CS 393/682 Network Security

Transcription

1 CS 393/682 Network Security Nasir Memon Polytechnic University Module 9 Wireless LAN Security Course Logistics Start working on HW 6 Final homework. To be posted today. HW6 - Points for defending and attacking. Questions regarding homework are best answered by Vikram vikram@vip.poly.edu 4/29-31 Module 9 - Wireless 2 1

2 Wireless Dimension Access to Medium: Unlike wired medium (cables) wireless medium (air) is ubiquitous hence access restrictions to the medium must be handled explicitly, where as in wired environments it is implicit. War Dialing: Attacker gains access to wired medium by exhaustive dialing of phone numbers War Driving: Attacker gains access to wireless medium by just driving by the network coverage area. 4/29-31 Module 9 - Wireless 3 How is wireless different? The Medium Wireless medium has no explicit boundary This property weakens privacy and authentication mechanisms adopted from wired environment Portability Wireless devices are smaller in size and portable Data in portable devices require more protection than data on non-portable devices Mechanisms to recover stolen or lost devices are important Mechanisms for self-destruction of data are also important 4/29-31 Module 9 - Wireless 4 2

3 How is wireless different? Mobility - brings even bigger challenges Trust in infrastructure Wired networks assume certain level of trust in local infrastructure (we trust our routers) In wireless networks this is a weak assumption Would you put same level of trust on an Access Point in JFK as you put on your home AP? Security mechanisms should anticipate these variances in trust Security mechanisms should be independent of location. 4/29-31 Module 9 - Wireless 5 How is wireless different? Mobility Trust in location Wired networks implicitly assume network address is equivalent to physical location ( x.x is Poly s resources) In wireless networks physical location not tied to network address. Physical location may change transparent to end nodes. Privacy of location On wired network privacy of location is not a concern In wireless networks location privacy of the user is a serious issue because users can be tracked, their travel behaviors can be used for marketing purposes etc. Similar scenario exists on the Web: A user s web surfing pattern can be tracked and this raised several privacy issues in 1999 (Double Click s Cookie Tracking) 4/29-31 Module 9 - Wireless 6 3

4 How is wireless different? Processing power, memory & energy requirements Handheld devices have stringent processing power, memory, and energy requirements Current security solutions require expensive processing power & memory Handheld devices mandate inexpensive substitutes for Crypto algorithms (AES instead of 3-DES) Authentication schemes Better one-time password schemes with feasible remote key updates 4/29-31 Module 9 - Wireless 7 How is wireless different? Network Topologies Wired networks usually rely on network topology to deploy security solutions E.g: firewall is installed on a machine where all traffic is visible Wireless networks (esp. ad-hoc) have dynamic topologies Wireless networks may not have single point of convergence (hidden host problem!) Wireless networks put emphasis on host based solutions e.g: distributed firewalls 4/29-31 Module 9 - Wireless 8 4

5 Wireless LAN (WLAN) Technology Based on b technology Uses Direct Sequence Spread Spectrum (DSSS) modulation at 2.4 Ghz Free, public frequency Serves as an Ethernet-to-wireless bridge Speed between 1 and 11 Mbps (shared bandwidth) Most Access Points (APs) include: DHCP Client (LAN Port) DHCP Server (Wireless Port) NAT 4/29-31 Module 9 - Wireless Components Components of BSS (1) STA 1 (AP) DS STA 2 (AP) BSS (2) A MAC, PHY layer specification Serves mobile and portable devices Provides transparency of mobility Appears as 802 LAN to LLC Basic Service Set (BSS) Extended Service Set (ESS) Distribution System (DS) Station (STA) STA that is providing access to Distribution System Service (DSS) is an Access Point (AP) supports Ad-hoc networking Provides link level security 4/29-31 Module 9 - Wireless 10 5

6 WLAN Basic Service Set AP Connects WLAN to/ extends wired network More units deliver higher speed and greater area coverage Client Many media (PCI, PCMCIA, etc.) Support for multiple operating systems 4/29-31 Module 9 - Wireless 11 WLAN Extended Service Set Two or more wired networks connected by wireless bridge 4/29-31 Module 9 - Wireless 12 6

