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Wireless Security Overview: WEP, WPA, RSN (802.11i)

This overview provides information on the weaknesses and insecurities of WEP, the improvements made in WPA, and the robust secure network of RSN (802.11i). Topics discussed include attacks on WEP, RADIUS and EAP authentication, AES-CCMP encryption, and case studies.

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Wireless Security Overview: WEP, WPA, RSN (802.11i)

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  1. Wireless Security Overview Paul Cychosz March 2005

  2. Wireless Security • Topics: • WEP • - attacks • WPA / TKIP • RSN (802.11i) • - RADIUS • - EAP • - AES-CCMP • Small Case study

  3. WEP • Goals: • Integrity: No tampering with messages • Confidentiality: No eavesdropping • Access Control: No unauthorized access

  4. WEP Encryption • RC4 Stream Cipher • CRC-32 Integrity checking • 40 or 104-bit key Decryption

  5. WEP Insecurities -- Initial Vector (IV) problem RC4 Encryption = E(...), C = ciphertext, P = plaintext, k = key • C1 = P1  E(IV, k) • C2 = P2  E(IV, k) • then XOR ciphertext together, C1  C2 = (P1 E(IV, k))  (P2 E(IV, k)) = P1  P2 so knowing one plaintext will get you the other - but usually just having the XOR of two plaintexts is good enough XOR cancels keystream

  6. WEP Insecurities -- Initial Vector (IV) problem Why is IV reused? IV only 24-bits in WEP, IV must repeat after 2^24 or ~ 16.7M packets -practical? -IV sent in clear with ciphertext, easy collision detection • yes, since WEP key rarely changes • yes, usually less than 16 million packets (some keys filtered) • yes, implementations make it worse • IV reset, multi-user shared key

  7. WEP Insecurities -- Checksum (ICV) • CRC-32 is NOT a hash function! • Still can be malicious • Already a CRC in network stack to detect errors Linear Properties: CRC-32(P  C) = CRC-32(P)  CRC-32(C) - Bit flipping

  8. WEP Insecurities Combining Ideas A  B: (IV || C) = RC4(IV,k)  (M || CRC-32(M) ) -- hash collision similarities Oscar calculates C’ such that it decrypts to M’. WhereM’ = M  X, X is specially selected. O  B: (IV || C’) = RC4(IV,k)  (M’ || CRC-32(M) )

  9. WEP Insecurities Combining Ideas Leads to message injection (math omitted) WEP authentication: Challenge supplicant that they really know k. M (IV || M || CRC-32(M))  E(IV,k)) -- Worthless, unless Oscar only one using network

  10. WEP Insecurities • Even more! • IP Redirection • Double-Encryption • Decryption Dictionaries (~ 24GB via FMS / DHCP / Parallel attacking, more about this in a minute…) • -- Some vendors make it worse. (nonsequential IV, constant IV, etc.) • See: Mobicom ’01: Borisov, Goldberg, Wagner. • Intercepting Mobile Communications: The Insecurity of 802.11.

  11. WEP Insecurities Don’t even have to understand how WEP works! Airsnort, WEPcrack, kismet, dwepcrack, aircrack, many others

  12. WEP2 • 128-bit IV • Never fully supported • Still not secure, still uses CRC-32 • key/IV size doesn’t even matter! • WEP2 barely exists, no one uses. . . • . . . • . . . • Moving on!

  13. WEP 2001: Fluhrer, Mantin, Shamir Weaknesses in the Key Scheduling Algorithm of RC4. • completely passive attack • Inductive chosen plaintext attack • Takes 5-10M. packets to find secret key • linear complexity attack (2048-bit? No problem!) • Showed that WEP is near useless

  14. WEP FMS stats http://www.securityfocus.com/infocus/1814

  15. WEP Since FMS: Optimized attacks via statistical analysis, defeats dynamic re-keying of WEP (previous proposed solution) • Attack only takes several thousand packets • Looks at packets w/ unique IV, exploits DHCP and ICMP echo (ping) • Optimizations on this: WEP key cracked in minutes • http://www.securityfocus.com/infocus/1824

