WEP is supposed to protect data sent over a wireless link. As discussed in the t
ID: 3822857 • Letter: W
Question
WEP is supposed to protect data sent over a wireless link. As discussed
in the text, WEP has many security flaws, one of which involves its use
of initialization vectors, or IVs. WEP IVs are 24 bits long. WEP uses
a fixed long-term key K. For each packet, WEP sends an IV in the
clear along with the encrypted packet, where the packet is encrypted
with a stream cipher using the key Kjy = (IV, K), that is, the IV is
pre-pended to the long-term key K. Suppose that a particular WEP
connection sends packets containing 1500 bytes over an 11 Mbps link.
a) If the IVs are chosen at random, what is the expected amount of time until the first IV repeats? What is the expected amount of time until some IV repeats?
b) If the IVs are not selected at random but are instead selected in sequence, say, IV j = i, for i = 0,1,2,... ,22 4 — 1, what is the expected amount of time until the first IV repeats? What is the expected amount of time until some IV is repeated?
c) Why is a repeated IV a security concern?
d) Why is WEP "unsafe at any key length" [321]? That is, why is WEP no more secure if K is 256 bits than if K is 40 bits?
Explanation / Answer
a) If the IVs are chosen at random, what is the expected amount of time until the first IV repeats? What is the expected amount of time until some IV repeats?
--> Should repeat after about like 5000 packets or so.
b) If the IVs are not selected at random but are instead selected in sequence, say, IV j = i, for i = 0,1,2,... ,22 4 — 1, what is the expected amount of time until the first IV repeats? What is the expected amount of time until some IV is repeated?
--> Should be around 3000 packets or so.
c) Why is a repeated IV a security concern?
--> Always not good to be able to predict security and then break into it.
d) Why is WEP "unsafe at any key length" [321]? That is, why is WEP no more secure if K is 256 bits than if K is 40 bits?
--> There is a weakness in the key scheduling algorithm of RC4, so that no matter how long,it can be exploited. And so, always exploited and broken into, no matter the length with the same easiness only variable by the length (factor of length n, still n time)
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