7 WLAN Ad Hoc (Peer-to-Peer) Service Set Two or more wireless-enabled devices create own wireless network without an AP 4/29-31 Module 9 - Wireless 13 Wired Equivalent Privacy (WEP) Wired equivalence privacy? Wireless medium has no packet boundaries WEP controls access to LAN via authentication Wireless is an open medium Provides link-level security equivalent to a closed medium No end-to-end privacy Security Goals of WEP Access Control Provide access control to the underlying medium through authentication Confidentiality Provide confidentiality to data on the underlying medium through encryption Data Integrity Provide means to determine integrity of data between links 4/29-31 Module 9 - Wireless 14 7

8 Wired Equivalent Privacy (WEP) An attack on WEP should compromise at least one of these properties Three levels of security Open system WEP is disabled in this mode. No security. Shared Key Authentication provides access control to medium Encryption provides confidentiality to data on network You can have confidentiality on an open system! That is, you can encrypt all the traffic and not have access control to the medium! Also means a wily hacker can have all his traffic encrypted on our network so that no one see what he is doing! 4/29-31 Module 9 - Wireless 15 Claimed Properties of WEP It is reasonably strong Withstands brute force attacks and cryptanalysis It is self-synchronizing Uses self-synchronizing stream cipher It is efficient Hardware/software implementation It is exportable (Does not matter anymore). Rest of the world needs security too! It is optional WEP layer should be independent of other layers NOT REALLY!! 4/29-31 Module 9 - Wireless 16 8

9 Current Security SSID MAC address filtering WEP Unfortunately, most of these mechanisms are off by default All of these mechanisms have problems 4/29-31 Module 9 - Wireless b Network Security Issues Out-of-the-Box settings create open network No real user identification and authentication No support for SecureID, PKI, etc. Encryption methods vulnerable to known plain text attack WEP implementation of RC4 Default keys proliferation Service Set Identifiers (SSID) broadcast in the clear Unauthorized users can join network and inject traffic 4/29-31 Module 9 - Wireless 18 9

10 802.11b Issues (cont.) Wireless cards broadcast their MAC address Easily captured and programmed onto another card Use of network name as the shared-secret for authentication 1. Sniff network name 2. Reconfigure device to show membership 3. Reboot 4. Access target network Rogue APs Shared-key authentication one-way only User cannot authenticate an AP Unauthorized access can serve as launch pad for DOS attacks by hijacking legitimate clients 4/29-31 Module 9 - Wireless 19 Vernam Ciphers The WEP encryption algorithm RC4 is a Vernam Cipher: Encryption Key K Pseudo-random number generator Random byte b Plaintext data byte p Ciphertext data byte p Decryption works the same way: p = c b 4/29-31 Module 9 - Wireless 20 10

11 pad (6) Key id (2) WEP crypto function IV init. vector secret key plaintext seed 64 integrity algorithm WEP PRNG key sequence ICV + cipher text message WEP uses RC4 PRNG CRC-32 for integrity algorithm IV is renewed for each packet (usu. iv++) actual key size = (vendor advertised size 24) ICV is Integrity Check Value (CRC-32) 4/29-31 Module 9 - Wireless 21 WEP Frame IV 4 PDU >=1 ICV 4 IV 3 Key id is used to choose between four secret keys ICV is integrity check sum (CRC-32) Pad is zero. Unused. 4/29-31 Module 9 - Wireless 22 11

12 RC4 algorithm Proprietary algorithm belonging to RSADS Inc. Secret key stream cipher. Variable length key (up to 2048 bits). Fairly fast (1Mbyte/sec on 33MHz processor). Claimed to be very strong. Exportable outside the U.S. Algorithm leaked onto the Internet in /29-31 Module 9 - Wireless 23 Properties of Vernam Ciphers What happens when p 1 and p 2 are encrypted under the same random byte b? Then: c 1 = p 1 b c 2 = p 2 b c 1 c 2 = (p 1 b) (p 2 b) = p 1 p 2 Conclusion: it is a very bad idea to encrypt any two bytes of data using the same byte output by a Vernam Cipher PRNG. Ever. 4/29-31 Module 9 - Wireless 24 12

