802.1X | Wi-Fi Radio Types

802.1X, also known as EAPOL, for EAP over LAN, is a basic protocol supported by enterprise-grade Wi-Fi networks, as well as modern wired Ethernet switches and other network technologies. The idea behind 802.1X is to allow the user's device to connect to the network as if the RADIUS server and advanced authentication systems did not exist, but to then block the network link for the device for all other protocols except 802. IX, until authentication is complete. The network's only requirements are twofold: prevent all data traffic from or to the client except for EAPOL (using Ethernet protocol 0×888E) from passing; and taking the EAPOL frames, removing the EAP messages embedded within, and tunneling those over the RADIUS protocol to the AAA server.
The job of the network, then, is rather simple. However, the sheer number of protocols can make the process seem complex. We'll go through the details slowly. The important thing to keep in mind is that 802.1X is purely a way of opening what acts like a direct link between the AAA server and the client device, to allow the user to be authenticated by whatever means the AAA server and client deem necessary. The protocols are all layered, allowing the highest-level security protocols to ride on increasingly more specific frames that each act as blank envelopes for its contents.
Once the AAA server and the client have successfully authenticated, the AAA server will use its RADIUS link to inform the network that the client can pass. The network will tear down its EAPOL-only firewall, allowing generic data traffic to pass. In the same message that the AAA server tells the network to allow the client (an EAP Success), it also passes the PMK—the master key that the client also has and will be used for encryption—to the network, which can then drop into the four-way handshake to derive the PTK and start the encrypted channel. This PMK exchange goes in an encrypted portion of the EAP response from the RADIUS server, and is removed when the EAP Success is forwarded over the air. The encryption is rather simple, and is based on the shared password that the RADIUS server and controller or access point have. Along with the PMK comes a session lifetime. The RADIUS server tells the controller or access point how long the authentication, and subsequent use of the keys derived from it, is valid. Once that time expires, both the access point and the client are required to erase any knowledge of the key, and the client must reauthenticate using EAP to get a new one and continue using the network.
For network administrators, it is important to keep in mind that the EAP traffic in EAPOL is not encrypted. Because the AAA server and the client have not agreed on the keys yet, all of the traffic between the client and the RADIUS server can be seen by passive observers. This necessarily limits the EAP methods—the specific types of authentication—that can be used. For example, in the early days of 802.1X, an EAP method known as EAP-MD5 was used, where the user typed a password (or the client used the user's computer account password), which was then hashed with the MD5 one-way cryptographic hash algorithm, and then sent across the network. Now, MD5 is flawed, but is still secure enough that an attacker would have a very hard time reverse-engineering the password from the hash of it. However, the attacker wouldn't need to do this, as he could just replay the same MD5 hashed version himself, as if he were the original user, and gain access to the network. For this reason, no modern wireless device supports EAP-MD5 for wireless authentication.

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