07-08-2012, 04:36 PM
Wireless Security
1Wireless Security.pdf (Size: 304.24 KB / Downloads: 147)
Wireless Architecture
The obvious architecture is pure peer-to-peer
— each machine has a radio, and talks directly
to any other machine
n In fact, 802.11 (WiFi) can work that way, but
rarely does
n More common scenario: base stations (also
known as access points)
Access Points
An ordinary wireless node associates with an
access point (AP)
More precisely, it associates with the AP
having a matching network name (if specified)
and the strongest signal
If another AP starts sending a stronger signal
(probably because the wireless node has
moved), it will reassociate with the new access
point
All transmissions from the laptop go to the
access point
All transmissions to the laptop come from the
access point
Why This Works
Conventionally, we worry about authenticating
the client to the server Here, we need to authenticate the server to
the client
The infrastructure wasn’t designed for that;
more important, users don’t expect to check
for it (and have no way to do so in any event)
Key Setup for WEP
Each WEP node keeps a 24-bit packet counter
(the IV)
Actual cipher key is configured key
concatenated with counter
Two different flaws. . .
224 packets isn’t that many — you still get key
reuse when the packet counter overflows
RC4 has a cryptanalytic flaw
Packet Redirection
Suppose you know (or can guess) the
destination IP address of a packet
n Because RC4 is a stream cipher, you can make
controlled changes to the plaintext by flipping
ciphertext bits
n Flip the proper bits to send the packet to you
instead, and reinject it
n But it’s worse than that
1Wireless Security.pdf (Size: 304.24 KB / Downloads: 147)
Wireless Architecture
The obvious architecture is pure peer-to-peer
— each machine has a radio, and talks directly
to any other machine
n In fact, 802.11 (WiFi) can work that way, but
rarely does
n More common scenario: base stations (also
known as access points)
Access Points
An ordinary wireless node associates with an
access point (AP)
More precisely, it associates with the AP
having a matching network name (if specified)
and the strongest signal
If another AP starts sending a stronger signal
(probably because the wireless node has
moved), it will reassociate with the new access
point
All transmissions from the laptop go to the
access point
All transmissions to the laptop come from the
access point
Why This Works
Conventionally, we worry about authenticating
the client to the server Here, we need to authenticate the server to
the client
The infrastructure wasn’t designed for that;
more important, users don’t expect to check
for it (and have no way to do so in any event)
Key Setup for WEP
Each WEP node keeps a 24-bit packet counter
(the IV)
Actual cipher key is configured key
concatenated with counter
Two different flaws. . .
224 packets isn’t that many — you still get key
reuse when the packet counter overflows
RC4 has a cryptanalytic flaw
Packet Redirection
Suppose you know (or can guess) the
destination IP address of a packet
n Because RC4 is a stream cipher, you can make
controlled changes to the plaintext by flipping
ciphertext bits
n Flip the proper bits to send the packet to you
instead, and reinject it
n But it’s worse than that