Wireless Security.pdf - PDF Archive

Wireless LAN Security 421
with (or without) modification such that the receiving station has no way to tell that the
packet it is receiving is an old (replayed) packet. To see how this attack can be exploited,
consider a hypothetical scenario where Alice is an account holder, Bob is a bank and
Eve is another account holder in the bank. Suppose Alice and Eve do some business and
Alice needs to pay Eve $500. So, Alice connects to Bob over the network and transfers
$500 from her account to Eve. Eve, however, is greedy. She knows Alice is going to
transfer money. So, she captures all data going from Alice to Bob. Even though Eve
does not know what the messages say, she has a pretty good guess that these messages
instruct Bob to transfer $500 from Alice’s account to Eve’s. So, Eve waits a couple of
days and replays these captured messages to Bob. This may have the effect of transferring
another $500 from Alice’s account to Eve’s account unless Bob has some mechanism for
determining that he is being replayed the messages from a previous session.
Replay attacks are usually prevented by linking the integrity protection mechanism to
either timestamps and/or session sequence numbers. However, WEP does not provide for
any such protection.
18.7 Loopholes in 802.11 Security
To summarize, here is the list of things that are wrong with 802.11 security:
1. 802.11 does not provide any mechanism for key establishment over an unsecure
medium. This means key sharing among STAs in a BSS and sometimes across
BSSs.
2. WEP uses a synchronous stream cipher over a medium, where it is difficult to
ensure synchronization during a complete session.
3. To solve the previous problem, WEP uses a per-packet key by concatenating the
IV directly to the preshared key to produce a key for RC4. This exposes the base
key or master key to attacks like FMS.
4. Since the master key is usually manually configured and static and since the IV
used in 802.11 is just 24 bits long, this results in a very limited key-space.
5. 802.11 specifies that changing the IV with each packet is optional, thus making
key reuse highly probable.
6. The CRC-32 used for message integrity is linear.
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