(ISC)2 CISSP-ISSAP Exam Adam works as a Network Administrator. He discovers that the wireless AP transmits 128 bytes of plaintext, and the station responds by encrypting the plaintext. It then…

Question

Options

• APre-shared key authentication
• BOpen system authentication
• CShared key authentication
• DSingle key authentication

• A
  17% (4)
• B
  4% (1)
• C
  70% (16)
• D
  9% (2)

Explanation

Shared Key Authentication (SKA) is the correct answer because it uses a four-step challenge-response handshake: the AP sends a plaintext challenge (128 bytes), the client encrypts it using the pre-configured WEP key, sends it back, and the AP verifies the result. The use of WEP's RC4 cipher and the same key for both authentication and subsequent traffic is the defining characteristic of SKA.

Why the distractors are wrong:

• A (Pre-shared key): PSK is associated with WPA/WPA2-Personal, not WEP's challenge-response process - it's a different protocol family entirely.
• B (Open system): Open system authentication requires no cryptographic challenge at all - the AP accepts any client that requests access, making it essentially null authentication.
• D (Single key authentication): This is not a defined 802.11 standard authentication type; it's a fabricated distractor with no formal specification.

Memory tip: Associate Shared Key = Challenge-Response - the AP challenges you to prove you hold the shared key by encrypting its plaintext. If you can encrypt it correctly, you must have the key. This also doubles as a security weakness to remember: because the plaintext and ciphertext are both transmitted, an attacker can derive the keystream, making SKA less secure than Open System authentication in practice.

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