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Network Security

Network Security. Chapter 7. Security in Wireless Local Area Networks. Objectives. WEP(802.11) Key Establishment Anonymity Authentication Confidentiality Data Integrity Loopholes WPA(Wi-Fi protected access) WPA2(802.11i). WEP(Wire Equivalent Privacy). Key Establishment in 802.11.

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Network Security

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  1. Network Security Chapter 7. Security in Wireless Local Area Networks

  2. Objectives • WEP(802.11) • Key Establishment • Anonymity • Authentication • Confidentiality • Data Integrity • Loopholes • WPA(Wi-Fi protected access) • WPA2(802.11i)

  3. WEP(Wire Equivalent Privacy)

  4. Key Establishment in 802.11 • Rely on “pre-shared” keys between STA and APs. • Problems • Manual configuration of keys. • So, open to manual error. • Can not be expect to choose a “strong” key. • No way for each STA to be assigned a unique key. AP has no way of uniquely identifying a STA in a secure fashion. Two group of STA : allowed group, non-allowed group • 802.11 allows each STA( and AP) in a BSS to be configured with 4 different keys. 4 user group  little finer control over reliable STA recognition • In practice, use the same key across BSSs over the whole ESS. • Makes roaming easier and faster. More susceptible to compromise.

  5. Anonymity in 802.11 • 802.11 – IP based networks • For given the IP address, it is extremely difficult to determine the identity of the subscriber. • DHCP(Dynamic Host Configuration Protocol) • NAT(Network Address Translation) ( private IP addr.  NAT  Globally valid IP addr.)

  6. Authentication in 802.11 • AP periodically broadcast beacons(management frames) which announce the existence of a network - SSID(service set ID). • STA network scan • passive scan : scan the channels to find beacon. • active scan • STA Net. : Probe request ( SSID, SSID=0(any) ) • AP  STA : probe response. • STA : choose a network wish to join. • Authentication process (Open System Auth. or Shared Key Auth.)

  7. Open System Authentication • Default Authentication Algorithm • Allows any and all station to join the network (no authentication). • AP can enforce the use of SKA.

  8. Shared Key Authentication • Based on challenge-response system. • Two groups of STA • group 1 : access allowed – shared a secret key with AP • group 2 : access not allowed

  9. Authentication and Handoffs • Roaming in 802.11 • Inside BSA (Inter-BSA) : static • between two BSAs (inter-BSA) : 802.11 deals with • between two ESA (Inter-ESAs) : requires Layer 3 support • Inter-BSA roaming • STA : tracks RSS (received signal strength) • RSS < threshold : start to scan for stronger beacon signals. • until RSS (current beacon) > threshold - stops scanning and stay current cell. or RSS (current beacon) < break-off threshold – move to another stronger beacon • The prior-AP and the post-AP do not co-ordinate for handoff • Requires re-authentication – contribute big delay to handoff

  10. Problems with 802.11 Authentication • Authentication with shared key. • No way for the AP to reliably determine the exact identity of STA • Share keys across APs • Makes it difficult to remove STAs from the allowed set of STAs. (changing new key to all stations) • One-way Authentication • STA can not authenticate Network • Suffers all drawbacks that WEP suffers.

  11. Pseudo-Authentication Schemes • Network can use other scheme instead of SKA • One schemes • STA  AP : probe-request (SSID) • APSTA : respond if only probe-request contains SSID. • Very weak authentication, protect causal eavesdroppers • SSID send in clear text (beacon) by APs. • AP can hide SSID, but visible with a wireless network analyzer tool Kismet ( www.kismetwireless.net ) • MAC address filtering • AP maintains a list of MAC addresses of all the access allowed STAs. • Station allows the user to change their MAC address via software. • MAC spoofing • In UNIX/Linux OS : ifconfig eth0 hw ether 00:01:02:03:04:05 • Windows system: SMAC, MAC Makeup

