in the CWSP book on page 391,it is mentioned that "the IV has a third byte component that is used in addition to TSC0 and 1 to help prevent creation of weak IVs and to create the 24bit IV length".
which one out of the remaining 4 bytes is the third byte: TSC 2,3,4,5?
The third byte of the 24 bit sequence is actually an 8-bit value that helps prevent the resulting IV from being "weak." Some resources will list this as a Dummy Byte value, but the intent is to ensure that the combination of TSC0, TSC1, and this byte result in a non-weak IV value.
where does this third byte come from?
The "third byte" is the WEP seed described in d) below.
IEEE 802.11 section 184.108.40.206 TKIP overview
"The TKIP is a cipher suite enhancing the WEP protocol on pre-RSNA hardware. TKIP modifies WEP as follows:
"a) A transmitter calculates a keyed cryptographic message integrity code (MIC) over the MSDU SA and DA, the MSDU priority (see 220.127.116.11), and the MSDU plaintext data. TKIP appends the computed MIC to the MSDU data prior to fragmentation into MPDUs. The receiver verifies the MIC after decryption, ICV checking, and defragmentation of the MPDUs into an MSDU and discards any received MSDUs with invalid MICs. TKIP?¡é?€??s MIC provides a defense against forgery attacks.
"b) Because of the design constraints of the TKIP MIC, it is still possible for an adversary to compromise message integrity; therefore, TKIP also implements countermeasures. The countermeasures bound the probability of a successful forgery and the amount of information an attacker can learn about a key.
"c) TKIP uses a per-MPDU TKIP sequence counter (TSC) to sequence the MPDUs it sends. The receiver drops MPDUs received out of order, i.e., not received with increasing sequence numbers. This provides replay protection. TKIP encodes the TSC value from the sender to the receiver as a WEP IV and extended IV.
"d) TKIP uses a cryptographic mixing function to combine a temporal key, the TA, and the TSC into the WEP seed. The receiver recovers the TSC from a received MPDU and utilizes the mixing function to compute the same WEP seed needed to correctly decrypt the MPDU. The key mixing function is designed to defeat weak-key attacks against the WEP key."
I hope this helps. Can you add your location to your forum profile? Thanks. /criss