Network Security and Privacy
Wireless networks are, by nature, much less secure than their more mature wired cousins. Since wireless NICs use the air as their data transport medium, they are vulnerable to unauthorized use and eavesdropping. A network "sniffer" could be used to monitor and steal network information with a heightened sense of ease versus a wired LAN. Without the need for a physical connection to access a wireless networks, they can be easily infiltrated. All the would-be hacker needs is a wireless NIC and knowledge of the current weaknesses of wireless network security to guide them.
In an attempt to curb attacks from would-be hackers, the standards implement what is called the wired equivalency protocol (WEP). In theory, the idea is that this protocol will protect network privacy. As a secondary function, WEP is used to prevent unauthorized access to the wireless network. Analysis performed by several researchers has shown this protocol to fall short of these two fundamental goals. It has been found that this protocol is subject to the following attacks:
- Passive attacks to decrypt traffic based on statistical analysis
- Active attacks to inject new traffic from unauthorized mobile stations, based on known plaintext.
- Active attacks to decrypt traffic, based on tricking the access point (AP).
- A "Dictionary-building" attack where a day's traffic is monitored and analyzed providing automated real-time decryption of all traffic.
The WEP protocol relies on a secret key that is shared in a basic service set (BSS) - a wireless AP and a set of associated nodes. This key is used to encrypt data packets before they are transmitted. The packets are also checked for integrity to ensure that they have not been modified in transit. One flaw of the 802.11 standard is that it does not address the issue of how shared keys are to be established. In most implementations of wireless networks this is a single key that is shared between each node and access point and is manually set.
The problems with this encryption method lie in the heart of the encryption algorithm. WEP uses the RC4 algorithm, which is a stream cipher. A stream cipher expands a short key into an infinite pseudo-random key stream. The sender uses this key stream by XORing the key stream with the plaintext of the message to produce the ciphertext. The function of an XOR or the "exclusive or" of two bits produces a 1 if either one or the other bit, but not both, being compared is a 1, else it produces a zero. With this in mind, the receiver uses its copy of the key to generate the identical key stream. XORing the ciphertext received with this key stream produces the original plaintext.
In operating in this manner, the stream ciphers lend themselves to several types of attacks. One such attack is the changing of a bit by an attacker in an intercepted packet. In doing so, the data that will be decrypted will be corrupted. Another can lead to the ability to recover all plaintexts sent. In this attack, the eavesdropper need only to intercept two ciphertexts encrypted with the same key stream. With this, it is possible to obtain the XOR of the two plaintexts. Knowledge of this XOR can enable statistical attacks that can recover the plaintexts. As more ciphertexts with the same shared key are known, this attack becomes more convenient. Once one of the plaintexts is known, it is trivial to decipher the others.