wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Industrial Wireless Communications Security (IWCS)/C42 28-5
Base key
Transmit
MAC address
Packet
sequence
4 most
significant bits
2 least
Mixer, Phase 1
significant
bits
Mixer, Phase 2
Intermediate key
Per-packet key
Plaintext
Software
Hardware
RC4
+
Ciphertext
FIGURE 28.4
Temporal key integrity protocol.
28.2.2.3 WiFi Protected Access 2
WiFi protected access 2 (WPA2) is based on the fully ratified version of IEEE 802.11i. WPA2 is WPA
with a new advanced encryption standard (AES) based algorithm known as Counter Model with CBC-
MAC protocol (CCMP). AES supports multiple encryption key sizes: 128, 196, and 256 bits. WPA2 provides
enhanced security over WPA [10].
The temporal key integrity protocol (TKIP) depicted in Figure 28.4 is a link layer encryption protocol initially
called WEP2. TKIP has security features that improve upon WEP and is designed as a software upgrade
to WEP-based solutions; i.e., a wrapper around WEP. Some of the improvements made by TKIP include
1. Using multiple master keys instead of a single key as used in WEP
2. Deriving a unique RC4 key for each frame generated from a master key
3. Numbering of each frame with a sequence number to mitigate against replay attacks
4. Using a new integrity check hashing algorithm called Michael to detect frame forgeries.
28.2.2.4 tKIP, CCMP, WAPI
TKIP adds new keying and message integrity check (MIC) mechanisms to WEP to offer additional
security. These new mechanisms are key hierarchy and automatic key management, per-frame keying,
sequence counter, new MIC, and countermeasures on MIC failures [11].
Key hierarchy and automatic key management: Keys used to encrypt frames are derived from master
keys that are refreshed in a secure manner.
Per-frame keying: A unique RC4 key is derived for each frame from the master key to lessen the likelihood
of key attacks.
Sequence counter: Each frame is numbered with a sequence number to eliminate replay attacks (capturing
traffic and transmitting the traffic at another time).
New MIC: Michael, a cryptographic integrity check hashing algorithm, is used to defeat forgeries.
Countermeasures on MIC failures: Two failed forgeries in a second cause a station to invoke a rekeying
procedure.
TKIP dynamically changes keys as the system is used. A large IV used with TKIP defeats the key recovery
attacks that were possible in WEP. TKIP temporarily solves four problems: forgeries with the MIC, replays
with IV sequence enforcement, weak key attacks with key mixing, and collision attacks with rekeying. CCMP
© 2011 by Taylor and Francis Group, LLC