Operation of the WEP Protocol
If a user activates WEP, the NIC encrypts the payload, which consists of the frame body and cyclic redundancy check (CRC), of each 802.11 frame before transmission using an RC4 stream cipher provided by RSA Security. The receiving station, such as an access point or another radio NIC, performs decryption when it receives the frame. As a result, 802.11 WEP only encrypts data between 802.11 stations. Once the frame enters the wired side of the network, such as between access points, WEP no longer applies.
As part of the encryption process, WEP prepares a key schedule (“seed”) by concatenating the shared secret key supplied by the user of the sending station with a randomly-generated 24-bit initialization vector (IV). The IV lengthens the life of the secret key because the station can change the IV for each frame transmission. WEP inputs the resulting “seed” into a pseudo-random number generator that produces a key stream equal to the length of the frame’s payload plus a 32-bit integrity check value (ICV).
The ICV is a checksum that the receiving station later recalculates and compares to the one sent by the sending station to determine whether the transmitted data underwent any form of tampering while in transit. In the case of a mismatch, the receiving station can reject the frame or flag the user for potential security violations.
With WEP, the sending and receiving stations use the same key for encryption and decryption. WEP specifies a shared 40- or 104-bit key to encrypt and decrypt data (once the 24-bit IV is added in, this matches FortiWLC (SD)’s 64- or 128-bit WEP specification, respectively). Each radio NIC and access point, therefore, must be manually configured with the same key.
Before transmission takes place, WEP combines the key stream with the payload and ICV through a bit-wise XOR process, which produces cipher text (encrypted data). WEP includes
the IV in the clear (unencrypted) within the first few bytes of the frame body. The receiving station uses this IV along with the shared secret key supplied by the user of the receiving station to decrypt the payload portion of the frame body.
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