| Title | Author | Created | Published | Tags | | --------------------------------- | ---------------------------- | ----------------- | ----------------- | -------------------------------------------------- | | Dynamic Encryption Key Generation | <ul><li>Jon Marien</li></ul> | February 11, 2025 | February 11, 2025 | [[#classes\|#classes]], [[#SYST44998\|#SYST44998]] | # 802.11 Layer 2 Dynamic Encryption Key Generation ## 802.1X/EAP and Dynamic Keys - Dynamic keys are a **by-product** of the 802.1X/EAP process. - EAP protocols that utilize mutual authentication provide the seeding material used to generate the keys. - Benefits of dynamic keys: - They cannot be compromised by SE. - Every user has a different key. - **Outcome**: if the key is compromised, other supplicants will not be exposed. ![[image-1.png]] ## Robust Security Network (RSN) - A **robust security network** (RSN) is a network that allows the creation of only **robust security network associations** (RSNAs). - Two 802.11 radios must have established procedure to: - **Authenticate**. - **Associate**. - **Create** unique dynamic keys by means a 4-way handshake process. - **CCMP/AES** is the mandated encryption method and TKIP is optional. - Anything else would be called a legacy secure network. ## RSNA within a BSS - Terminology and procedures: - **Pairwise Transient Key** (PTK): - **Unicast** traffic. - **Unique** between a client STA and the AP. - **CCMP/AES** or **TKIP/ARC4**. - **Group Temporal Key** (GTK): - Broadcast and multicast traffic. - Determined by the **lowest common denominator** (LCD) method. - E.g.: - *LCD (CCMP, CCMP)* = **CCMP** - *LCD (CCMP, TKIP)* = **TKIP** - *LCD (CCMP, CCMP, WEP)* = **WEP** - Weakest encryption technique among the client STAs is chosen as the GTK. ![[image-2.png]] ## Robust Security Network Criteria - In a **RSN**, only **RSNAs** are allowed: - **TKIP/ARC4**. - **CCMP/AES**. *GTK=LCD (CCMP, TKIP) = TKIP* ![[image-3.png]] ## Transition Security Network (TSN) - Allows **WEP**. GTK=LCD (CCMP, TKIP,WEP) = WEP ![[image-4.png]] ## Pre-RSN, RSN, TSN Coexistence - BSS is featured by: - Service set identifier (SSID) = **logical name** - Basic service set identifier (BSSID) = **MAC address** - It is possible to have multiple SSIDs within the same AP. - Each SSID is associated with a virtual MAC address. - Each SSID and vBSSS might be associated to a security level.    ## The RSN Information Element - AP send frames to inform client STAs about its RSN capabilities. ![[image-5.png]] - Client STA use the association frame request to inform the AP about its security capabilities. ![[image-7.png]] - When a client STA is performing a handoff, it uses the re-association frame to inform the new AP about its security capabilities. ![[image-8.png]] ## Authentication and Key Management (AKM) - **AKM** is defined in **802.11-2012** standard. - Consists of a set of algorithms designed to provide: - **Authentication**. - **Key management**: - Individually. - In combination with higher-layer authentication and key-management algorithms. - **Key point:** - **Authentication** and **Key generation** are mixed. - An **authentication** process is necessary to generate dynamic encryption keys. - **802.1X/EAP** and **PSK** authentication processes generate the seeding material needed to create dynamic encryption keys. - **Authorization** is not finalized until encryption keys are created, and encryption keys **cannot be created without authentication**. ### AKM Steps 1. Discovery 2. Authentication 3. Master Key Creation 4. Temporal Key Creation 5. Authorization 6. Encryption *(steps can be seen in the image below)* ![[image-9.png|419x321]] ## Discovery Component ![[image-10.png]] ## Authentication and Master Key Generation ![[image-11.png]] ### Temporal Key Generation and Authorization ![[image-12.png]] ## RSNA Key Hierarchy - Each upper tier key acts as the seeding material for lower keys. - [**Master Session Key (MSK)**](#**Master%20Session%20Key%20(MSK)**) - [**Group Master Key (GMK)**](#**Group%20Master%20Key%20(GMK)**) - [Temporal Keys](#Temporal%20Keys) - [**Pairwise Master Key (PMK)**](#**Pairwise%20Master%20Key%20(PMK)**) - **Pairwise Transient Key** ![[image-23.png]] --- ### Master Keys #### **Master Session Key (MSK)** - **MSK** is also called **AAA key**. - It is generated either from the **802.1X/EAP** or **PSK authentication**. - **64 octets** in length. - **MSK generation is out of the scope** of the 802.11-2012 standard and is EAP specific. ![[image-22.png]] --- #### **Pairwise Master Key (PMK)** - MSK is the seeding material for the PMK. - PMK resides on supplicant and authentication server. - PMK is unique and is generated every time there is an authentication or re-authentication. - It is sent from the server to the authenticator. ##### **Group Master Key (GMK)** - Randomly generated at the authenticator. ![[image-20.png]] --- #### **Master Keys Overview** ![[image-16.png]] ### Temporal Keys - Outcome of the 4-way handshake. - **Pairwise Temporal Key (PTK)** - It is **created from the PMK**. - It is used to **protect the payload of the 802.11 data frame**. - **Unicast traffic** from/to STA to/from AP. - **Unique** for every STA. - **Group Temporal Key (GTK)** - It is **derived from the GMK**. - Encrypt all the broadcast and multicast traffic. - Shared among all client STAs and a single authenticator. ![[image-17.png]] ## Purpose of 4-Way Handshake Process - It uses 4 EAPOL‐Key frame messages between the authenticator and the supplicant for six major purposes: 1. **Confirms the existence of the PMK** at the peer station. 2. **Ensures that the PMK is current**. 3. **Derives a new Pairwise Transient Key (PTK)** from the PMK. 4. **Installs the PTK** on the supplicant **and** the authenticator. 5. **Transfers the GTK** from the authenticator to the supplicant and **installs the GTK** on the supplicant **and, if necessary**, the authenticator. 6. **Confirms the selection** of the cipher suites. ### 4‐Way Handshake > Final process used to generate PTK for encryption of unicast transmissions and a GTK for encryption of broadcast/multicast transmissions #### Key Terms - **Pseudo-random function (PRF)**: - It hashes input values to create a pseudo-random value. - **Nonce**: - Random numerical value generated one time only. - Authenticator nonce (**ANonce**) - Supplicant nonce (**SNonce**) - Authenticator MAC add (**AA**) - Supplicant MAC add (**SPA**) - **Pairwise Transient Key** is given by this formula: $ PTK=PRF(PMK+ANonce+SNonce+AA+SPA)$ - To **generate the PTK**, both the supplicant and authenticator need all 5 parameters. ### How the Handshake Works - **MSG #1:** AP sends the `ANonce`, then STA can generate the PTK. - **MSG #2:** - STA sends the `SNonce`. - MIC. - **MSG #3:** - AP sends the GTK encrypted using the PTK. - Command to install the PTK. - MIC. - **MSG #4:** - Ack that the temporal keys are installed. - MIC. ![[image-24.png]] ## Group Handshake - The Group Key Handshake is used only to issue a new GTK to client STA that have previously formed security associations. - E.g., the GTK should change: - **after a client STA disassociation or de-authentication**. - **in a certain period of time**. - **MSG #1:** - AP sends the new generated GTK to STA encrypted using its PTK. - MIC. - **MSG #2:** - ACK the installation of the new GTK. - MIC. ![[image-25.png]]