ieee 802.11i TKIP CCMP

1. IEEE 802.11i
Robust Security Networks
TKIP
CCMP

2. WEP Cryptographic Operations
• Confidentiality and integrity are handled simultaneously in WEP
WEP Data Processing

3. • 802.1X addresses two of the major flaws in WEP
1. authentication
2. key management
• The major remaining flaw to be addressed
– Lack of confidentiality
• idea to overcome this problem
– Link Layer encryption technique.

4. 802.11i ?
• 802.11i defines 2 protocols for link layer protection
1. Temporal Key Integrity Protocol (TKIP)
2. Counter Mode with CBC-MAC Protocol (CCMP)

5. • First new link layer encryption technique.
• upgraded the security of WEP-based hardware
• Retains the basic architecture and operations of
WEP.
• Initially called “WEP2”
The Temporal Key Integrity Protocol (TKIP)

6. The Temporal Key Integrity Protocol (TKIP)
1. Key hierarchy and automatic key management
– Use of master keys for deriving key for frame encryption.
– key management operations automatically refreshes key.
2. Per-frame keying
– Every frame has a unique RC4 key from the master key.
– This process is called key mixing
Differences from WEP
(Features of TKIP)

7. The Temporal Key Integrity Protocol (TKIP)
3. Sequence counter
Use:
1. out-of-order frames can be flagged,
2. mitigating against replay attacks
4. New message integrity check
– CRC replaced with Michael integrity check
– more robust cryptographic Algorithm
– easier to detect frame forgeries
Differences from WEP

8. The Temporal Key Integrity Protocol (TKIP)
5. Countermeasures on message integrity check failures
– Michael can be compromised in an active attack
– so TKIP includes countermeasures
Differences from WEP

9. The Temporal Key Integrity Protocol (TKIP)
• Doubles the length of the IV from 24 to 48 bits.
• This made attackers difficult to predict the keys
• key mixing
– RC4 key unique to each frame
– key mixing calculation is done by including
temporal key+transmitter address+ sequence
counter .
TKIP initialization vector use and key mixing

10. The Temporal Key Integrity Protocol (TKIP)
• TKIP IV also serves as a sequence counter.
• When a new master key is installed it sets sequence counter to 1 and
increments as the frames are transmitted.
• following are the steps to defend replay attacks:
– TKIP maintains the most recent sequence counter
– The sequence counter is checked against the most recently received
sequence counter
– If it is larger than any previous value, the frame is accepted
– If smaller, it is rejected.
– If equal, duplicate frame for error.
– Duplicate sequence numbers may represent an error.
TKIP sequence counter and replay protection

11. The Temporal Key Integrity Protocol (TKIP)
• WEP uses CRC which proved un suitable.
• Major challenge of TKIP is to strengthen integrity check and
also able to run on a low power processor
– Michael is implemented entirely with bitwise operations
– Can run on any processor without harming performance
– MIC better than CRC , but fails against a sustained and
determined attack
• countermeasures detect the active attack and shut
down the network and refresh the keys
The Michael Integrity Check and countermeasures

12. TKIP Data Processing and Operation
• Like WEP, TKIP provides confidentiality and integrity together.
• Confidentiality is achieved through encryption using RC4
hardware with string security belts of key management.

13. TKIP Data Processing and Operation
TKIP Inputs
1. The frame
2. A temporal key
3. A MIC key
4. The transmitter
address
5. A sequence counter

14. TKIP Design – Key Mixing

15. TKIP data transmission
1. The 802.11 frame is queued for transmission.
2. The Message Integrity Check (MIC) is computed.
3. Sequence numbers are assigned to fragments.
4. Each frame is encrypted with a unique per-frame WEP key.
5. The frame plus Michael message integrity check value from
step 2 and the RC4 key from step 4 are passed to WEP.

16. TKIP Data Processing and Operation

17. TKIP reception process

18. 1. When a frame is received by the wireless interface and passes
the frame check sequence .
2. The first step TKIP takes is to check the sequence number to
prevent replay attacks.
3. The WEP seed used to encrypt the packet is recovered.
4. With the WEP seed in hand, the outer WEP layer around the
frame can be removed and the contents recovered.
5. If fragmentation was applied, it may be necessary to wait for
further frames to arrive before reassembling a complete
payload.
6. Once the frame is reassembled, Michael is calculated over the
contents of the frame.
TKIP reception process

19. The Michael Integrity Check
• Michael operates on frames passed down to it at the MAC
service layer from higher-layer protocols
• Michael is not a secure cryptographic protocol and it does not
protect individual 802.11 frames
• It protects the reassembled data unit given to 802.11 for
transmission
• Several attacks on WEP served as the motivation for Michael.
• Message integrity check (MIC) value calculated on data,
destination address (DA) and source address (SA). It also adds
four zero bytes before the unencrypted data.

