1、1,IEEE 802.11 Network Security,Rohit Tripathi Graduate Student. University of Southern California.,2,Presentation Overview,IEEE 802.11 Network Security 802.11 Basics 802.11 Architecture 802.11 Security (WEP) WEP Vulnerabilities. Practical Implementation of Attacks. Recent security advancements for W
2、i-Fi Networks. Summary,3,What is IEEE 802.11?,IEEE Standard for Wireless LANs. Ethernet(networking) capability over radio waves. Increased Mobility and Flexibility. Sometimes even more economical. More practical.(e.g. Large halls, atriums,etc.) 802.11b 11 Mbps, 802.11g up to 54Mbps, 802.11a up to 54
3、Mbps. .11b/g at 2.4GHz radio frequencies. .11a at 5-6GHz. Uses CSMA/CA (MAC protocol for shared Media). Different Modulation schemes provides different data speeds. Range 100 -150 feet (approx.),4,Terminology,WLAN - Wireless Local Area Network. AP Access Point. Station Any device on wireless network
4、. SSID Service Set Identifier(identifies a network name.). MAC Address Wireless LAN card address, 6 octets (xx.xx.xx.xx.xx.xx),5,Picture from http:/www.cse.ohio-state.edu/jain/,Infrastructure Network Mode,6,802.11 Architecture (cont.),Frame Types. Data Frames: Used for Data Transmission. Control Fra
5、mes: Used for Media Access Control (RTS, CTS, ACK) Management Frames: Used to exchange Management Information. (Beacon, Probe, Association, Authentication.),7,802.11 Standard Security Goals,Create Privacy as achieved by a wired network (WEP Wired Equivalent Privacy). Parking Lot Attack!. Should Prov
6、ide Confidentiality: No eavesdropping. Access Control: Deny access to unauthenticated stations. Data Integrity: Prevent tampering with transmitted messages.Research shows that none of the goals are achieved!,8,WEP Protocol,Wired Equivalent Privacy. Link Layer Protocol. Two subsystems. WEP Authentica
7、tion Technique. Provides Access Control. WEP Data Encapsulation Technique. Data Integrity and confidentiality.,9,WEP Authentication,STA,AP,Decrypted nonce?,From IEEE: Overview of 802.11 Security.,10,WEP Data Encryption,Message,CRC,Key stream = RC4(IV,K),Ciphertext,IV,XOR,Transmitted Data,Compute Int
8、egrity Checksum c(M) and append to original message M.P = Key-stream generated using RC4 encryption algorithm on a 24 bit IV concatenated to a pre-shared key (40-bit or 104-bit).XOR “P” with Key-stream to produce ciphertext.C = P RC4(IV,K)Transmit IV and Ciphertext.,11,WEP Data Decryption,Decryption
9、 (Reverse Process) Extract IV P = C RC4(IV, K) = (P RC4(IV,K) RC4(IV,K) = P Split P into and re-compute the checksum c(M) to see if it matches c. Establishes message integrity.,12,Attacks on WEP,WEP allows IV to be reused with any frame Same IV produces same key stream (RC4(IV,K). Risks of Key-strea
10、m Reuse. Now If C1 = P1 RC4(IV, K)and C2 = P2 RC4(IV, K)then C1 C2 = (P1 RC4(IV, K) (P2 RC4(IV, K)= P1 P2 (i.e. XOR of two plain texts) Key-stream reuse to read encrypted traffic. Known techniques to find out P1 and P2 given P1 P2. Becomes easier with more intercepted packets using same IV value.,13
11、,Key-stream reuse (continued),Decryption Dictionaries Over time attacker can build a table of key-stream corresponding to each IV (Need known plaintext). Helped by well defined protocol structures(IP headers, login sequence,etc.) Attacker can transmit known plaintext (e.g. email spam) and intercept
12、the cipher text to gain the key-stream. Table size depends only on the IV length and not the key size. Standard fixes IV size as 24 bits. Gives a modest space requirement. E.g. 1500 bytes for each of the 224 possible IVs needs 24GB. Even first few thousands IVs table can be effective in most cases b
13、ecause of most PMCIA cards reset IV value to 0 when reinitialized.,14,Attacks on WEP (continued),Message Modification CRC is insufficient to detect message tampering (not a cryptographically secure authentication code). Controlled modification of cipher-text possible without disrupting the checksum.
