The Trouble with WEP

Or, cracking WiFi networksfor fun & profit (not really)

Jim Owens

Overview

Background and a little history How WEP works WEP’s major weaknesses A short course in wardriving

Using kismet to scout out the wireless landscape Zeroing in with the aircrack-ng suite

airodump, to capture traffic aireplay, to replay weakly encrypted packets aircrack, to find the key using statistical methods

Background & history…

Wireless Equivalent Privacy Adopted in 1999 as part of 802.11 standard Later swallowed whole by 802.11b standard Initially, used only 40-bit encryption keys, due

to technology export restrictions Later, expanded to 104-bit keys when export

restrictions were eased Used 6 times as often as WPA/WPA2 despite

known fatal weakness* (85% / 14% / 1%)

*Based on a 2006 survey in Seattle area

How WEP works

1. Plain text gets CRC-32 checksum appended

2. 24-bit initialization vector pre-pended to key as a seed for RC4 key scheduling algorithm

3. RC4’s pseudo-random generation algorithm outputs keystream

4. Keystream XORed with plain text

5. IV in plain text pre-pended to message

6. On receipt, keystream regenerated and XORed with cipher text to produce plain text

WEP’s major weaknesses

IV space too small (224) On a busy network, IVs must repeat in <= 5 hours 50% probability that IV repeats in 5,000 packets

RC4 algorithm produces “weak” IVs that can be correctly guessed 5% or 13% of the time

No key management; typically just one key IP traffic contains much known plaintext data Open to injected traffic that is rebroadcast

Wardriving: Kismet

Network detector, sniffer, IDS Works on 802.11b, 802.11a, 802.11g

networks Uses passive monitoring, so hard to detect Logs sniffed packets in formats compatible

with Wireshark/Tcpdump, Airsnort Channel surfs automatically Optionally, supports GPS for network location

Kismet: Install & configure

Binary packages available for most systems Requires WiFi adaptor that supports monitor

mode as “capture source” Logs traffic in popular formats* Specify source in /etc/kismet/kismet.conf, as

driver,device,source_name

source=ipw2200,eth1,Stella

*Wireshark, Airsnort, etc.

Stella, the WiFi attack animal!

Wardriving: Recon phase

Use Kismet to survey WiFi landscape and to choose a target network

Record necessary data for Aircrack attack: Channel number? SSID? Access point MAC address?

Wardriving: Kismet

Wardriving: Attack phase

Aircrack-ng: Software for network detection, sniffing, WEP cracking, and analysis

Works on 802.11b, 802.11a, 802.11g Uses passive monitoring & packet injection Main tools

aircrack-ng: Cracking airdecap: Packet decryption airmon: Monitor mode switching aireplay: Packet injection (Linux only) airodump: Exports traffic to .cap files

Wardriving: Aircrack procedure

1. Bring up adapter on target’s channel in monitor mode:

# ifconfig wlan0 up# iwconfig wlan0 mode Monitor channel 9

2. Capture packets to file on channel, IVs only

# airodump wlan0 ./berlin_dump 9 1

Wardriving: Airodump

Wardriving: Aircrack procedure

3. Find weakly-encrypted packets to replay in interactive mode

# aireplay -2 -b 00:14:6C:40:BA:A6 \-x 512 wlan0

4. Finally, crack WEP key with captured IVs

# aircrack -n 64 berlin-dump.ivs

Wardriving: Aireplay

Wardriving: Aircrack

Summary

WEP has numerous serious flaws WEP's flaws are thoroughly documented WEP is readily exploitable in a short time, by

unskilled attackers, using readily available tools

Strong protection is readily available Bottom line:

Don't use WEP, period!

Questions?