13 Attacks on WEP Stream ciphers and keystream reuse Stream ciphers expand a secret key to a stream of pseudo random numbers Message is XORed (denoted by + here after) with random number stream to produce the cipher text Suppose two messages used the same secret key then stream cipher is easily broken so WEP uses an IV to extend the life of secret key But, reusing IV is same as reusing the secret key! C 1 = P 1 + RC4(IV, key) C 2 = P 2 + RC4(IV, key) but (C 1 +C 2 ) = (P 1 +P 2 ) and (P 1 +P 2 ) can be easily cryptanalyzed Given two cipher texts with the same IV, we can remove the effects of XORing with the RC4 stream! (for the same secret key) 4/29-31 Module 9 - Wireless 25 Attacks on WEP Two assumptions for this attack Availability of ciphertexts with same IV IV length is fixed 24 bits (2 24 = 16,777,216) Implementations make the reuse factor worse! Every time a card is initialized IV is set to zero! IV is collision usually after only 5,000 packets! So, obtaining cipher text with same IV is practical Partial knowledge of plaintexts Can use legitimate traffic to obtain known plain texts e.g: Login:, password: prompts in a telnet session Bouncing Spam off a mail server through wireless network 4/29-31 Module 9 - Wireless 26 13

14 Dictionary Attack Once plaintext corresponding to ciphertext obtained, it is possible to decrypt any ciphertext for same key and IV. Can be done in real time! A dictionary of IVs (~2 24 entries) can be built For each IV find the associated key stream C i = P i + RC4(IV i, key) Tabulate these two fields searchable by IV For each packet, scan the table to find the IV first and then XOR the message with corresponding keystream in the dictionary to decrypt the message. C n = P n + RC4(IV, key) we know RC4(IV, key) from the dictionary, we know C n so we can find P n! Size of the dictionary depends on size of the IV, which is fixed by the standard at 24 bits! Increasing key size has no affect on this attack! 4/29-31 Module 9 - Wireless 27 Key Management WEP does not specify how keys are to be managed. Assumes array of four possible keys is somehow populated. Each message contains index of key used. Most installations use single key for entire network. Changing keys requires every single user to reconfigure their wireless drivers!! Hence keys seldom changed. 4/29-31 Module 9 - Wireless 28 14

15 Message Modification ICV is not key based - CRC-32. It is linear, i.e. crc(x + y) = crc(x) + crc(y). Given C = RC4(v,k) + (M, crc(m)) is is possible to find new ciphertext C which decrypts to M = M + d where d is arbitrary!! To do this XOR (d, crc(d)) with C. C = C + (d, c(d)) = RC4(v,k) + (M, crc(m)) + (d, crc(d)) = RC4(v,k) + (M + d, crc(m) + crc(d)) = RC4(v,k) + (M + d, crc(m+d)) = RC4(v,k) + (M, crc(m )) So you can change first bit of plaintext by choosing d to be And so on 4/29-31 Module 9 - Wireless 29 Message Injection If you know one (C, P) ciphertext, plaintext pair for a IV and key, then you can inject any message with the same IV. This is because P + C = P + (P + RC4(v, k)) = RC4(v,k). Which is the key stream!!! And it can be reused indefinitely (almost)!! Key seldom gets changed and IV reuse happens often so this is not a problem. 4/29-31 Module 9 - Wireless 30 15

16 Attack on Access Control nonce+rc4(iv, key) Request.Authentication 128 nonce nonce+rc4(iv, key) IV Request received Decrypt the packet and verify nonce Normal session nonce+rc4(iv, key) client hacker Request.Authentication 128 nonce nonce+rc4(iv, key) IV server Request received Decrypt the packet and verify nonce Hacker Using Data Obtained From Previous Session It is possible to get authenticated without knowing the secret key! (shown in red) We only need a plaintext, ciphertext pair of one legitimate authentication. (shown in black) and we can authenticate ourselves for the same key. 4/29-31 Module 9 - Wireless 31 ARP Spoofing Wireless AP is a transparent bridge New hardware, same old problems (and a few new ones Subject to a man-in-the-middle attack I am A B --> A Attacker Victim B Switch Copy and Forward Victim A 4/29-31 Module 9 - Wireless 32 16

