THE AUTHENTICATOR DECRYPTS THE CHALLENGE TEXT AND COMPARES THE VALUESAGAINST THE ORIGINAL
4. The authenticator decrypts the challenge text and compares the valuesagainst the original. If they match, the requestor is authenticated. Onthe other hand, if the requestor does not have the shared key, thecipher stream cannot be reproduced. Therefore, the plaintext cannot bediscovered, and theoretically, the transmission is secured.
Figure 9.6
Shared-Key AuthenticationAuthentication RequestAuthentication Response(Challenge)Wired NetworkWireless Client(Encrypted Challenge)(Success)Client WEP Key : 12345AP WEP Key : 12345Continued
One of the greatest weaknesses in shared-key authentication is the fact thatit provides an attacker with enough information to try to crack the WEP secret key.The challenge, which is sent from authenticator to requestor, is sent in the clear.The requesting client then transmits the same challenge, encrypted using the WEPsecret key, back to the authenticator. An attacker who captures both of thesepackets has two pieces to a three-piece puzzle: the cleartext challenge and theencrypted ciphertext of that challenge. The algorithm, RC4, is also known. All thatis missing is the secret key. To determine the key, the attacker simply tries a brute-force search of thepotential key space using a dictionary attack. At each step, the attacker tries todecrypt the encrypted challenge with a dictionary word as the secret key. The resultis then compared against the authenticator’s challenge. If the two match, theattacker has determined the secret key. In cryptography, this attack is called aknown plaintext attackand is the primary reason that shared-key authentication isconsidered slightly weaker than open authentication.Understanding WEP Vulnerabilities
Like any standard or protocol,WEP has some inherent disadvantages.The focus of securityis to allow a balance of access and control while juggling the advantages and disadvantagesof each implemented countermeasure for security gaps.WEP’s disadvantages include:■
The RC4 encryption algorithm is a known stream cipher.This means that it takesa finite key and attempts to make an infinite pseudorandom key stream in orderto generate the encryption.■
Altering the secret must be done across the board. All APs and all clients must bechanged at the same time.■
Used on its own,WEP does not provide adequate WLAN security.■
To be effective,WEP has to be implemented on every client as well as on every AP.WEP is part of the 802.11 standard defined for wireless networks in 1999.WEP differsfrom many other kinds of encryption employed to secure network communication in thatit is implemented at MAC sublayer of the Data Link layer (Layer 2) of the OSI model.Security can be implemented at many layers of the model. IPSec, for example, is imple-mented at the Network layer (Layer 3) of the OSI model; PPTP creates a secure end-to-end tunnel using the Network layer (GRE) and Transport layer protocols to encapsulateand transport data; HTTP-S and SSH are Application layer (Layer 7) protocols forencrypting data. Due to the complexity of the 802.11 MAC and the amount of processingpower it requires, the 802.11 standard made 40-bit WEP an optional implementation.