Hacking - The Art of Exploitation, 2nd Edition

Cryptology 439

exist in the packet in the binary form of high- and low-order 16-bit words.

The conversion is fairly simple:

Src IP = 192.168.2.57

SH= 192 · 256 + 168 = 50344

SL =2·256+57=569

Dst IP = 192.168.2.1

DH= 192 · 256 + 168 = 50344

DL= 2 · 256 + 1 = 513

New IP = 123.45.67.89

NH = 123 · 256 + 45 = 31533

NL = 67 · 256 + 89 = 17241

The checksum will be changed by NH + NL − DH − DL, so this value must

be subtracted from somewhere else in the packet. Since the source address is

also known and doesn’t matter too much, the low-order 16-bit word of that

IP address makes a good target:

S'L= SL− (NH+NL −DH −DL)

S'L=569−(31533 + 17241− 50344 − 513)

S'L= 2652

The new source IP address should therefore be 192.168.10.92. The

source IP address can be modified in the encrypted packet using the same

XORing trick, and then the checksums should match. When the packet is

sent to the wireless access point, the packet will be decrypted and sent to

123.45.67.89, where the attacker can retrieve it.

If the attacker happens to have the ability to monitor packets on an

entire class B network, the source address doesn’t even need to be modified.

Assuming the attacker had control over the entire 123.45.X.X IP range, the

low-order 16-bit word of the IP address could be strategically chosen not to

disturb the checksum. If NL = DH + DL − NH, the checksum won’t be changed.

Here’s an example:

NL =DH+DL− NH

NL = 50,344 + 513− 31,533

N'L= 82390

The new destination IP address should be 123.45.75.124.

0x785 Fluhrer, Mantin, and Shamir Attack

The Fluhrer, Mantin, and Shamir (FMS) attack is the most commonly

used attack against WEP, popularized by tools such as AirSnort. This attack