WEP Cracking

WEP is infamously known as the broken wireless security protocol. A design flaw was discovered in 2001 and after several cascading discoveries it's now possible to crack a WEP protected network within minutes. WPA is the successor to WEP and features a better but not perfect security protocol.

Background

There are now many sources that describe the vulnerability in detail and APR replay to generate traffic, but this is a short summary. For an throughout explaination on how WEP is implemented and the vulnerabilities, see the link below.

Types attacks:

• Passive attacks to decrypt traffic based on statistical analysis.
• Active attack to inject new traffic from unauthorized mobile stations, based on known plaintext.
• Active attacks to decrypt traffic, based on tricking the access point.
• Dictionary-building attack that, after analysis of about a day's worth of traffic, allows real-time automated decryption of all traffic.

Original discoveries and paper:

• (In)security of the WEP algorithm by Nikita Borisov, Ian Goldberg, and David Wagner
• Intercepting Mobile Communications:The Insecurity of 802.11 paper

Way of attack

This is the most efficient method to crack a WEP protected network. The attack can be performed in under a minute. The result is the WEP hex/ascii-key used.

• WEP encryption:

1. 24-bit unencrypted initialization vector + 104-bit key (13 characters/bytes), 128-bit key
2. Used to generate RC4 cipher stream
3. XOR the message
4. Encrypted network packets

• ARP replay:

1. On the basis that the first 12-bytes of ARP packets always stays the same
2. Capture one ARP packet
3. Continously inject packet back to into the network to stimulate traffic
4. Capture 10-20 000 ARP (or other data) packets

• Key attack:

1. Find Initialization Vector collisions where two cipher texts (12-bytes ARP) are the same (only 2^24 possibilities)
2. XOR back first 12-bytes using the known plain-text ARP data
3. RC4 stream cipher is revealed for that specific IV
4. Find enough collisions
5. Data used to build a table of Initialization Vectors and RC4 stream ciphers
6. Use table to gain a statistical factor for the remainder missing key bytes, 104-bit (13 character) key

References

• How to Crack a Wi-Fi Network's WEP Password with BackTrack
• SpoonWEP for Noob
• Part 10: WPA-PSK Cracking with a dictionary attack in Backtrack 3
• Cracking WEP Using Backtrack: A Beginner’s Guide
• Remote-Exploit Backtrack
• Cracking WPA FAST with video cards