The WEP has defenses against the playback attack. To avoid encrypting two
cipher texts with the same key stream, an Initialization Vector (IV) is used to
augment the shared WEP key (secret key) and produce a different RC4 key for each
packet. The IV is also included in the package. The WEP keys (secret key) are
available in two types, 64-bit and 128-bit. Often you will see them referenced as
40-bit and 104-bit instead. The reason for this misnomer is that the WEP key (40/104
bits) is concatenated with the initialization vector (24 bits) resulting in a 64/128 bit
total key size.
