Shared vs. Open Authentication

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When a wireless client (or station) connects to a wireless access point, there are 2 steps involved. First the station must be authenticated. If that passes, the station can then be associated. Once that’s done, traffic can pass.

As a part of using WEP, there is a type of authentication called Shared Key that can be used (not available if you’re not doing WEP). Shared Key Authentication is considered to be a BIG security hole! In fact, the upcoming 802.11i amendment prohibits its use.Here’s why:

For shared key authentication, the Access Point (AP) generates some random string of ASCII, sends it CLEARTEXT to the Station, the station encrypts it using his configured WEP key, sends it ENCRYPTED to the AP, and the AP then decrypts it to see if the starting ASCII string is produced. This is meant to insure both sides have the same key. The problem is that 2 of 3 parts of the encryption scheme are sent over the air, and makes it much easier for a hacker to figure out the WEP key. Therefore it’s generally recommended to not use shared key authentication anymore.

(For the curious Using Open Authentication ends up being better than shared key, because the authentication step ends up being a NULL step – the station is just automatically authenticated, and subsequently associated. But they still have to have the right WEP key for the encryption/decryption on the actual packets to work. A station that is allowed to associate, but can’t pass traffic is considered a lesser threat.