WPA2 and AES | Security for Wi-fi Radio

WPA2 introduces a new encryption algorithm, using the Advanced Encryption Standard (AES). This cipher was produced to be used as a standard algorithm wherever encryption is needed.

AES is a block cipher, unlike RC4. A block cipher takes blocks of messages—fixed chunks of bytes—and encrypts each block, producing a new block of the same size. These are nonlinear ciphers, and so the bit-flip attacks are significantly harder. AES was specifically designed and is believed to be practically impervious to those styles of attacks. With block ciphers, each block starts off independently, a bit of a downside compared to stream ciphers. To remove that independence, WPA2 also uses what is called Counter mode, a simple concept where later blocks are made to depend on previous blocks.

The MIC used is also based on AES, but is used as a cryptographic hash. This use is called cipher block chaining (CBC), and essentially uses the same concept of making later blocks depend on earlier ones, but only outputting the last block as the result. This small block (128 bits) is dependent on every bit of the input, and so works as a signature, or hash.

The overall algorithm used is known as Counter Mode with Cipher Block Chaining-Message Authentication Code (CCMP).

Table 1 shows the frame body used with WPA2. As with WPA, WPA2 has essentially the same expanded IV. Because WPA2 isn't using TKIP, the name has been changed to the packet number (PN), but serves the same purpose, starting at 0 and counting up. The PN is used for replay detection, as well as ensuring per-frame keying. The MIC is also eight bytes, but uses CBC-MAC rather than Michael. With new hardware, the last vestige of WEP can be dropped, and the old ICV is removed.

Table 1: 8.02.11 Fram Body with WPA2 

PN = IV	Data	MIC
8 bytes	n—8 bytes	8 bytes

Because the WPA2 MIC is considered to be cryptographically strong, the designers of WPA2 eliminated the countermeasures that WPA has. It is still true that no frame should come in with an invalid MIC; however, the administrator can be alerted to deal with it in his own time, as there are not any known exploits that can be successfully mounted against WPA2 using an invalid MIC to date.