Wi-Fi WPA Reconnaissance

• WPA

  • Improvement over WEP with TKIP encryption and message integrity checks (MIC)
  • TKIP dynamically generates a new 128-bit key for each packet with MIC

• WPA2

  • Uses AES
  • AES keys can be 128, 192 or 256 bits

• Two modes

  • WPA personal
    • Uses Pre-Shared key
      • Encryption key is derived from the passphrase
      • Can be vulnerable to dictionary attacks if weak passphrase is used
  • WPA Enterprise
    • Uses 802.1x security
    • Uses Extensible Authentication Protocol (EAP)- TLS
    • RADIUS server for authentication

4-Way handshake

1. The AP sends the ANonce (authenticator nonce) in an EAPOL-key message to the client.
2. The client generates an SNonce (Supplicant nonce), derives the Pairwise Transient Key (PTK) from the Pairwise Master Key (PMK) + ANonce + SNonce + AP MAC + Client MAC and sends the SNonce plus a MIC in an EAPOL-Key message to the AP.
3. The AP derives the same PTK, to validate the message verifies the client's MIC, installs the PTK and sends an EAPOL-Key message with it's own MIC and key installation/ack flags to the client.
4. The client verifies the AP MIC, installs the PTK and sends a final EAPOL-Key ACK to confirm successful key installation.

Pairwise Master Key

• The PMK is derived from the PSK + the SSID of the network.
• PBKDF2 is used with HMAC-SHA1
• The SSID is the SALT and 3096 iterations of the PBKDF2 function is used.
• The PMK is never transmitted between the client and the AP.
• It is used to derive the PTK

PMK = PBKDF2(HMAC-SHA, PSK, SSID, 4096, 256)

Pairwise Transient Key

PTK = PMK + ANonce + SNonce + AP MAC + Client MAC

Message Integrity Codes

MIC = HMAC-SHA1(PMK, ANonce || SNonce || AP MAC || Client MAC || Message Length)

Capture the PSK methods

• Handshake capture
• PMKID capture
• WPS
• Evil-twin
• Social Engineering