Collecting the Frames for Cracking WEP, etection of the Sniffers, Wireless Spoofing, MAC Address Spoofing by gorvam saddar

Collecting the Frames for Cracking WEP

The goal of an attacker is to discover the WEP shared-secret key.  Often, the shared key can be discovered by guesswork based on a certain amount of social engineering regarding the administrator who configures the wireless LAN and all its users.  Some client software stores the WEP keys in the operating system registry or initialization scripts.  In the following, we assume that the attacker was unsuccessful in obtaining the key in this manner.  The attacker then employs systematic procedures in cracking the WEP.  For this purpose, a large number (millions) of frames need to be collected because of the way WEP works.

The wireless device generates on the fly an Initialization Vector (IV) of 24-bits.  Adding these bits to the shared-secret key of either 40 or 104 bits, we often speak of 64-, or 128-bit encryption. WEP generates a pseudo-random key stream from the shared secret key and the IV. The CRC-32 checksum of the plain text, known as the Integrity Check (IC) field, is appended to the data to be sent.  It is then exclusive-ORed with the pseudo-random key stream to produce the cipher text.   The IV is appended in the clear to the cipher text and transmitted. The receiver extracts the IV, uses the secret key to re-generate the random key stream, and exclusive-ORs the received cipher text to yield the original plaintext.

Certain cards are so simplistic that they start their IV as 0 and increment it by 1 for each frame, resetting in between for some events.  Even the better cards generate weak IVs from which the first few bytes of the shared key can be computed after statistical analyses.  Some implementations generate fewer mathematically weak vectors than others do. 

The attacker sniffs a large number of frames from a single BSS.  These frames all use the same key.  The mathematics behind the systematic computation of the secret shared key from a collection of cipher text extracted from these frames is described elsewhere in this volume.  What is needed however is a collection of frames that were encrypted using “mathematically-weak” IVs. The number of encrypted frames that were mathematically weak is a small percentage of all frames.  In a collection of a million frames, there may only be a hundred mathematically weak frames.  It is conceivable that the collection may take a few hours to several days depending on how busy the WLAN is.

Given a sufficient number of mathematically weak frames, the systematic computation that exposes the bytes of the secret key is intensive.  However, an attacker can employ powerful computers.  On an average PC, this may take a few seconds to hours.  The storage of the large numbers of frames is in the several hundred-mega bytes to a few giga bytes range.

An example of a WEP cracking tool is AirSnort ( http://airsnort.shmoo.com ).

3.5           Detection of the Sniffers

Detecting the presence of a wireless sniffer, who remains radio-silent, through network security measures is virtually impossible.  Once the attacker begins probing (i.e., by injecting packets), the presence and the coordinates of the wireless device can be detected.

4.  Wireless Spoofing

There are well-known attack techniques known as spoofing in both wired and wireless networks.  The attacker constructs frames by filling selected fields that contain addresses or identifiers with legitimate looking but non-existent values, or with values that belong to others.  The attacker would have collected these legitimate values through sniffing.

4.1           MAC Address Spoofing

The attacker generally desires to be hidden.  But the probing activity injects frames that are observable by system administrators.  The attacker fills the Sender MAC Address field of the injected frames with a spoofed value so that his equipment is not identified.

Typical APs control access by permitting only those stations with known MAC addresses.  Either the attacker has to compromise a computer system that has a station, or he spoofs with legitimate MAC addresses in frames that he manufactures.  MAC addresses are assigned at the time of manufacture, but setting the MAC address of a wireless card or AP to an arbitrary chosen value is a simple matter of invoking an appropriate software tool that engages in a dialog with the user and accepts values.  Such tools are routinely included when a station or AP is purchased.  The attacker, however, changes the MAC address programmatically, sends several frames with that address, and repeats this with another MAC address.  In a period of a second, this can happen several thousand times.

When an AP is not filtering MAC addresses, there is no need for the attacker to use legitimate MAC addresses.   However, in certain attacks, the attacker needs to have a large number of MAC addresses than he could collect by sniffing.  Random MAC addresses are generated.  However, not every random sequence of six bytes is a MAC address.  The IEEE assigns globally the first three bytes, and the manufacturer chooses the last three bytes.  The officially assigned numbers are publicly available.  The attacker generates a random MAC address by selecting an IEEE-assigned three bytes appended with an additional three random bytes.