wireless ad hoc networking

Wireless LAN Security 415
Octet: 4 4 >=1 8 4 4
MAC
header
IV/ KeyID Extended IV Data MIC ICV
Encrypted
FCS
Figure 15.19
TKIP MPDU formats.
to improve WEP’s security, an implementation of TKIP is optional for RSN.
However, CCMP is mandatory for RSN compliance. CCMP is based on CCM
of the AES encryption algorithm. It provides confidentiality, authentication,
integrity, as well as replay protection.
15.3.3.1 Temporal Key Integrity Protocol
The TKIP is a data confidentiality protocol that is designed to enhance the
WEP’s security level. TKIP improves the security of WEP encapsulation with
several functions, for example, the Michael algorithm 15 and a key mixing
function.
WEP uses ICV for integrity check but ICV offers little real protection.
However, ICV is still checked in TKIP. Besides, TKIP adopts the Michael
algorithm as its data integrity mechanism. An 8-byte MIC is computed
using the Michael algorithm and combines a data integrity key, which
is a part of the PTK for TKIP. The frame format for TKIP is shown in
Figure 15.19. Since the Michael algorithm is simple, it is vulnerable to bruteforce
attacks. TKIP adopts a series of countermeasures, such as changing
the encryption key or stopping data transmission to make up for the
vulnerability.
As mentioned earlier, the RC4 key in WEP is the concatenation of a
40-bit WEP key and a 24-bit IV. The IV is very short, so IV values are
frequently reused in a busy network. TKIP increases the IV size to 48 bits.
In addition, IV may serve as a sequence number, called the TKIP sequence
counter (TSC), which could prevent replay attacks. TSC starts from 0 and
is incremented by 1 for each frame sent. TKIP discards all frames whose
TSCs are no greater than the last frame’s. Next, TKIP applies a key-mixing
function to generate an RC4 key for each frame. The key-mixing function
consists two phases: Phase 1 uses the MAC address, the higher-order 32
bits of the IV, and the data encryption key (which is a part of the PTK)
to calculate the phase-1 key. In phase 2, TKIP uses the phase-1 key, the
lower-order 16 bits of IV, and the data encryption key to create the 128-bit
RC4 encryption key.
All known security weaknesses of WEP have been solved in TKIP. The
TKIP resolutions for WEP weaknesses are summarized in Table 15.1.