Wi-Fi Link Security

To summarize, 802.11 security is provided by three different grades of technology: the outdated and broken WEP, the transition-providing WPA, and the secure and modern WPA2.

WPA and WPA2 are both built on the same framework of 802. Hi, which provides a rich protocol for 802.11 clients and access points to communicate and negotiate which over-the-air encryption and integrity algorithms should be used.

Networks start off with a master key—either a preshared key, entered as text by the user into the access point and mobile device, or generated in real time by enterprise-grade authentication systems. This master key is then used to derive a per-connection key, called the PTK. The PTK is then used to encrypt and provide integrity protection for each frame, using either TKIP for WPA or AES for WPA2.

It bears repeating that preshared keys, for all grades of 802.11 security, have problems that cause both security and management headaches. The biggest security headache is that the privacy of the entire network is based on that PSK being kept private for eternity. If a PSK is ever found out by an attacker—even if that key has been retired or changed a long time ago—then the attacker can use that key to decrypt any recordings of traffic that were taken when the PSK had been in use. Furthermore, because preshared keys are text and are common for all devices, they are easy to share and impossible to revoke. Good users can be fooled into giving the PSK away, or bad users—such as employees who have left the organization—can continue to use the preshared keys as often as they desire.

These problems are solved, however, by moving away from preshared keys to using 802.1X and EAP. Recently, some vendors have been introducing the ability to create per-user preshared keys. The advantage of having per-user keys is that one user's access can be revoked without allowing that user to compromise the rest of the network. The problem with this scheme, however, is the continued lack of forward secrecy, meaning that a user who has his password stolen can still have decrypted every packet ever sent or will send using that key. For this reason, 802.1X is still recommended, using strong EAP methods that provide forward secrecy.