Real 802.11 Security: Wi-Fi Protected Access and 802.11i - page 89

What Is TKIP and Why Was It Created?

TKIP stands for Temporal Key Integrity Protocol, but that's not important right now (or probably ever). TKIP exists for one reason: to allow WEP systems to be upgraded to be secure . This is the reason TKIP was created and this requirement guided the design throughout the standardization process. TKIP has now been adopted as part of the WPA certification and also is included as part of RSN in IEEE 802.11i. In 2001, when WEP was blown apart, there were millions of installed Wi-Fi systems, all suddenly without a viable link layer security solution. Originally, the new security measures of the IEEE 802.11i standard were expected to come into effect gradually, starting with applications where very high security was needed. However, when WEP was broken, in an instant, all these millions of systems were rendered insecure . A solution was needed that would allow them to be upgraded and become secure again.

The requirement that TKIP should run on legacy hardware (that is, hardware already installed using WEP) was a severe restriction on the approach to be taken. In the case of AES (see Chapter 12), the solution was designed from scratch; the designers could focus on the best solution possible within the general constraints that it should be practical and cost-effective to implement. In some ways, this ability to start from scratch mode made the AES approach a simpler problem to solve. But TKIP had the requirement to be secure and available as an upgrade to WEP systems.

To help understand why upgrading existing WEP systems is a significant problem, we need to take a look at the internals of Wi-Fi LAN systems and how they are built. We start with a Wi-Fi adapter card. There are not too many manufacturers of silicon chips for Wi-Fi LAN. In fact, the majority of existing WEP-based Wi-Fi LAN systems are based on the chips of only two or three companies. There are essentially four parts to a Wi-Fi LAN card:

• Radio Frequency (RF) section

• Modem section

• MAC (Medium Access Control) section

• Host interface to connect to your computer—PC card or USB, for example

Roughly speaking, the RF section deals with receiving and transmitting through the antenna; the modem deals with extracting data from the received signals; and the MAC deals with protocol issues, including WEP encryption. The four components are shown in Figure 11.1.

The RF section requires very special design and the use of exotic semiconductor materials. However, the remaining parts can be implemented in standard run-of-the-mill integrated circuit (IC) technology. The key to reducing cost in electronics is to cram everything you can into a minimum number of integrated circuits and then produce a huge quantity of them. Therefore, successive generations of Wi-Fi LAN designs used fewer and fewer components. In the latest systems, Host Interface, MAC, and modem blocks are combined into a single IC. Eventually, we might expect that the RF section will also be included, to produce a single IC solution.

The part we want to look at is the MAC section of the IC. This is the part that implements most of the IEEE 802.11 protocol. On one side (the Host Interface side), it receives, from the computer, packets of data for transmission and instructions for activities such as "look for a new AP" or "issue a request connection to that AP." It also delivers to the computer packets of data that have been received. On the other side (by the modem), it delivers a stream of bits containing all the various IEEE 802.11 control and data frames, including special functions like sleep modes, data acknowledgment, and retransmission of lost data. Importantly (for us), it also encrypts and decrypts the data frames .

Because the MAC operations are rather complex, all the implementations are built around a small microprocessor embedded inside the IC. The microprocessor is programmed to handle all the formatting and timing operations to control the protocol. Typically, however, this processor is not very powerful and certain operations are just too fast for it to handle. Therefore, the MAC is implemented as a combination of firmware and hardware, as shown in Figure 11.2.

Figure 11.2 shows a block called Hardware Assist. If you want to go to the store for a loaf of bread, you can walk. But if you want to go at 70 mph, you need hardware assistance (in other words, your automobile). So it is with the MAC. The small microprocessors in the Wi-Fi cards shipped from 1997 to 2003 need help to go at 11Mbps, and that comes in the form of custom hardware in the IC.

If all the MAC functions were done only by the microprocessor, it would be possible to change the security system just by downloading new firmware. However, because encryption and decryption requires a fair bit of computation, the implementation of WEP almost always depends on the hardware assist functions and, of course, these functions cannot be changed after manufacture.

You can see now why TKIP is necessary. The hardware assist functions in these earlier systems cannot support AES-CCMP. They can support only RC4 (WEP). Therefore, the designers set out to find a way to implement real security using the existing RC4 implementation, and in a way that can be done through firmware upgrades. This is TKIP.