- Running AirSnort
Once launched, airsnort must be configured to work with your wireless nic and to make crack attempts according to your desires. In order to properly capture packets, first indicate the name of your wireless networking device in the "Network device" field. This will be something like "wlanX" for cards that use the wlan-ng drivers and "ethX" for other cards. Next select the type of card that you are using in the "Card type" drop down box. Available choices are Prism2, Orinoco, and other. Cisco cards fall into the other category. The purpose if this field is primarily to inform airsnort how to place your nic into monitor mode. In monitor mode a wireless nic gathers all packets indiscriminately, and no association with an access point is required. For wlan-ng and orinoco_cs based nics, monitor mode is entered automatically when the 'Start' button is clicked to initiate a capture session. Other card types must be put into monitor mode outside of airsnort, prior to clicking Start.
Choose between "scan" mode to scan through all 11 802.11b channels at a regular interval, or "channel mode to monitor a specific channel. Note that in either case it is quite possible to receive packets that bleed through from neighboring channels.
- Capture Details
Capture uses the pcap library to receive monitor mode packets. The packets go through two filters. First, non-encrypted packets are filtered out. Then, if they are encrypted, useless packets (those without a weak IV) are discarded. All non-data packets are discarded with the exception of 802.11b Beacon and probe response packets which are examined in order to obtain access point SSID data.
To distinguish encrypted and non-encrypted packets, capture examines the first two bytes of the output. Since unencrypted IP packets have a first pair value of 0xAAAA (part of the SNAP), all of these packets get dropped.
For a description of what constitutes an interesting packet please refer to the FMS paper and its discussion of "weak IVs"
- Cracking Details
Cracking attempts are made in parallel with packet capture. Currently, the cracker attempts to crack the captured packets for both a 40 bit and 128 bit key each time 10 new weak IVs are seen for a given access point. Airsnort uses a probabalistic attack, so, the best guess may not be the right one. With limited captured data and enough CPU power, you can perform more exaustive searches. The search for a key involves a depth first traversal of an n-ary tree. The depth of tree is 5 for 40 bit key attempts and 13 for 128 bit key attempts. The breadth of the trees is governed by the 40 and 128 bit crack depth fields in the airsnort gui. A breadth parameter of 'n' instructs airsnort to try the n most likely values at each key position using statistics derived from the IVs that have been collected. Large breadth setting can result in very slow processing time for crack attempts default values of 3 for 40 bit cracks and 2 for 128 bit cracks are recommended for starters. If a large number of weak IVs have been gathered (> 1500 if a 40 bit key is suspected, > 3000 if a 128 bit key is suspected), you may want to try increasing the breadth values.
The number of interesting packets needed to perform a successful crack depends on two things; luck and key length. Assuming that luck is on your side, the key length is the only important factor. For a key length of 128 bits, this translates to about 1500 packets. For other key lengths, assume 115 packets per byte of the key. Some keys are more resistant to this technique than others and may require far more packets. If you have a lot of packets and no key, either wait for more packets or try a larger breadth.
In any case, if the cracker believes it has a correct password, it checks the checksum of a random packet. If this is successful, the correct password is printed in ASCII and Hex, and the successful crack is indicated by an 'X' in the leftmost column of the display.
When executing the cracking operation, crack operates with a partial key search from the given data. Since it is a probabalistic attack, The best guess may not be the right one, so, with limited captured data and enough CPU power, you can perform more exaustive searches. By setting the breadth parameter, you can specify to search "worse" guesses. It is not suggested that you specify a breadth of more than three or four.
- Save and Restore
Airsnort saves data in two formats. All packets captured by aisrnort can be saved in pcap dump file format by selecting the "Log to file" option from the File menu. This must be done before a capture session is initiated. Airsnort can also save a much smaller amount of data of data about a capture session in the form of "crack" files. These files represent the minimum amount of data that airsnort maintains for each access point that it discovers. Crack files contain summary data of those packets that airsnort has seen that actually use weak IVs. Airsnort will always ask the user to save data to a crack file whenever the program is terminated. By using save files, airsnort session can effectively be paused and resumed at a later time by first loading the save file, then starting a capture session. Restoration of data from a pcap dump file amounts to replaying the entire capture session from which the dump file was created, all statistics will reflect what was seen during the live capture session. Restoration of data from a crack file will only display statistics about packets that use weak IVs, thus packet counts are likely to be much smaller than seen during the live capture. It is possible to load a pcap dump file and create a corresponding crack file in order to reduce the amount of stored data.