"With 802.11 and WEP, data integrity is provided by a 32-bit integrity check value (ICV) that is appended to the 802.11 payload and encrypted with WEP. Although the ICV is encrypted, you can use cryptanalysis to change bits in the encrypted payload and update the encrypted ICV without being detected by the receiver.

With WPA, a method known as Michael specifies a new algorithm that calculates an 8-byte message integrity code (MIC) using the calculation facilities available on existing wireless devices. The MIC is placed between the data portion of the IEEE 802.11 frame and the 4-byte ICV. The MIC field is encrypted together with the frame data and the ICV.

Michael also helps provide replay protection. A new frame counter in the IEEE 802.11 frame helps prevent replay attacks."