intext Initialisation Secret Keystream vector Ciphertext Figure 8-3: WEP Encryption Process Step (2) the ICV is appended to the data block Step (3) The initialisation vector is combined with the secret key to generate the full encryption key Step (4) the RC4 algorithm is used to transform the encryption key into a key stream Step (5) an XOR operation is performed between the key stream and the output of Step (2) Step (6) the initialisation vector is combined with the cipher text. Although WEP provides a reasonable level of security against casual eavesdroppers, its weaknesses were recognised soon after its release as part of the 802.11 standard. In 2001, cryptographers Scott Fluhrer, Itsik Mantin and Adi Shamir realised that, because of a weakness in the RC4 key scheduling algorithm, the output key stream was significantly non-random. This allows the encryption key to be determined by analysing a sufficiently large number of data packets encrypted using the key. In essence, WEP transmits information about the encryption key as part of the encrypted message SO that a determined hacker, equipped with the necessary tools, could collect and analyse transmitted data to extract the encryption key. This requires several million packets to be intercepted and analysed, but could still be accomplished in under an hour on a high traffic network. WEP also uses a static shared key, as there is no mechanism for changing the key other than manual re-entry of a new key or passphrase into every device that operates on the WLAN. Chapter Eight It was not technological limitations that limited the strength of the encryption algorithm used in 802.11 but, interestingly, US export controls. The export of data encrypting technology was considered by the US government to be a threat to national security and as a result the WEP scheme could not be the strongest available if it was to be adopted as an international standard. These restrictions have since been lifted and new, more powerful encryption methods have been developed, such as the