  16. WPA • April 2003 • Snapshot of “in progress” 802.11i standard • Only temporary solution • Fixes many WEP problems • Based on TKIP • Same Encryption as WEP (RC4)

  17. WPA • Designed to work with a 802.1x authentication server • 104-bit key  128-bit • 24-bit IV  48-bit IV • MIC: CRC-32  “Michael” • Frame counter (TSC)

  18. WPA • 2 modes: WPA-Personal, WPA-Enterprise • PSK • pass phrase • other improvements: • -key generated from pw + salt + PRNG 802.1x Authentication

  19. TKIP • Temporal Key Integrity Protocol • Cryptographic message integrity code (MIC) forgery • New IV sequencing (TSC) replay • Per-Packet mixing function weak IV • Re-keying key reuse

  20. TKIP Encryption Graph

  21. TKIP Decryption Graph

  22. TKIP • Key Mixing: Use temporal key instead of base key • Key regenerated frequently • Per packet temporal key S-box “d” = dummy byte created in a way to prevent weak keys Feistel structure intermediate key

  23. TKIP Michael – replacement MIC for CRC-32 • Made to be fast • A bit problematic • Requires addtl. countermeasures: Rekeying, Rate limit rekeying, etc.

  24. TKIP • Just a wrapper around WEP, overhead • Long term security questionable TKIP WEP

  25. TKIP • Main goal achieved: Backward compatibility • Fixed major vuln. without changing hardware • Underappreciated

  26. 802.11i (WPA 2) • Current flagship heavyweight solution • Robust Secure Network (RSN) • Ratified June 2004 • Based on newer AES encryption • Can use authentication server or PSK • Backward compatibility modes, need new hardware for AES

  27. 802.11i • Terms: • 802.1x: Authentication standard • RADIUS: Authentication Server • EAP: Extensible Authentication Protocol • CCMP: Encryption based on AES counter mode with CBC-MAC

  28. 802.11i Parts Robust Secure Network (RSN) 802.1x / EAPoL CCMP / TKIP / WRAP Encryption / Integrity EAP RADIUS EAP-TLS Outside of 802.11i, but de facto standard Authentication / Key Dist.

  29. 802.11i - Auth. Goals 1. Mutual authentication 2. Identity privacy 3. Dictionary attack resistance 4. Replay attack resistance 5. Derivation of strong session keys 6. Tested implementation 7. Delegation: Allow guests through clients 8. Fast reconnect: Mobile IP, different auth. procedure, see 802.11r, modified handshaking

  30. 802.1x / EAPoL • 802.11i process starts with EAP • Port-based security • Does not use IP Terms: AS: Authentication server STA: Station / Supplicant / Client AP: Access Point

  31. 802.1x • - Link Security • Can only communicate with AS, e.g. RADIUS, until “EAP-Success” message received • DHCP Blocked

  32. 802.11i – First half STA AP AS Capability Discovery 802.1x Authentication 802.1x Key Management Keygen & Distribution Encryption + Additional handshaking

  33. 802.11i – Init First: Capability discovery, any point on proceeding? AP  client: RSN Information Element (RSN IE) “1” means: 802.1x and CCMP support Pre-Auth, GK for unicast, etc. WEP-40/104, TKIP, CCMP, WRAP, Vendor specific 802.1x auth, key mgmt, vendor spec.

  34. EAP • Extensible Authentication Protocol - The transport protocol to authenticate users • "EAP is used to select a specific authentication mechanism, typically after • the authenticator requests more information in order to determine the • specific authentication method to be used." –RFC 3748, page 3 (Step 0) Link Control Phase w/ AP to initiate “EAP-Start” (EAPoL-Start) - AP usually just a “pass-through” until end of EAP • 4 message types: • EAP-Request • EAP-Response • EAP-Success • EAP-Failure

  35. EAP General EAP Message Flow

  36. EAP • Layered Stack Model – 4 Levels • Lower layer: Responsible for transmitting and receiving EAP frames between the station and authenticator. Variations of lower layer include UDP, TCP, etc. • EAP layer: Duplicate detection, retransmission • EAP Peer/Auth: Sets up packet based on Code field • EAP Method: Implement authentication algorithms, fragmentation

