WPA2 and the Four-Way Handshake

The handshake is why WPA2 cracking works at all, and it works offline. You capture four small frames once, then you can guess passwords against them forever on your own hardware with the AP none the wiser. Understanding what those four frames carry tells you exactly why a weak passphrase is dead and a strong one is fine.

WPA2-Personal starts from a shared secret. The passphrase everyone types plus the SSID get run through PBKDF2 with 4096 iterations to produce the PMK (Pairwise Master Key), a 256-bit value. Same passphrase, same SSID, same PMK every time. That PMK never travels over the air. Neither does the passphrase. So what does the handshake actually move?

The four messages

The handshake derives a fresh per-session key, the PTK (Pairwise Transient Key), from the PMK plus a few public values. Here is the exchange after a client associates:

1. AP to client: the AP's random nonce, the ANonce. Sent in the clear.
2. Client to AP: the client's nonce, the SNonce, plus a MIC (message integrity code) proving the client computed the PTK.
3. AP to client: the group key and another MIC.
4. Client to AP: an ACK that the keys are installed.

The PTK is a hash of the PMK, both MAC addresses, and both nonces. The two MACs and both nonces cross the air in plaintext. The MIC is the tell: it is computed with the PTK, which means it is computed with the PMK, which means it is computed with the passphrase.

Why that lets you crack it

You now hold everything except the passphrase: both MACs, both nonces, and a MIC that is a function of the passphrase. So you run the loop yourself. Take a candidate passphrase, derive its PMK for this SSID, derive the PTK from the captured nonces and MACs, compute the MIC, and compare it to the captured MIC. Match means you found the passphrase. No match, try the next word.

None of this touches the AP. The AP already gave you the answer key when you captured the handshake. This is why WPA2 cracking is an offline dictionary attack, and why capturing message 2 (which carries the client's MIC) is the part that actually matters.

The cost that protects a good passphrase is the PBKDF2 work. Those 4096 iterations mean each guess is expensive, so throughput is thousands to a few hundred thousand guesses per second per GPU, not billions. Against rockyou.txt that finishes fast. Against a random 12-character passphrase it never finishes in your lifetime. The math punishes lazy passwords and rewards strong ones, nothing in between.

Personal is not Enterprise

Everything above is WPA2-Personal, the pre-shared-key mode home and small-office networks use. WPA2-Enterprise (802.1X) is a different animal: each user authenticates through a RADIUS server, there is no shared passphrase, and the PMK is derived per-session from that exchange. The handshake capture-and-crack in the next lesson targets PSK networks. Enterprise resists it because there is no single shared password to guess.