ANonce from the AP, SNonce from the client):

PTK = PRF(PMK, "Pairwise key expansion",
          min(AA, SPA) || max(AA, SPA) ||
          min(ANonce, SNonce) || max(ANonce, SNonce))

It is then split into sub-keys:

KCK (Key Confirmation Key): authenticates handshake messages (MIC)
KEK (Key Encryption Key): encrypts the GTK sent to the client
TK (Temporal Key): encrypts the actual data frames

Because the PTK depends on the PMK and the nonces, capturing the 4-way handshake lets an attacker brute-force the passphrase offline (the basis of WPA2 cracking).

Source

IEEE 802.11i