A common reason WEP is crackable is the reuse of the same initialization vector across frames.

• A. True
• B. False
• C. Not applicable
• D. Not sure

Answer: (A)

Reusing the same initialization vector across frames lets an attacker observe the same RC4 keystream for multiple packets. In WEP, each frame is encrypted by XORing the plaintext with a keystream that results from the shared secret key combined with the frame’s IV, which is sent in the clear. If two frames use the same IV, the keystream portion for both frames is identical, so the relationship between the two ciphertexts is C1 xor C2 = P1 xor P2. If the attacker can guess or know part of one plaintext, they can reveal corresponding parts of the other, and with enough such collisions, they can progressively recover plaintext or even the key. The problem is exacerbated by the 24-bit IV, which is too small to avoid repeats in real networks; with typical traffic, IV collisions happen frequently, making this vulnerability practical to exploit. Thus the statement is true: the reuse of the same initialization vector across frames is a common reason WEP is crackable.