Temporal Key Integrity Protocol

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Temporal Key Integrity Protocol or TKIP is a security protocol used in the IEEE 802.11 wireless networks. Wi-Fi Protected Access (WPA) is used for WiFi networks to correct deficiencies in the older Wired Equivalent Privacy (WEP) standard.

[edit] Background

TKIP (pronounced "tee-kip") was designed by the IEEE 802.11i task group and the Wi-Fi Alliance as a solution to replace WEP without requiring the replacement of legacy hardware. This was necessary because the breaking of WEP had left WiFi networks without viable link-layer security, and a solution was required for already deployed hardware.

On October 31, 2002, the Wi-Fi Alliance endorsed TKIP under the name Wi-Fi Protected Access (WPA).^[1] The IEEE endorsed the final version of TKIP, along with more robust solutions such as 802.1X and the AES based CCMP, when they published IEEE 802.11i-2004 on 23 July 2004.^[2] The Wi-Fi Alliance soon afterwards adopted the full specification under the marketing name WPA2.^[3]

[edit] Technical Details

TKIP is a "wrapper" that goes around the existing WEP encryption. TKIP comprises the same encryption engine and RC4 algorithm defined for WEP. However, the key used for encryption in TKIP is 128 bits long. This solves the first problem of WEP: a too-short key length.

An important part of TKIP is that it changes the key used for each packet. This is the "Temporal" part of the picture. The key is created by mixing together a combination of things, including a base key (called a Pairwise Transient Key in TKIP parlance), the MAC address of the transmitting station, and the serial number for the packet. The mixing operation is designed to put a minimum demand on the stations and access points, yet have enough cryptographic strength so that it cannot easily be broken.

Each packet transmitted using TKIP has a unique 48-bit serial number that is incremented every time a new packet is transmitted and used both as the Initialization Vector and part of the key. Putting a sequence number into the key ensures that the key is different for every packet. This solves another problem of WEP, called "collision attacks," which can occur when the same key is used for two different packets. With different keys, there are no collisions.

Having the serial number of the packet also be the initialization vector helps to reduce yet another WEP problem, called "replay attacks." Because a 48-bit sequence number will take thousands of years to repeat itself, no one can replay old packets from a wireless connection---they will be detected as out of order because the sequence numbers won't be right.

The last, and most important, piece that is mixed into the TKIP key is the base key. Without a way to generate unique base keys, TKIP would solve many of WEP's problems, but not its worst one: the constant reuse of a well-known key by everyone on the wireless LAN. To deal with this, TKIP generates the base key that is mixed into the per-packet key. Each time a wireless station associates to an access point, a new base key is created. This base key is built by hashing together a special session secret with some random numbers (called nonces) generated by the access point and the station as well as the MAC address of the access point and the station. With 802.1X authentication, the session secret is unique and transmitted securely to the station by the authentication server; when using TKIP with pre-shared keys, the session secret is the same for everyone and never changes---hence the vulnerability of using TKIP with pre-shared keys.