  12. Confidentiality in 802.11

  13. Problems in WEP • Use RC4 in synchronous mode for data packet encryption. • Data loss desynchronizes the key streams generators at the two endpoint. • Stream cipher is not suitable for wireless medium where loss is wide spread. • Solution - Apply encryption/decryption per packet basis. • Require to use unique key for each packet. • WEP : key = {IV || shared key}, 64/128 bits, • Per-packet key : simple concatenation of IV and shared key • IV : send in clear text, 24bits among 64/128 bits are revealed. • IV reuse, chance of duplicate IV

  14. RC4 weak Keys • Certain keys had bits that when changed had a greater effect on the XORed data than others. • There were also bits that when changed had no effect whatsoever on the output. • They called these keys “weak keys.” • Airsnort discovers WEP keys in a matter of hours in some cases. • wireless NIC be capable of RF monitor mode. • Cisco Aironet • Prism2 based cards using wlan-ng drivers or Host-AP drivers • Orinoco cards and clones using patched orinoco_cs drivers • Orinoc cards using the latest Orinoco drivers >= 0.15 with built in monitor mode support • And many others.

  15. Why the same key should not to be used? • When used with frequency analysis technique it is often enough to get lots of information about the two plaintext. • If P1(one of plaintext) is known, P2 can be calculated easily. • WAP Key space : for 64bit key • 40bit is fixed, 24bit IV  224 key space. • 1500 byte-packet @ 11Mbps : {(1500*8) * 224 } / 11*106 = 5.08 hr • RC4 in SSL : unique per session. • 10,000 sessions/day : 224 / 10,000 ≈ 3 years

  16. Data Integrity in 802.11 • ICV : CRC-32 • Linear, not cryptographically computed. • Change X  Z; , = • flapping a bit in (stream) ciphertext carries through into the plaintext. (proof?) • How can we protect this attack?

  17. Redirection Attack in 802.11 • 802.11 header : not protected by the ICV. • 802.11 STA( Alice) – (AP) – Bob • STA – AP : encrypted, AP decrypt and forward frame to Bob. • Eve capture wireless link frame and modify the destination address to Charlie. • Eve retransmit them to the AP • AP forward it Charlie • Attacker do not need to use WEP. • How can we protect this attack?

  18. Replay Attack in 802.11 • Scenario • Alice : account holder, • Bob : bank, • Eve : another account holder • Alice  Bob : $500 to EVE • (Eve know Alice is going to transfer money) • Eve : captures all data from Alice to Bob • Good guess for $500 transfer message. • Replay this massage a few days later. • How can we protect this attack?

  19. Loopholes in 802.11 Security • Does not provide any key establishment mechanism • WEP use synchronous stream cipher– difficult to synchronization during a complete session. • Use per-packet key. (IV || preshared key)=weak key; exposed to FMS attack • Limited key space. • Changing the IV with each packet is optional, making key reuse highly probable. • CRC-32 is linear • ICV does not protect integrity of 802.11 header – redirection attack • No protection against replay attack • No support for STA to authenticate the network.

  20. WPA( Wi-Fi Protected Access )

  21. WPA • IEEE Task group : 802.11i security standard • Use AES as default mode • WPA2 • Not backward compatible • Wi-Fi alliance (major 802.11 vendors) • Aim to ensure product interoperability • To improve the security of 802.11 network without requiring a hardware upgrade. – for temporary purpose • Temporal key Integrity Protocol(TKIP) – known as WPA • Include the key management and the authentication architecture(802.1X) specified in 802.11i. • WPA : TKIP(confidentiality), MICHAEL(integrity) • WPA2 : AES(confidentiality, integrity)

  22. Key Establishment in TKIP • WEP use hardware encryption engine on a WLAN NIC. • Want to compatible with WEP. • Two environment(hone network, enterprise network) in 802.11 network • Enterprise network : • 802.1x for key establishment and authentication. • Backend authentication server • 802.1x – can be used with upper layer Authentication protocols to provide access control to the network. • Kerboros, EAP-LEAP, EAP-MDS, EAP-PEAP, EAP-TLS, EAP-TTLA, EAP-SIM • Home network • Out-of-band mechanism (manual configuration) for key establishment

