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+---
+title: "4 Way Handshake - Wifi Authentication"
+summary: "In this article, you will learn how authentitcation works for WPA/WPA2 network"
+date: 2024-10-10T20:00:00+0200
+lastUpdate: 2024-10-10T20:00:00+0200
+tags: ["network", "wifi", "authentication"]
+author: clementS
+draft: false
+---
+
+# 4 Way Handshake
+
+## Abstract
+
+To connect to a WiFi network, the authentication depends on the WiFi protocol. The most common protocols currently in use are 
+1. **WEP**
+2. **WPA Personal**
+3. **WPA Entreprise**
+4. **WPA2-Personal**
+5. **WPA2-Enterprise**
+6. **WPA3**  
+
+(There are 2 different versions of WPA and WPA2 which work slightly differently in terms of authentication.)
+
+While WEP and WPA3 have their own authentication protocols, WPA and WPA2 (which are the most common) use the **4-Way Handshake** method to allow a device to connect to WiFi and secure connection.
+
+
+The 4-Way Handshake requires a PMK (Pair Master Key), a secret shared by both side (client & server). This can be a password (network key) or a username + password as it's the case for Enterprise authentication.
+
+In this article, you will learn how this method works from a network-oriented approach.
+
+## Configuration
+
+We will consider a WPA-Personal/WPA2-Personal WiFi, so the PMK will just be the WiFi password encrypted. We will go into further explains soon.  
+
+But remember that except getting PMK before authentication starts threw a WiFi password(secret shared by both sides), WPA2-Entreprise & WPA-Entreprise works the same way.
+Indeed, they use 4 way handshake to secure communication.  
+
+There is a device that knows the key (password) and wants to connect to a WPA/WPA2-PSK WiFi network.
+
+## 4 Way Handshake Initiation
+
+**The device sends an authentication request to the network.**  
+### This request is not included in the 4-Way Handshake.
+
+## 4 Way Handshake
+
+![schema](schema.png)
+
+### *"STA" stands for Station and "AP" stands for Access Point*  
+### This schema shows the different requests and what they contain.
+
+## First Request
+
+Firstly, the AP sends a nonce, which is a large random integer.
+
+When the station receives the packet, the station creates the PTK (Pairwise Transient Key) using the key derivation function (KDF). This key is created with:
+
+1. The PSK (Pre-Shared Key).
+2. The ANonce received previously.
+3. The SNonce.
+
+The SNonce is similar to the ANonce, a large random number generated on the station side. Each time the station tries to authenticate, the SNonce and ANonce are generated again. These numbers ensure that the PTK is temporary.  
+**The KDF algorithm returns the PTK. We don’t use the password or the PSK directly because we don’t want the explicit key to go through the network.**  
+**This is quite obvious; we don’t want others to easily intercept the password.**
+
+### Pre-Shared Key
+
+To get the PSK, the device only needs the SSID (network MAC address) and the password.  
+=> The process is really simple:  
+**Hash the password with the SSID as salt using the SHA-1 algorithm.**
+
+Now, we have the PSK (the AP also performs this operation).
+
+**The PSK is calculated before authentication starts (the station has all infos it needs to)**  
+**The PMK is exactly the same as PSK for both WPA-Personal and WPA2-Personal**
+
+**VERY IMPORTANT**
+
+*For WPA-Entreprise & WPA2-Entreprise, they use here the PMK created by RADIUS authentication*
+## Second Request
+
+Let’s make a short recap.  
+At the beginning, we only know the **PSK=PMK**, which we derive from the **network password** and **SSID**.  
+*PMK derived from RADIUS authentication in case of WPA-Entreprise & WPA2-Entreprise*  
+
+We received the AP's nonce from the first request, and the station generated the SNonce to create the **PTK** from the **ANonce**, **SNonce**, and **PSK**.
+
+Now we need to tell the AP that we know the password without explicitly sending the password.
+
+The station sends a packet containing the SNonce, allowing the AP to generate the same PTK. At the end of the packet, a MIC is added.  
+**Here’s the trick:**  
+
+**We add a signature, which is called the MIC (Message Integrity Code).**  
+
+The MIC is just a SHA-1 hash of the packet content with the PTK as salt.  
+When the AP receives the second request, it will generate its PTK (it has all the necessary information: SNonce, ANonce, PSK). However, at this moment, the AP does not know if the station has the correct password.
+
+The AP generates the MIC from its newly generated PTK and compares it with the provided MIC.  
+**If the MICs are identical => the PTKs are identical => the PSKs are identical => the passwords are identical.**
+
+## Third Request
+
+To tell the station that it has the correct password, the AP will not do so explicitly.  
+**The AP will use the same strategy -> MIC.**
+
+To optimize the request, the AP will send useful information: the GTK. The **GTK** (Group Temporal Key) is directly generated by the AP.  
+To check if the station has the correct password, the AP generates the MIC corresponding to the GTK (packet content) using the PTK.
+
+If both match, the station and AP are sure that the other has the correct password.
+
+## Last Request
+
+The station sends an acknowledgment packet to tell the AP that everything is OK.  
+**The encrypted communication can now begin.**
+
+## Encryption
+
+WPA2 uses **AES** while WPA uses **TKIP** to encrypt packet content. Even if both are encryption's methods, AES is more secure.This makes it impossible for a hacker to read a captured packet.
+
+The PTK contains several keys.  
+Several keys are derived from the PTK:
+
+1. **KCK** = PTK bits 0-127, the key confirmation key -> used for MIC (verifying message integrity).
+2. **KEK** = PTK bits 128-255, the key encryption key -> used for encrypting the key data in the authentication packets.
+3. **TEK** = PTK bits 256-383, the temporal encryption key for unicast traffic -> used for AES encryption.
+
+We simplify things by saying that we use the PTK for something, but in reality, we often use just a part of it.
+
+## Recap of Key Derivation
+
+![key_derivation](key_derivation.png)
+
+In WPA2-PSK & WPA-PSK, the PMK is the same as the PSK.
+
+## Conclusion
+
+Remember that even if we use WPA/WPA2, if a hacker catch all packets, he can brutforce PTK and get WiFi; Indeed, he has all infos he needs (2 nonces, bssid, protocols, ...)
+
+You learned how the 4 way handshake works. Now you can discover the way to get PMK for WPA(2)-Entreprise.