Encryption in the 802.11 Standard

The 802.11 specification provides data privacy with the WEP algorithm. WEP is based on the
RC4 symmetric stream cipher. The symmetric nature of RC4 requires that matching WEP keys, either 40 or 104 bits in length, must be statically configured on client devices and access points (APs). WEP was chosen primarily because of its low computational overhead. Although 802.11-enabled PCs are common today, this situation was not the case back in 1997. The majority of WLAN devices were application-specific devices (ASDs). Examples of ASDs include barcode scanners, tablet PCs, and 802.11-based phones. The applications that run on ASDs generally do not require much computational power, so as a result, ASDs have meager CPUs. WEP is a simple-to-implement algorithm that you can write in as few as 30 lines of code, in some cases. The low overhead incurred by WEP made it an ideal encryption algorithm to use on ASDs.

To avoid the ECB mode of encryption, WEP uses a 24-bit IV, which is concatenated to the key
before being processed by the RC4 cipher. Figure 4-5 shows a WEP-encrypted frame, including the IV.

The IV must change on a per-frame basis to avoid IV collisions. IV collisions occur when the same IV and WEP key are used, resulting in the same key stream being used to encrypt a
frame. This collision gives attackers a better opportunity to guess the plaintext data by
seeing similarities in the ciphertext. The point of using an IV is to prevent this scenario, so it is important to change the IV often. Most vendors offer per-frame IVs on their WLAN devices.


The 802.11 specification requires that matching WEP keys be statically configured on both
client and infrastructure devices. You can define up to four keys on a device, but you can use only one at a time for encrypting outbound frames. Figure 4-6 shows a Cisco Aironet client configuration screen for WEP configuration.



In addition to data encryption, the 802.11 specification provides for a 32-bit value that functions as an integrity check for the frame. This check tells the receiver that the frame has arrived without being corrupted during transmission. It augments the Layer 1 and Layer 2 frame check sequences (FCSs), which are designed to check for transmission-related errors.

The ICV is calculated against all fields in the frame using a cyclic redundancy check (CRC)-32 polynomial function. The sender calculates the values and places the result in the ICV field. The ICV is included in the WEP-encrypted portion of the frame, so it is not plainly visible to eavesdroppers. The frame receiver decrypts the frame, calculates an ICV value, and compares what it calculates against what has arrived in the ICV field. If the values match, the frame is considered to be genuine and untampered with. If they don't match, the frame is discarded. Figure 4-8 diagrams the ICV operation.

Overview of Encryption

Data encryption mechanisms are based on cipher algorithms that give data a randomized
appearance. Two type of ciphers exist:

• Stream ciphers
  

• Block ciphers

Both cipher types operate by generating a key stream from a secret key value. The key stream is mixed with the data, or plaintext, to produce the encrypted output, or ciphertext. The two cipher types differ in the size of the data they operate on at a time.

A stream cipher generates a continuous key stream based on the key value. For example, a stream cipher can generate a 15-byte key stream to encrypt one frame and a 200-byte key stream to encrypt another. Figure 4-2 illustrates stream cipher operation. Stream ciphers are small and efficient encryption algorithms and as a result do not incur extensive CPU usage. A commonly used stream cipher is RC4, which is the basis of the WEP algorithm.

Figure 4-3. Block Cipher Operation

The process of encryption described here for stream ciphers and block ciphers is known as Electronic Code Book (ECB) encryption mode. ECB mode encryption has the characteristic that the same plaintext input always generates the same ciphertext output. The input plaintext always produces the same ciphertext. This factor is a potential security threat because eavesdroppers can see patterns in the ciphertext and start making educated guesses about the original plaintext.

Some encryption techniques can overcome this issue:

• Initialization vectors

• Feedback modes

Initialization Vectors

An initialization vector (IV) is a number added to the key, which has the end result of altering the key stream. The IV is concatenated to the key before the key stream is generated. Every time the IV changes, so does the key stream. Figure 4-4 shows two scenarios. The first is stream cipher encryption without the use of an IV. In this case, the plain text DATA when mixed with the key stream 12345 always produces the ciphertext AHGHE. The second scenario shows the same plaintext mixed with the IV augmented key stream to generate different ciphertext. Note that the ciphertext output in the second scenario is different from the ciphertext output from the first. The 802.11 world recommends that you change the IV on a per-frame basis. This way, if the same frame is transmitted twice, it's highly probable that the resulting ciphertext is different for each frame.

Figure 4-4. Encryption and Initialization Vectors