  23. Key Hierarchy in 802.11

  24. TKIP per-packet Data Encryption Key TSC : protect replay attack Key Mixing - IEEE Std 802.111/D3.0” Nov 2003. Overview[p://www.cs.umd.edu/~mhshin/doc/802.11/802.11i-D3.0.pdf

  25. 802.1X/EAP Authentication • Enterprise network : • 802.1x for key establishment and authentication. • Controlled port : open only when the device has been authorized by 802.1X. • Uncontrolled port : • provide a path for EAPoL. • MAC filtering to access control and deter DoS Attacks.

  26. EAP over WLAN

  27. EAP-TLS • Authentication(TLS) : STA Server • Server AP : Shared key (PMK) • PTK derivation (EAPoL Encryption-key, MIC-key)

  28. Alternatives to EAP-TLS • EAP-TLS : • requires server and STA have certificate. • Requires deployment of PKI • Alternatives • EAP-TTLS(tunneled TLS) • PEAP (Reference URL) • Use certificate to authenticate the network(server) to the STA(client). • Do not use certificate to authenticate STA(client) to the network(server). • Instead ST can use CHAP, PAP and so on to authenticate. • Phase 1 : Authenticate network –establish a TLS tunnel. • Phase 2 : carryout password based authentication protocol to authenticate the client.

  29. Confidentiality • WEP : • weak per-packet key • See slide #17 of this PPT for details. • Enhancement • IV : 24 bits  48 bits • per-packet key : • {IV(24) || per master(40/104)} = {64/128} bits : WEP • Key mixing function (IV(48), MAC, Encryption key(PTK) ) • Increase IV bit size and still compatible with existing WEP HW.

  30. Integrity • WEP : CRC-32 • linear, not cryptographically secure integrity protocol. • Enhancement • Use new MIC protocol – MICHAEL(no multiply operation, just shift and add operation), not as secure as MD5 or SHA • Use IV( increment from 0 for each packet during association-802.11 session) as sequence counter – protect replay attacks. • MSDU(MAC service data unit) : L3 packet • MPDU (MAC protocol data unit) : L2 frame MICHAEL-IEEE Std 802.111/D3.0” Nov 2003. http://www.cs.umd.edu/~mhshin/doc/802.11/802.11i-D3.0.pdf

  31. TKIP – the complete picture

  32. WPA2(802.11i)

  33. Key Establishment & Authentication • Key-establishment and the key hierarchy architecture • WPA and WPA2 are almost identical • WPA2 use the same key for encryption and integrity protection. • Authentication • Identical with WPA. • Pre-shared or 802.1X.

  34. Confidentiality • AES counter mode • Ci = Mi XOR EK( i ) • Security lies on the counter. • Counter value should not be repeated with same key, the system is secure. - fresh key for every session.

  35. Integrity • AES CBC-MAC protocol. • AES-CCMP(counter-mode CBMC-MAC)

  36. The Overall Picture : confidentiality + Integrity • PTK MSB 256 bit: EAPoL MIC key and encryption key. • LSB 128bits : data key (encryption and Integrity) • Ctr counts up to 216 - allowable 128 bits data blocks • MPDU : can be 223 (27 x 216) bits long

  37. AES-CCMP Encryption-detail

  38. AES-CCMP Integrity-detail

  39. Criticisms • CCMP succeeds in combining the encryption and integrity protocol in one process. • Encryption of the various message blocks can no longer be carried out in parallel, because of CBC-MAC calculation. Slowdown the protocol. • CBC-MAC require the massage block to be divided into exact number of blocks.  require data padding.

  40. Resources • IEEE Std 802.11i/D3.0, November 2002 • 802.11i –SANS report • 802.11 overview - SANS report • Eaton, Dennis “Dividing into the 802.11i Spec: A tutorial”, Nov. 2004. • MITIGATING DENIAL OF SERVICE ATTACKS IN COMPUTER NETWORKS – Doctoral Dissertation(2006-Helsinki Univ of Tech.)

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