Wireless Security is covered in this lecture session. The components of 802.11 or also known as Wi-Fi are wireless station and access point. The Wi-Fi focuses on Layer 1 and 2 in the OSI model. It has two modes namely infrastructure and ad-hoc mode.
RTS/CTS (Request to Send / Clear to Send) is the optional mechanism used by the 802.11 wireless networking protocol to reduce frame collisions introduced by the hidden terminal problem. Originally the protocol fixed the exposed terminal problem as well, but modern RTS/CTS include ACKs and do not solve the exposed terminal problem.
In Wi-Fi we have three kinds namely 802.11a, 802.11b and 802.11g.
When a Wi-Fi client enters range of one or more APs, APs does the following:
• Send beacons
• Beacon includes SSID
• AP chosen on signal strength and observed error rates
• Accepts client – client tunes to AP channel
Wi-Fi client checks for stronger and more reliable APs and re-associate with the new AP.
Re-associations with APs are done when moving out of range, high error rates, and high network traffic.
AP has 14 channels and only three channels don’t overlap.
Open System Authentication (OSA) is a process by which a computer can gain access to a wireless network that uses the Wired Equivalent Privacy (WEP) protocol. With OSA, a computer equipped with a wireless modem can access any WEP network and receive files that are not encrypted.
Access points have Access Control Lists (ACL), a list of allowed MAC addresses. However, they are sniffable and spoofable.
As we all know that wireless LAN uses radio signal. This signal is usually weakened by walls, floors and other radio interferences. The way to solve this problem is by using a directional antenna since it allows interception over longer distances and provides focused reception.
Three basic services provided for the Wi-Fi environment are authentication, integrity and confidentiality.
Security services provided by the 802.11b are shared key authentication and Wired Equivalence Privacy (WEP encryption).
Wired Equivalent Privacy (WEP) is a deprecated algorithm to secure IEEE 802.11 wireless networks. Wireless networks broadcast messages using radio and are thus more susceptible to eavesdropping than wired networks.
RC4 (also known as ARC4 or ARCFOUR meaning Alleged RC4, see below) is the most widely-used software stream cipher and is used in popular protocols such as Secure Sockets Layer (SSL) (to protect Internet traffic) and WEP (to secure wireless networks).
In two devices that use Shared Key Encryption, the steps that occur are as follows:
1. The station sends an authentication request to the access point.
2. The access point sends challenge text to the station.
3. The station uses its configured 64-bit or 128-bit default key to encrypt the challenge text, and it sends the encrypted text to the access point.
4. The access point decrypts the encrypted text using its configured WEP key that corresponds to the station's default key. The access point compares the decrypted text with the original challenge text. If the decrypted text matches the original challenge text, then the access point and the station share the same WEP key, and the access point authenticates the station.
5. The station connects to the network.
In WEP safeguards, shared secret key is required for association with access point, sending and receiving data. Messages are encrypted for confidentiality and possess checksum for integrity. But the management traffic still broadcasts SSID.
An initialization vector (IV) is a block of bits that is required to allow a stream cipher or a block cipher to be executed in any of several streaming modes of operation to produce a unique stream independent from other streams produced by the same encryption key, without having to go through a (usually lengthy) re-keying process.
To commit a passive WEP attack, the attacker collects all traffic. By this, he/she could collect two messages:
• Encrypted with same key and IV
• Statistical attacks to reveal plaintext
• Plaintext XOR Ciphertext = Keystream
For active WEP attack, if the attacker knows plaintext and ciphertext pair, the keystream is known, the attacker can create correctly encrypted messages and the AP is deceived into accepting the message.
A bit-flipping attack is an attack on a cryptographic cipher in which the attacker can change the ciphertext in such a way as to result in a predictable change of the plaintext, although the attacker is not able to learn the plaintext itself.
Some vendors allow limited WEP keys from the passphrase created by users into only 21 bits of entropy in 40bit key. But this key is weaker and easier to be cracked.
A brute force attack is a strategy used to break the encryption of data. It involves traversing the search space of possible keys until the correct key is found. With this, ciphertext is captured with IV and 240 possible secret keys are searched. Lastly ciphertext is decrypted.
WEP has extended to 128 bits, which means 104 bit secret key and 24 bit IV is available. So, this makes It harder to decrypt.
Weakness of WEP:
• The initialization vector is too small (16 million IV values).
• The integrity check value (ICV) algorithm is not appropriate.
• WEP’s use of RC4 is weak.
• Authentication messages can be easily forged.
WEP cracking tools include:
• WEPCRACK – first tool to demonstrate attack using IV weakness
• AIRSNORT – Automated tool that sniffs, searches for weaker IVs, records encrypted data until key is derived.
Ways to generate WEP traffic include:
• Capture encrypted ARP request packets
• Anecdotally lengths of 68, 118 and 368 bytes appear appropriate
• Replay encrypted ARP packets to generate encrypted ARP replies.
The above ways are implemented into Aireplay.
For this entire lesson, we know that wireless LAN is not reliable and cannot be trusted. We still need firewall between WLAN and Backbone, extra authentication, IDS at WLAN/Backbone junction, and assessments on vulnerabilities in WLAN.
It is pretty easy to search for unauthorized APs, ad-doc networks and clients. We can easily do port-scanning for unknown SNMP agents and web/telnet interfaces, plus war-walking where you can walk around with your laptop and sniff data packets, identify IP addresses and detect signal strength.
There are also wireless intrusion detection tools to counter this above attacks, such as Airmagnet, AirDefense, Trapeze, Aruba, etc.
AP’s security should be reviewed. Firewalls and router ACLs should be utilized and limit the usage of AP administration interfaces. Configuration of APs should be done at SSID, WEP keys, and community string and password policy.
Station protection includes personal firewalls, VPN from station into Intranet, host intrusion detection, configuration scanning.
Locations of APs should be set at the center of buildings. Radio signal should be pointed using a directional antenna.
Temporal Key Integrity Protocol (TKIP) implements a key mixing function that combines the secret root key with the initialization vector before passing it to the RC4 initialization.
Wi-Fi Protected Access (WPA and WPA2) is a certification program created by the Wi-Fi Alliance to indicate compliance with the security protocol created by the Wi-Fi Alliance to secure wireless computer networks. This protocol was created in response to several serious weaknesses researchers had found in the previous system, WEP (Wired Equivalent Privacy).
WPA has two modes being the:
• pre-shared mode which uses pre-shared keys
• Enterprise mode which uses the Extensible Authentication Protocol (EAP), the transport for authentication.
Practical WPA attacks include:
• Dictionary attack on pre-shared key mode
• Denial of Service (DoS) attack
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