WEP (IV) (40 bits) - (104 bits)

shared key (40 bits) + IV (initialization vector) (24 bits) 
= 64 bits 

ICV = CRC-32 + IV

shared key (same) + IV (evolution)
 
Prediction de (SharedKey, IV) (IP)

Attaques passives (p.ex. airsnort, wepcrack...)