20. Michael data processing
 Operates on 32-bit blocks of data.
 Padding is used , if required, and only for the computation of the MIC, but not
transmitted.
 MIC is added on to the tail of the data frame
 The data-plus-MIC is given to 802.11 for transmission
 During the fragmentation process, the MIC value may be split across multiple
802.11 fragments

21. Michael countermeasures
If an attacker is able to bypass replay protection and the WEP integrity check,
it would be possible to mount a brute-force attack on the Michael integrity check.
When a station detects a MIC failure
1. The MIC failure is noted and logged. Before the MIC is validated, the frame
must pass through the replay protection hurdle as well as the legacy WEP
integrity check. Getting a frame to Michael for validation is not a trivial
undertaking. Therefore, any MIC validation error is likely to be an extremely
security-relevant matter that should be investigated by system
administrators.
2. If the failure is the second one within a 60-second window, countermeasures
dictate shutting down communications for a further 60 seconds. When the
second MIC failure within 60 seconds is detected, all TKIP communication is
disabled for 60 seconds. Instituting a communication blackout makes it
impossible for an attacker to mount a sustained attack quickly.
3. Keys are refreshed. Stations delete their copies of the master keys and
request new keys from the authenticator; authenticators are responsible for
generating and distributing new keys.

22. Counter Mode with CBC-MAC Protocol
(CCMP)
So far interpretation ?? –TKIP is better than WEP
Still Problem ?? – TKIP relies on WEP encryption
technique which is again proved insecure.
What is the solution ?? – IEEE began working with AES
technique for encryption.
CCMP is the protocol that uses AES for encryption.

23. Counter Mode with CBC-MAC Protocol
(CCMP)
CCMP is basically a combination of counter(CTR) mode privacy and Cipher
Block Chaining(CBC) message authentication with AES technique.
The CCM mode combines CTR for confidentiality and CBC-MAC for
authentication and integrity.
Basically AES is flexible to use with any key size and block size. But all AES
processing used within CCMP mandates AES with a 128 bit key and a 128
bit block size.
Like TKIP, CCMP uses a fresh temporal key (TK) for every session.
CCMP also requires a unique nonce value for each frame protected by a
given TK, and CCMP uses a 48-bit packet number (PN) for this purpose.

24. CCMP Data Processing-
encryption(Transmission)
CCMP Inputs
• The frame
• A temporal key
• A key identifier
• A packet number

25. CCMP data transmission.
1. 802.11 frame is queued for transmission MAC header + payload.
2. A 48 bit packet number is assigned:
3. The Additional Authentication Data (AAD) field is constructed using MAC
header of the frame:
4. Construct CCMP Nonce block : Packet number + sender address
5. CCMP Header is constructed: Packet number + key id
6. Run CCM encryption using the temporal key (TK), AAD, Nonce and data to
form the ciphertext and Message Integrity Check (MIC):
7. The Encrypted frame is formed by concatenating the original MAC Header, the
CCMP header, the Encrypted Data and the MIC.
CCMP Data Processing

26. CCMP reception
It’s the reverse of encryption and transmission process
1. When a frame is received by the wireless interface and checks Frame check
sequence and if valid passes to CCMP.
2. The additional authentication data (AAD) is recovered from the received
frame.
3. The CCMP nonce is also recovered from the frame.
4. The receiver decrypts the ciphertext.
5. The integrity check is calculated over the plaintext data and the additional
authentication data.
6. Finally replay detection is done.

27. Data Transfer Summary
WEP TKIP CCMP
Cipher RC4 RC4 AES
Key Size 40 or 104 bits 128 bits 128 bits
encryption,
64 bit auth
Key Life 24-bit IV, wrap 48-bit IV 48-bit IV
Packet Key Concat. Mixing Fnc Not Needed
Integrity
Data CRC-32 Michael CCM
Header None Michael CCM
Replay None Use IV Use IV
Key Mgmt. None EAP-based EAP-based

28. • These are the standard operations that will set
the procedure for key derivation and
distribution.
• Defines two keys :
1. Pairwise keys
2. Group keys
Robust Security Network (RSN) Operations

29. Robust Security Network (RSN) Operations
• 802.11i pairwise Key Hierarchy
Key Confirmation Key : to compute integrity checks on keys
Key Encryption Key : to encrypt keying messages

30. • Group key hierarchy
(for broadcast and multicast transmissions)
Robust Security Network (RSN) Operations

31. 802.11i Key Derivation and Distribution
• This section explains the technique of key
derivation and distribution securely- pairwise
key and group key
• The process is often called key exchange
process.
• Also explains the method of updating the
keys.

32. 802.11i Key Derivation and Distribution
Updating pairwise keys: the four-way handshake

33. Updating pairwise keys: the four-way handshake
Step1:
i. Authenticator sends nonce(random value) to the supplicant. Nonce
prevents the replay attack
ii. After receiving this supplicant expands pairwise master key .
Expansion = MAC address supplicant + MAC address MAC address of
Authenticator + PMK + two nonces.
Step 2:
i. Supplicant sends supplicant nonce and a copy of security parameters
from initial association with network. Whole message is authenticated
by EAPOL KCK.
ii. Authenticator extracts supplicant nonce which allows authenticator to
derive full pairwise key through this. Authenticator validates the
message. If invalid handshake fails.

