This section gets into the details of how the TKIP algorithm is implemented. This may be of interest only if you yourself are a designer. If you don't need to know these details but still want to read on, we admire your dedication!
The first assumption we make is that master keys have been distributed and session keys derived on both sides of the communications link. The master keys could have been obtained using an upper layer authentication method based on EAP, or preshared master keys could be in use. The latter case is analogous to the WEP approach, in which keys are distributed out of band and programmed, or simply typed, into the devices. This approach could be used for smaller networks or ad-hoc mode (IBSS) operation.
Chapter 10 describes the way in which the keys are derived. As noted in that chapter, three types of key are derived for TKIP:
Key to protect the EAPOL-Key exchange messages
Pairwise key to be used for actual message protection using TKIP
Group key to protect broadcasts using TKIP
It is the second two types of key information that we are interested in here. From the pairwise key information are derived temporal keys:
Temporal Encryption key (128 bit): This is used as an input to the key mixing stage prior to actual RC4 encryption.
Temporal Authenticator TX MIC key: This is used with the Michael authentication method to create the MIC on frames transmitted by the authenticator (access point in ESS network).
Temporal Authenticator RX MIC key: This is used with Michael to create the MIC on frames transmitted by the supplicant (typically the mobile device).
For the group keys, only the first two types of key need be derived because broadcasts (and multicasts) are sent out only by the authenticator and never by the supplicant.
TKIP's task is to provide a security service for validating the integrity of received data and to obscure transmitted data beyond recognition. To accomplish this, TKIP employs a set of tools:
IV generation and checking
MIC generation and checking
Encryption and decryption
For the transmit side, the position of these components relative to other MAC activities is shown in Figure 11.4. The four processes of TKIP are shown as follows:
Note that the integrity check value is computed over, and appended to, the MSDU prior to fragmentation. As a result, the check value bytes are present only in the last MPDU and are within the encrypted payload. The original (WEP) checkword, the ICV, is still computed and appended to each MPDU, although it is not included as part of TKIP packet integrity checking.
Because the MIC is computed at the MSDU level, it is not possible to include the IV value in the MIC computation for two reasons. First, because the MSDU might be fragmented, there might be multiple values of IV used to send the fragments of the MSDU, so which value of IV would you choose? Second, the value for the IV must not be selected until after the fragment is removed from the transmission queues. In future to support multimedia, IEEE 802.11e could have up to eight priority queues for outbound frames and the order in which fragments are selected for transmission depends on many factors related to priority and real-time constraints. Therefore, MSDUs of higher priority can overtake a previous MSDU or even interrupt it between fragments. In TKIP, we have only one IV counter for the whole link (and not one per queue), and so the assignment of the IV value has to wait until the last moment, when a fragment is selected for transmission. As a result, the value cannot be known when the MIC is calculated
Computing the MIC at the MSDU level, and not protecting the IV, allows an attacker to block a station by replaying old frames with a new IV value. The problem arises because the IV doubles up as the TKIP sequence counter (TSC) used to avoid replay attacks. Such forged frames will, of course, fail to decrypt correctly and be discarded. They do not threaten the integrity of the protocol. However, the effect is to make subsequent valid frames look like a replay attack. When a valid frame arrives, it might be rejected because the TSC value has been "used up" by the attacking station. This is a class of denial-of-service attack. In wireless, there are many simple ways to deny service that cannot be prevented and do present a potential nuisance.
The Encryption box shown in Figure 11.4 is assumed to be the same RC4 encryption used in WEP. Most manufacturers implemented this box in a way that can't be changed by firmware upgrades. Existing WEP equipment often includes hardware initialization of the RC4 S-Box. The fact that this unit could not be changed was the biggest problem for the design of TKIP.
Before looking at each of the TKIP elements in detail, let's look at the corresponding receive chain, as shown in Figure 11.5.
The receive chain is not quite the reverse of the transmit chain. For one thing, decryption is not the first operation. Instead, the TSC (derived from the IV) is checked as a replay defense. Note that the ICV value is checked and used to reject the packet. This is not technically an integrity check, but it is a quick indication of whether the decryption has been successful: Decrypting a packet with the wrong key or IV values is almost certain to produce a bad ICV value.
The MIC is checked after all the fragments have been received and reassembled into an MSDU. Note that if the MIC fails, not only will the MSDU be discarded but countermeasures may be invoked. Although possible, it is extremely unlikely that random errors in transmission would be such that the frame would pass the CRC check and then decrypt to produce an acceptable ICV. If we receive a MIC failure, we can be very sure that it is due to intentional tampering and not just random interference or transmission errors.