u8.8 Weaknesses P2.

u8.8 Weaknesses P2.

1. Key Scheduling Weakness

   • Practical attack on WEP is possible through 2 weaknesses
     • Certain keys leak into key stream
       • Large classes of weak keys exist, in which a small part of the secret key determines a large number of bits of the initial bit stream from the PRNG.
         • Known as the invariance weakness
     • Partially exposed key allows whole key to be determined
       • If a portion of the key is known, then analysis of the initial key stream allows the whole key to be determined. In 802.11b, part of the key is public as the IV is broadcast in the clear.
         • Known as the IV weakness.
     • The attack is passive, as an attacker can gather traffic without being detected. Once the key has been correctly derived the attacker can use the key to associate with an access point and enter the network with no warning.

2. IV is transmitted with each message so a portion of the key is known.

   • First few bytes of the PRNG can also be discovered by examining the first few bytes of each message and XOR them with fixed headers of 802.11b messages
     • Knowing these few bytes allows the keyspace to be reduced and brute force can be mounted

3. A crucial aspect of this attack is that the time taken for the attack increases linearly with the key size. Normally as an encryption key increases, the time to break the key increases exponentially. An exponential increase would cause an attack on a 41-bit key to take twice as long as a 40-bit key. The practical effect of this was seen in the predicted length of time to brute force a 104-bit WEP key versus a 40-bit WEP key (1021 years versus 200 days). But as the attack time for this 802.11b attack increases linearly, then the time taken to break a 104-bit WEP key is only 2.6 times the length of an attack on a 40-bit WEP key. This leaves the so-called 'enhanced' WEP implementations as vulnerable as the standard WEP implementations.

4. WEP Crack

   • first tool to demonstrate an attack using the IV weakness
     • a generator that creates a list of all the weaker IVs;
     • a search tool that searches the output of a network sniffer for weaker IVs and records the first byte of encrypted data;
     • a cracker tool to combine the list of weaker IVs and the selected first bytes.

5. Airsnort

   • Automated version of WepCrack
     • sniffs, searches for the weaker IVs and records the corresponding encrypted data until there is enough data for the key to be derived