Four-Way Handshake

The Problem

You want to connect to a WiFi network. You know the password. The router knows the password.

But you can’t just send the password over the air. Anyone listening would capture it.

How do you prove you know the password, without revealing it?

That’s what the four-way handshake solves.

The Goal

Both sides need to end up with the same encryption keys, without ever transmitting:

• The actual password
• The final encryption key

Instead, they exchange random numbers and use math to derive the keys independently.

The Four Messages

Step by Step

Before the handshake:

Both the AP and Client already know:

• SSID (network name)
• Password (the pre-shared key)

From these, both independently calculate the PMK (Pairwise Master Key).

The PMK is never transmitted. Both sides derive it from the password.

Message 1: AP → Client

The AP sends a random number called ANonce.

• Just a random value
• Not encrypted (doesn’t need to be)
• Starts the handshake

Message 2: Client → AP

The Client now has:

• PMK (from password)
• ANonce (from AP)
• SNonce (its own random number)

It combines these to create the PTK (Pairwise Transient Key) — the actual encryption key.

The Client sends:

• SNonce (its random number)
• MIC (Message Integrity Code) — proof it calculated the PTK correctly

The MIC proves the Client knows the password without revealing it.

Message 3: AP → Client

The AP now has everything it needs. It calculates the same PTK.

It sends:

• GTK (Group Temporal Key) — for broadcast/multicast traffic
• Another MIC to prove it also knows the password

The Client installs the keys after receiving this message.

Message 4: Client → AP

The Client sends an ACK (acknowledgment).

The AP installs the keys after receiving this.

Both sides now have identical encryption keys. The connection is secured.

Key Hierarchy

The handshake creates multiple keys:

Why This is Secure

The password is never sent:

• Both sides derive PMK independently
• Attacker can’t extract password from captured handshake

Random numbers prevent replay:

• ANonce and SNonce are fresh each time
• Can’t reuse a captured handshake

MIC proves knowledge:

• Only someone with the correct PMK can generate valid MIC
• Proves identity without revealing secrets

The Weakness (KRACK Attack)

The handshake has one vulnerability:

• If Message 4 is lost, the AP retransmits Message 3
• Client reinstalls the same key
• This resets the nonce counter
• Reused nonce + same key = encryption can be broken

WPA3’s SAE handshake was designed to avoid this entire class of attacks.

For WPA2: Keep your devices patched. The KRACK vulnerability was fixed in 2017 updates.