17 ARP Spoofing This attack also translates to a wireless network It also may compromise the integrity of a wired network (from the parking lot ) So much for wired security I am A B --> A Access Point b Attacker Victim B Switch Copy and Forward Victim A 4/29-31 Module 9 - Wireless 33 Other Mechanisms If these mechanisms are insufficient, what else can we do? Don t trust the wireless network Put it behind a firewall Place the AP on its own network segment Use a VPN to secure the connection Encrypted application protocols SSH, SSL/TLS, etc. 4/29-31 Module 9 - Wireless 34 17

18 General authentication requirements for access to networks Unique identification of users at the edge of the network Identity take-over must be impossible Ease of use for the end-user Per-institution provisioning of users in one database of the institutions network Low maintenance Ease of use for guests Enabling various authentication-mechanisms 4/29-31 Module 9 - Wireless 35 WEP2 Some Proposed Modifications Increases size of IV space to 128 bits Possible Exam Question: How does increasing IV size help? VPN Use of EAP for authentication within IEEE 802.1X (Off line password guessing attacks) Key may be changed periodically via IEEE 802.1X reauthentication to avoid staleness No keyed MIC No authentication for re-associate, disassociate (Denial of Service attacks) No IV replay protection 4/29-31 Module 9 - Wireless 36 18

19 VPN-based Security Children s Hospital Enterprise Network Private Network - Unencrypted Contivity Public Network - Encrypted APs terminals 4/29-31 Module 9 - Wireless 37 RC4 and IPSec Use IPSec Security WEP Security AP VPN Device Wireless client 4/29-31 Module 9 - Wireless 38 19

20 IEEE 802.1X Access solution (Layer 2) between client and AP Several available authentication-mechanisms (EAP- MD5, EAP-TLS, EAP-TTLS, PEAP) Standardised Also encrypts all data, using dynamic keys RADIUS back end: Scaleable Re-use existing Trust relationships Client software necessary (OS-built in or thirdparty) 4/29-31 Module 9 - Wireless X x x is sometimes used to summarise all ethernet standards (i.e a, b) but it is not a standard! 802.1X is a standard from the 802.1a, 1b series, developed by 3Com, HP, and Microsoft 802.1X is a transport mechanism. The actual authentication takes place in the EAP-protocol on top of 802.1X. 4/29-31 Module 9 - Wireless 40 20

21 802.1x Framework 4/29-31 Module 9 - Wireless 41 Pre-Authentication State 4/29-31 Module 9 - Wireless 42 21

22 Post-Authentication State 4/29-31 Module 9 - Wireless 43 EAP over 802.1x Extensible Authentication Protocol (RFC 2284) provides an architecture in which several authentication-mechanisms can be used EAP-MD5 Username/Password (unsafe) EAP-TLS PKI (certificates), strong authentication EAP-TTLS Username/Password (safe) MS-CHAPv2 Microsoft Username/Password (not safe) LEAP/PEAP Microsoft/Cisco tunnel module for safe transport of MS-CHAPv2 4/29-31 Module 9 - Wireless 44 22

23 Protocol Overview PAP PAP CHAP CHAP EAP EAP EAP EAP MD5 TLS TLS TTLS PEAP MS-CHAPv2 EAP EAP 802.1X PPP PPP /29-31 Module 9 - Wireless 45 EAP Messages 4/29-31 Module 9 - Wireless 46 23

24 LEAP Lightweight Extensible Authentication Protocol An authentication protocol based on IETF RFC2284, Extensible Authentication Protocol, or EAP Provides mutual authentication between Cisco Aironet client cards and a backend RADIUS server Developed by Cisco Systems Also called, EAP-Cisco Wireless Implemented in Cisco Aironet wireless NICs 4/29-31 Module 9 - Wireless 47 Simplified Architecture of LEAP Radio transmissions Wired Ethernet LAN Radius Server Auth db Access Point User Machine (with client adapter) 4/29-31 Module 9 - Wireless 48 24

