How to Decrypt 802.11
Wireshark can decrypt WEP and WPA/WPA2 in pre-shared (or personal) mode. WPA/WPA2 enterprise mode decryption is not yet supported.
You can add decryption keys using Wireshark's 802.11 preferences or by using the wireless toolbar. Up to 64 keys are supported.
Adding Keys: 802.11 Preferences
Go to Edit->Preferences->IEEE 802.11. You should see a window that looks like this:
Note that the key examples mention WPA, and that each key item is labeled "Key". If your preferences window doesn't mention WPA, like this
then your version of Wireshark only supports WEP decryption. This might be the case with older versions of Wireshark, particularly the 64-bit Windows version.
In all versions WEP keys can be specified as a string of hexadecimal numbers, with or without colons:
In versions that support WPA decryption you should use a prefix to tell Wireshark what kind of key you're using:
wep The key is parsed as a WEP key.
wpa-pwd The password and SSID are used to create a raw pre-shared key.
wpa-psk The key is parsed as a raw pre-shared key.
Adding Keys: Wireless Toolbar
If you are using the Windows version of Wireshark and you have an AirPcap adapter you can add decryption keys using the wireless toolbar. If the toolbar isn't visible, you can show it by selecting View->Wireless Toolbar. Click on the Decryption Keys... button on the toolbar:
This will open the decryption key managment window. As shown in the window you can select between three decryption modes: None, Wireshark, and Driver:
Selecting None disables decryption. Selecting Wireshark uses Wireshark's built-in decryption features. Driver will pass the keys on to the AirPcap adapter so that 802.11 traffic is decrypted before it's passed on to Wireshark. Driver mode only supports WEP keys.
Along with decryption keys there are other preference settings that affect decryption.
Make sure Enable decryption is selected.
You may have to toggle Assume Packets Have FCS and Ignore the Protection bit depending on how your 802.11 driver delivers frames.
The WPA passphrase and SSID preferences let you encode non-printable or otherwise troublesome characters using URI-style percent escapes, e.g. %20 for a space. As a result you have to escape the percent characters themselves using %25.
WPA and WPA2 use keys derived from an EAPOL handshake, which occurs when a machine joins a Wi-Fi network, to encrypt traffic. Unless all four handshake packets are present for the session you're trying to decrypt, Wireshark won't be able to decrypt the traffic. You can use the display filter eapol to locate EAPOL packets in your capture.
In order to capture the handshake for a machine, you will need to force the machine to (re-)join the network while the capture is in progress. One way to do this is to put the machine to sleep (for smartphones and tablets, "turning off" the machine puts it to sleep) before you start the capture, start the capture, and then wake the machine up. You will need to do this for all machines whose traffic you want to see.
WPA and WPA2 use individual keys for each device. Older versions of Wireshark may only be able to use the most recently calculated session key to decrypt all packets. Therefore, when several devices have attached to the network while the trace was running, the packet overview shows all packets decoded, but in the detailed packet view, only packets of the last device that activated ciphering are properly deciphered.
The "password" key preference has the form wpa-pwd:password:ssid. You can optionally omit the SSID, and Wireshark will try to decrypt packets using the last-seen SSID. This may not work on busy networks, since the last-seen SSID may not be correct. For the key "Induction" and SSID "Coherer", the following key preferences are equivalent:
The file SampleCaptures/wpa-Induction.pcap has WPA traffic encrypted using the password "Induction" and SSID "Coherer".