An attacker is attempting to exploit a vulnerability in a WPA2-Personal network that uses a weak passphrase. They have captured a limited number of frames from the 4-way handshake. Instead of a full dictionary attack, they decide to use a precomputed rainbow table that maps common passphrases directly to their corresponding Pairwise Master Keys (PMKs). What is the primary limitation of using rainbow tables for cracking WPA2-Personal, especially compared to their effectiveness against other hash types?
- AThe use of a salt (SSID) in WPA2-Personal makes generic rainbow tables impractical.
- BRainbow tables are only effective against WEP, not WPA2.
- CThe 4-way handshake nonce values change too frequently for rainbow tables to be useful.
- DWPA2-Personal uses AES encryption, which is resistant to rainbow table attacks.
Show answer & explanationAnswer & explanation
Correct answer: A. The use of a salt (SSID) in WPA2-Personal makes generic rainbow tables impractical.
WPA2-Personal uses the SSID as a salt during the PBKDF2 derivation of the PMK. This means a unique PMK is generated for the same passphrase on different SSIDs. Generic rainbow tables, which precompute hashes without considering a dynamic salt, become impractical because a separate table would be needed for every possible SSID, making them prohibitively large.
Why the other options are wrong
- B. While rainbow tables aren't ideal for WPA2, the limitation isn't that they only work for WEP; WEP has different vulnerabilities.
- C. The nonce values are part of the PTK derivation, but the PMK (which rainbow tables would store) is derived from the passphrase and SSID, not the nonces directly for the initial PMK lookup.
- D. AES is an encryption algorithm, not directly related to the precomputation of passphrase hashes for rainbow tables; the issue lies in the key derivation function.
Rainbow Table Limitations in WPA2-Personal
Rainbow tables are generally inefficient for cracking WPA2-Personal due to the use of the SSID as a salt in the PBKDF2 key derivation function. This requires a unique rainbow table to be precomputed for each potential SSID, rendering the approach impractical for universal attacks.
- SSID acts as a salt during PMK generation.
- Different SSIDs produce different PMKs for the same passphrase.
- Requires a separate rainbow table for each SSID, making it unscalable.
Memory trick: SSID's salt makes Rainbow Tables impractical for WPA2.