14、 CRC is linear function i.e. c(x y) = c(x) c(y). Original Cipher-text C = RC4(IV, K) Possible to find C that decrypts to a modified message M = M C = C = RC4(IV, K) = RC4(IV, K) = RC4(IV, K) = RC4(IV, K) C decrypts to P , but with right checksum!,15,Authentication Spoofing,A challenge-response pair
15、gives the key-stream.,STA,AP,Decrypted nonce OK?,This key-stream can be used to encrypt any subsequent challenge.,16,IP Redirection,For decrypting a ciphertext without knowing the key. Trick the AP into decrypting ciphertext for the attacker. Sniff an encrypted packet. Modify the encrypted message s
16、o that the destination address is that of the attacker.(already shown that message modification is possible without any alarm.),17,Practical Implementation of Attacks,Till now we talked about retrieving the Key-stream. But is it possible to recover the secret key(Ultimate Break)? Fluhrer, Mantin and
17、 Shamir Attack. RC4 Key Scheduling Algorithm, Output Generator (PRGA). Knowledge of IV and and first output byte gives information about the key bytes (called Resolved Cases). Practical Implementation: AT&T Labs Tech Report. Passive Attack. Eased by the clear text transfer of IV. Coding for the simu
18、lated Attack 2 hours. Full key recovered with 256 probable resolved cases. Off the shelf hardware and software to capture packets.,18,Practical Attacks (continued),Mounting the Attack P C = P (P RC4(IV,K) = RC4(IV,K) i.e. Key-stream. Got “P” from 802.2 Encapsulation header added to IP/ARP traffic. 5
19、/6 *106 packets to recover 128 bit key.(Very practical on a busy network.) Improving the Attack User Memorable pass-phrase used as key. Check if the decoded key byte is an ASCII, number or punctuation. Key recovered in reasonable time - Ultimate Break of WEP!,19,Conclusions,Assume that the 802.11 li
20、nk layer WEP offers no security. Use E2E higher-level security mechanisms (IPSec, SSH). Anyone within the physical range can access the network as a valid user. Other kinds of Attacks: MAC spoofing, Jamming(DoS), Insertion Attacks(Rogue APs).This paved the way for further research to create more sec
21、ure WLANs.,20,Wi-Fi Protected Access(WPA),Specification from Wi-Fi Alliance(2003) Strong, standards-based, interoperable security for todays Wi-Fi networks. What needs to be addressed ( flaws in WEP). Weak User Authentication. Weak Key Management. Weak IV selection. CRC is good for detecting random
22、errors and not message tampering. Additionally the new system should be backward compatible.,21,Security Mechanisms in WPA,Authentication (and Key Management). Encryption.,22,Authentication and Key Management,Based on Extensible Authentication Protocol (EAP). Supports many flavors of EAP. Uses exist
23、ing key (temporary keys) distribution methods such as KDC, Kerberos, Needham-Shroeder. Can handle any form of user credentials. Digital certificates, username and passwords, secure IDs,etc. Creates a framework where client workstations mutually authenticates with the Authentication Server (4 way han
24、dshake).,23,Authentication Process,STA,AP,Authentication Server,From IEEE: Overview of 802.11 Security.,24,Session Key Distribution,C (Kc),AP s(Ks),Authentication Server (KDC),c,s,n,Kcs,s,nKc AND Kcs,c,nKs,Kc and Ks: Long term Keys.(Only used once!) Kcs : Session Key. (Temporary),Kcs,c,nKs AND dataK
25、cs,25,Encryption,WPA uses Temporal Key Integrity Protocol (TKIP) Dynamic Key (against static WEP key) distributed by Authentication Server. Key size increased to 128 bits. Change in WEP key requires manual work. Already shown that intruder who collects enough encrypted data (with static WEP key) can
26、 exploit the system. Sets up an hierarchy to generate more data encryption keys using the pair-wise key.(per packet key construction) Uses Message Integrity Check(MIC) against CRC. Provides strong mathematical function to check msg integrity.,26,WPA Deployment.,Can be installed as software upgrade o
27、n most Wi-Fi devices. Clients require software upgrade to the NIC. Enterprise would require an authentication server. Typically Remote Authentication Dial-In-User Service (RADIUS) Small Office and Home Office (SOHO) can use pre-shared key. But still use strong TKIP encryption, per packet key constru
28、ction. All the upgrades (transition) can happen in steps. Allows mixed mode.(though not recommended),27,The Future: WPA-2,Existing WPA features. TKIP encryption. EAP authentication Additionally use Advanced Encryption Standard(AES) for encryption. Currently considered to be a very strong encryption
29、algorithm. Variable Key size of 128, 192 or 256 bits. Drawback: Requires hardware upgrade.,28,Summary,802.11 security does not meet any of its objectives. Current Work to replace. Authentication Scheme using 802.1X (EAP) and Kerberos. Encryption scheme using AES. Key management using established mec
30、hanisms. Wi-Fi Alliance working on WPA2.,29,References,Nikita Borisov, Ian Goldberg, David Wagner, Intercepting Mobile Communications: The Insecurity of 802.11, ACM Mobicom 2001. Adam Stubblefield, John Ioannidis, Aviel D Rubin, Using the Fluhrer, Mantin, and Shamir Attack to Break WEP, AT&T Labs Technical Report TD-4ZCPZZ, 2001. Wi-Fi Alliance, Wi-Fi Protected Access, 2003. http:/grouper.ieee.org/groups/802/11/Tutorial/,30,Questions,?,
