1. Kemal AkkayaWireless & Network Security 1 Department of Computer Science Southern Illinois University Carbondale Wireless and Network Security Lecture 8: IEEE 802.11 Security - 1 Dr. Kemal Akkaya E-mail: kemal@cs.siu.edu

2. Kemal AkkayaWireless & Network Security 2 Management Messages in IEEE 802.11  Authentication frame:  802.11 authentication is a process whereby the access point either accepts or rejects the identity of a radio NIC.  Deauthentication frame:  A station sends a deauthentication frame to another station if it wishes to terminate secure communications.  Association request frame:  802.11 association enables the AP to allocate resources for and synchronize with a radio NIC.  Association response frame:  An AP sends an association response frame containing an acceptance or rejection notice to the radio NIC requesting association.  Reassociation request frame:  If a radio NIC roams away from the currently associated access point and finds another AP having a stronger beacon signal, the radio NIC will send a reassociation frame to the new AP.  Reassociation response frame:  An AP sends a reassociation response frame containing an acceptance or rejection notice to the radio NIC requesting reassociation.  Disassociation frame:  A station sends a disassociation frame to another station if it wishes to terminate the association.  Beacon frame:  The AP periodically sends a beacon frame to announce its presence and relay information, such as timestamp, SSID, and other parameters regarding the AP to radio NICs that are within range.  Probe request frame:  A station sends a probe request frame when it needs to obtain information from another station.  Probe response frame:  A station will respond with a probe response frame, containing capability information, supported data rates, etc., when after it receives a probe request frame.

3. Kemal AkkayaWireless & Network Security 3 IEEE 802.11 Security  Probe: Look for a (better) AP  Authentication: Getting permission to access the AP  Association: Register with the AP  Data Flow: Encrypted messages Encrypted data messages

4. Kemal AkkayaWireless & Network Security 4 Wired Equivalent Privacy (WEP)  Primary built security for 802.11 protocol  Intended to make wireless as secure as a wired network  Uses RC4 Algorithm  Provides  Confidentiality: Encrypts data Through symmetric encryption using RC4 with a shared key  Integrity: Data received are the data sent Through message check sum using encrypted cyclic redundancy check (CRC)  Authentication: AP only allows authorized stations to associate Through challenge/response

5. Kemal AkkayaWireless & Network Security 5 RC4  Sender calculates Integrity Check Value (ICV) over data  four-byte hash/CRC for data integrity  Each side has 104-bit shared key  Sender creates 24-bit initialization vector (IV), appends to key: gives 128-bit key  Sender also appends keyID (in 8-bit field)  128-bit key inputted into pseudo random number generator to get keystream  Data in frame + ICV is encrypted with RC4:  Bytes of keystream are XORed with bytes of data & ICV  IV & keyID are appended to encrypted data to create payload  Payload inserted into 802.11 frame encrypted dataICVIV MAC payload Key ID

6. Kemal AkkayaWireless & Network Security 6 RC4 Encryption Process

7. Kemal AkkayaWireless & Network Security 7 Authentication with the Access Point  AP has two ways of initiating communication with a client  Shared Key  Open Key  Open key allows anyone to start a conversation with AP  No authentication  Shared Key is supposed to add an extra layer of security by requiring authentication info as soon as one associates  Shared Key Authentication  Client begins by sending an association request to the AP  AP responds with a challenge text (unencrypted)  Client, using the proper key, encrypts text and sends it back to the AP  If properly encrypted, AP allows communication with the client  Not secure!: Using passive sniffing, one can gather 2 of the three variables needed in Shared Key authentication: challenge text and the encrypted challenge text

8. Kemal AkkayaWireless & Network Security 8 Pros & Cons  Pros  Easy computation Fast – 10 times faster than DES Can use large bit blocks and keys  Stream based encryption  Key can be made to change at regular intervals using fancy programming  Implementation in Popular languages (C, perl) well documented.  Cons  Vulnerable to brute force attacks  Require a large data structure  Proven Breakable by researchers at ATT and Rice Univ. (August, 2001) “One hour of brute force computation to break standard WEP”  Once Key is broken all messages are easily readable.

9. Kemal AkkayaWireless & Network Security 9 Problems with WEP  1 static key  No encryption is strong if one key is used forever  Key length is short  Brute forcing is possible  Using CRC32 in ICV  Bit flipping attack: CRC(msg XOR delta) = CRC(M) XOR CRC(delta)  Bits cannot set or cleared, but could be flipped  No specification on key distribution  Lacks scalability  No protection against replay attack  Improper RC4 implementation  Protocol doesn’t actually specify IV’s use  2 existing attacks Numerical limitation FMS attack

10. Kemal AkkayaWireless & Network Security 10 Attacks  Numerical Limitation Attack  IV’s are only 24bit, and thus there are only 16,777,216 possible IV’s  A busy network will repeat IV’s often  By listening to the encrypted traffic and picking out the duplicate IV’s, it is possible to obtain the clear text  FMS Attack -- weak IV attack --  Some IV’s do not work well with RC4  Using a formula, one can take these weak IV and infer parts of the WEP key 5 % chance of guessing correctly  Once again, passively monitoring the network for a few hours can be enough time to gather enough weak IV’s to figure out the WEP key  4M ~ 6M packets to decrypt 40bit WEP key  The time needed to deploy the attack is linearly proportional to the key length 104bit key is just as useless as 40bits key  A lot of other attacks…

11. Kemal AkkayaWireless & Network Security 11 Conclusion: WEP  Confidentiality  FMS attack  Integrity  Bit-flipping attack  Authentication  Non existent  WEP is flawed by a technology weakness, and there is no simple solution to fix it  Attacks against WEP are passive and extremely difficult to detect NO MORE WEP