The messages that pass capabilities information include capability bits and Information Elements, as described in Chapter 5. RSN/WPA systems have a specific Information Element that is used to negotiate the type of security that will be used. This works as follows. If an access point supports either RSN or WPA (or both), it includes in its beacon and probe response an Information Element with the following information:
Whether the access point is using preshared key or authentication server (key management)
What group security mechanism is operating
A list of one or more pairwise key security mechanisms that are supported
For example, a company that is transitioning from WEP to WPA might use WEP for broadcast (group) security and allow either WEP or TKIP on a device-by-device basis. The Information Element would inform WPA devices and they would select to use WEP/TKIP. The older WEP stations would not understand the new Information Element and would continue to use WEP/WEP, which is acceptable in this case. Later, the company might discontinue the use of WEP and the Information Element would indicate TKIP for broadcasts and only TKIP for pairwise connections.
If that same company then migrated to RSN, it might start advertising TKIP for broadcast and a choice of AES or TKIP for pairwise connections. The Information Element for RSN is not quite the same as for WPA and may contain more information. RSN is indicated by a capability bit and, if this bit is set, the default is to use AES?CCMP for both group and pairwise connections. The Information Element would be needed only if, as in the example above, a choice was offered.
The Information Element (IE) described so far is sent by the access point in beacons and probe responses. The mobile device must also include an Information Element in its association request if it wants to use the security capabilities. Although the IE sent by the access point might have a list of protocols to choose, the one sent with the association request must indicate only a single choice. This is the selection made by the mobile device and defines the protocol that will be used from that point on.
If the access point advertises a choice of TKIP or WEP, the mobile device may legitimately select to use WEP. This would be pretty strange, though. If the mobile device understands the Information Element, it must support WPA or RSN, so why would it choose an inferior security system like WEP? The simple answer is that it would not?unless there had been foul play. This example leads us to a potential weakness that must be prevented.
Suppose an attacker watches an access point and makes a note of what information is sent in probe responses. Remember that these messages are not encrypted; they are open for all to see. Suppose the access point is offering both TKIP and WEP. Now a new mobile device arrives and issues a probe request. The access point responds, but the attacker goes into action and blocks the response by transmitting some well-timed garbage. The attacker now forges a message that looks exactly like the valid response except that it offers only WEP as a choice. The mobile device thinks the access point only supports WEP and associates with this choice. The access point might think this is strange, but it appears quite valid. What the attacker has achieved is to force the mobile device to use a weaker security method; he has successfully weakened the target system.
To prevent this type of attack, both the access point and the mobile device send another copy of the valid Information Element during the pairwise four-way handshake. The four-way handshake is protected against any sort of tampering so, although the attacker can substitute the modified Information Element in the original response, he can't substitute it in the four-way handshake. Therefore, by keeping a copy of the original message, both the mobile device and the access point can detect the attack and drop the connection.
In this example, protection of the Information Element sent by the mobile device seems less important. Suppose the mobile device selects TKIP and indicates this in its association request. There wouldn't be much point in an attacker changing the selection to WEP because, even if accepted by the access point, not much will happen when the mobile device sends TKIP-encrypted frames to an access point that is expecting WEP! However, there is another reason for protecting the mobile device's selection. This is a more subtle reason and is associated with the process of preauthentication described in the next section.