  37. EAP Layered Model EAP Method EAP Method Type X Type Y EAP Method EAP Method Type X Type Y EAP Peer Layer EAP Layer Lower Layer EAP Auth Layer EAP Layer Lower Layer EAPoL

  38. EAPoL EAP is a general protocol • EAPoL-Start • EAPoL-Key • EAPoL-Packet • EAPoL-Logoff • EAPoL-Encapsulated-ASF-Alert MAC addr Header Protocol Version Packet Type Packet Body Length Packet Body 1) Sent to special group multicast address reserved for 802.1X authenticators (this sometimes preempted, h/w)

  39. EAPoL • EAPoL-Start • EAPoL-Key • EAPoL-Packet • EAPoL-Logoff • EAPoL-Encapsulated-ASF-Alert MAC addr Header Protocol Version Packet Type Packet Body Length Packet Body 2) Key exchange. Vague in 802.1x. 802.11i modifies this message

  40. EAPoL • EAPoL-Start • EAPoL-Key • EAPoL-Packet • EAPoL-Logoff • EAPoL-Encapsulated-ASF-Alert MAC addr Header Protocol Version Packet Type Packet Body Length Packet Body • Just a container • Supplicant wishes to disconnect • Not used typically

  41. EAP Packet header: Code Identifier Length Data . . . Code: Message type Identifier: To pair up messages Length: Header + Data size - Integrity depends on lower layers

  42. EAP Packet header, 1(Request) or 2(Response): Code Identifier Length Type Type Data . . . Types: 1 Identity 2 Notification 3 Nak (Response only) 4 MD5-Challenge 5 One Time Password (OTP) 6 Generic Token Card (GTC) 254 Expanded Types 255 Experimental use

  43. EAP • If all goes well, EAP-Success sent • Authentication server gives AP the key to continue with 2nd half of 802.11i communication • EAP info can be sent insecurely if bad EAP mode chosen. Many flavors of EAP LEAP, PEAP, EAP-TLS (This is de facto standard), EAP-TTLS, Others…

  44. EAP EAP Types: PEAP (Protected EAP): Uses a digital certificate on AP side, password / certificate on station side. Mutual Authentication. Native support, 3rd-party packages. EAP-TLS (EAP with Transport Level Security): RFC 2716. Certificates on both client & AP side. Mutual Authentication. Well supported. EAP-TTLS (Tunneled Transport Layer Security): Certificate on AP side, password / token / certificate on client side. Mutual Auth. Encrypts exchange, including the username. Good support. LEAP: Cisco solution, vuln. to dictionary attack. “Asleap” cracking tool. Dropping support for PEAP. Combine EAP-TTLS and PEAP, no certificates needed.

  45. Full 802.1x: EAP / RADIUS

  46. RADIUS . . . AP Not Encrypted* RADIUS • RADIUS - Remote Authentication Dial In User Service, RFC 3597 • If Oscar is on inside, can easily ARP-Poison and interject forged messages to RADIUS server and get valid responses. • Widely deployed protocol for network access authentication, authorization and accounting (AAA) • Not part of 802.11i! • * standard doesn’t req. encryption, but can if needed and often is, IPsec, etc.

  47. RADIUS • Stores database of login info typically in relational DB or Unix /etc/passwd file • Access-Request. Network access connection attempt from a client • Access-Accept. Sent from RADIUS server when client is authenticated and authorized. • Access-Reject. Sent by a RADIUS if either the credentials are not authentic or the connection attempt is not authorized. • Access-Challenge. Sent by a RADIUS server in response to an Access-Request message. Client must respond to this • Accounting-Request. Sent by a RADIUS client to specify accounting information for a connection that was accepted. • Accounting-Response. This message acknowledges the successful receipt and processing of the Accounting-Request message.

  48. RADIUS Message format Code Identifier Length Authenticator Attributes • 1-byte: Type • 1-byte: Length • Rest: data, i.e. EAP messages(79) 128-bits. Nonce ICV(Nonce) Access-Request(..type..) Access-Accept OR Access-Reject OR Access-Challenge

  49. 802.1x: EAP-TLS / RADIUS (1) AP-RADIUS key

  50. 802.1x: EAP-TLS / RADIUS (2)

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