Imagine a Wi-Fi network protected by WEP/WPA/WPA2. Users can join the network in case they know the pre-shared key of the access point.
Is it possible for a user A (who has successfully logged into the network) to sniff on the data communication of another user B, entirely because A uses the same pre-shared key like B (and C, D, E....)?
Well, it depends on the which protocol is being used.
For WEP and WPA, the keys used are derived directly from the pre-shared keys; that means that as long as you know the pre-shared keys, you can immediately decrypt packets as well.
On the other hand, WPA2 is somewhat stronger; the two sides exchange nonces to derive the keys. Hence, unless you listen into the initial exchange, you're out of luck. On the other hand, depending on when you start listening, this might not be that great of an improvement.