34. Updating pairwise keys: the four-way handshake
Step3:
i. At this point keys are in place both sides but requires confirmation.
ii. Authenticator sends supplicant a message indicating sequence number+
GTK which is encrypted using KEK and entire message is authenticated
using KCK
Step 4:
i. Supplicant sends a final confirmation message that it has received the
keying messages so that authenticator can start using the keys
ii. Entire message is authenticated using KCK

35. 1. The authenticator sends the supplicant a nonce, which is a random value that
prevents replay attacks. There is no authentication of the message, but there
is no danger from tampering. If the message is altered, the handshake fails
and will be rerun.
At this point, the supplicant can expand the pairwise master key into the full
pairwise key hierarchy. Expansion requires the MAC addresses of the
supplicant and authenticator, the pairwise master key, and the two nonces.
2. The supplicant sends a message that has the supplicant nonce and a copy of
the security parameters from the initial association with the network. The
whole message is authenticated by an integrity check code calculated using
the EAPOL Key Confirmation Key.
The authenticator receives the message and extracts the supplicant nonce,
which allows the authenticator to derive the full pairwise key hierarchy. Part
of the key hierarchy is the key used to "sign" the message. If the
authenticator cannot validate the message, the handshake fails.
Updating pairwise keys: the four-way handshake

36. 3. Keys are now in place on both sides of the handshake, but need to be
confirmed. The Authenticator sends the supplicant a message indicating
the sequence number for which the pairwise key will be added. It also
includes the current group transient key to enable update of the group
key. The group transient key is encrypted using the EAPOL Key Encryption
Key, and the entire message is authenticated using the Key Confirmation
Key.
4. The supplicant sends a final confirmation message to the authenticator to
indicate that it has received the keying messages and the authenticator
may start using the keys. The message is authenticated using the Key
Confirmation Key.
Updating pairwise keys: the four-way handshake

37. Updating group keys: the group key handshake
Because the group transient key is encrypted with the Key
Encryption Key from the pairwise hierarchy, the group key
handshake requires that a successful four-way handshake has
already occurred.
1. The authenticator sends the group transient key (GTK),
encrypted with the Key Encryption Key from the pairwise key
hierarchy. The message is also authenticated with a code
calculated with the Key Confirmation Key.
2. The supplicant sends an acknowledgment message,
indicating the authenticator should begin to use the new key
for group frames. This message is also authenticated using
the Key Confirmation Key.

38. Improved 802.11i Architecture
Stage 1: Network and Security Capability Discovery
Stage 2: 802.1X Authentication
(mutual authentication, shared secret, cipher suite)
Stage 3: Secure Association (management frames
protected)
Stage 4: 4-Way Handshake
(PMK confirmation, PTK derivation, and GTK
distribution)
Stage 5: Group Key Handshake
Stage 6: Secure Data Communications
Michael MIC Failure or Other Security
Failures
Group Key Handshake
Timout
4-Way Handshake
Timout
Association Failure
802.1X
Failure

39. State 1
Unauthenticated
, Unassociated
State 2
Authenticated,
Unassociated
State 3
Authenticated,
and Associated
Successful
MAC layer
Authentication
Successful
Association or
Reassociation
Disassociation
Notification
DeAuthentication
Notification
Deauthentica
tion
notification
Class 1
Frames
Class 1 & 2
Frames
Class 1, 2 & 3
Frames
Classic 802.11 State Machine

40. State 1
Unauthenticated,
Unassociated
State 2
Authenticated,
Unassociated
State 3
Authenticated, and
Associated
Successful MAC
layer Authentication
Successful
Association or
Reassociation
Disassociation
Notification
DeAuthentication
Notification
Deauthentication
notification
Class 1 Frames + ESN
Class 2 frames
Class 1 & 2
Frames
Class 1, 2 & 3
Frames
802.11i State Machine
State 4
ESN Associated
ESN Association or
Reassociation
ESN
Disassociation
Notification
Successful upper
layer Authentication
Class 1, 2 & 3 Frames
except Authentication
& Deauthentication

41. 802.11i Fast Handoff
STAAPold APnew
Associate-Request
Associate-Response
ACK
DS
Notified
Reassociate-Request (Authenticated)
Reassociate-Response (Authenticated)
ACK
DS
Notified
Disassociate (Authenticated)
Transition Period ~ RTTSTA-AP
802.1X/Identity Request
EAP-Success
802.1X/Identity Response
EAP-Request
EAP-Response
Transition Period ~ nRTTSTA-AP
n =3.5 (TLS), 2.5 (TLS continuation)