25 LEAP / Cisco Solution 1 Client associates with access point 2 Access point blocks all user requests to access LAN 5 RADIUS server delivers session key to access point Auth db Access Point 4 RADIUS server and client mutually authenticate and derive WEP session key 3 User performs network log-on (User ID and password) 6 Client and access point activate WEP. 7 Client and access point use WEP and key for protection of transmissions. 4/29-31 Module 9 - Wireless 49 LEAP / Cisco Solution Sequence of Events Step 1. A wireless client performs an b association with an access point (AP). Step 2. The AP blocks all attempts to gain access to the network (access control is provided until successful authentication occurs). Step 3. The user supplies a user ID and password in the network logon box (or equivalent). Step 4. The wireless client and the RADIUS server mutually authenticate (several methods exist). If bilateral authentication is successful, the client and RADIUS server compute a pair-wise WEP session key. 4/29-31 Module 9 - Wireless 50 25

26 LEAP / Cisco Solution Sequence of Events Step 5. The RADIUS server communicates the user WEP key to the AP. Step 6. Both the client and AP activate WEP for encryption. Step 7. The client and AP use the WEP session key and WEP for encryption of radio traffic. 4/29-31 Module 9 - Wireless 51 Mutual Authentication Are you who you say you are? Are you who you say you are? Wired Ethernet LAN Auth db Access Point Radius Server User Machine (with client adapter) 4/29-31 Module 9 - Wireless 52 26

27 K 3 K 4 MD5 Authentication of User to Network Conceptually Auth db User ID Password Password Key Hash Hash Challenge Response Key Hash Hash =? Determine Action 4/29-31 Module 9 - Wireless 53 Secure Channels Unique Keys per User per Session Keys are shared between AP and all users Radius Server User 1 K 1 Wired Ethernet LAN Auth db K 2 K 5 User 2 User 5 User 3 User 4 4/29-31 Module 9 - Wireless 54 27

28 Anonymity within LEAP Radius Server Wired Ethernet LAN Auth db Access Point User Machine (with client adapter) User / client will disclose Disclosed some information Parameters: violating MAC Address anonymity. Programmed SSID User ID 4/29-31 Module 9 - Wireless 55 Wireless Snooping of Responses on Password-based schemes Radio transmissions Wired Ethernet LAN Radius Server Auth db Access Point User Machine (with client adapter) Adversary can eavesdrop on wireless link Capture: User IDs Random challenges Responses (Hashed passwords) 4/29-31 Module 9 - Wireless 56 28

29 MD5 Authentication of User to Network Conceptually Auth db User ID Password Password Key Hash Hash Challenge Response Key Hash Hash =? Stolen Parameters Determine Action 4/29-31 Module 9 - Wireless 57 Conceptual Dictionary Attack Beating Low Entropy passwords Select User User User ID, challenges, responses Dictionaries Index Index database Response Challenge Hash Possible password Increment Index Captured Response password cracked Password located Yes Hash =? Computed Response No Check next database entry Masquerade 4/29-31 Module 9 - Wireless 58 29

30 EAP-TTLS Auth db User Machine (with client adapter) Access Point TLS Server secure password authentication tunnel Radius Server secure data tunnel 4/29-31 Module 9 - Wireless 59 EAP-TTLS Protocol Overview Two phases TLS handshake phase TLS tunnel phase Phase 1 is used to authenticate TTLS server to client (and optionally, vice versa) Results is activation of cipher suite Allows Phase 2 to proceed (using TLS record) Phase 2 uses TLS record layer to tunnel information between client and server 4/29-31 Module 9 - Wireless 60 30

31 Simplified Architecture of EAP-TTLS Radius Server Auth db Radio transmissions Wired Ethernet LAN Access Point TTLS Server User Machine (with client adapter) 4/29-31 Module 9 - Wireless 61 What s Right Protection of the infrastructure Authentication mechanism can change as needed address flaws in existing wireless security Lightweight No encapsulation, no per packet overhead simply periodic authentication transactions 4/29-31 Module 9 - Wireless 62 31

32 What s Wrong Technical One way Authentication Gateway authenticates the client Client has no explicit means to authenticate the Gateway Rouge gateways put client at risk Remember the loudest access point wins Still no Authentication of management frames (assoc/deassoc/beacons/etc ) 4/29-31 Module 9 - Wireless 63 What s Wrong - Technical MITM Send Authentication Successful to client Client associates with malicious AP Hijacking Send deassociation message to client AP is in the dark Change MAC to client and have live connection 4/29-31 Module 9 - Wireless 64 32

33 What s Wrong Technical RADIUS uses shared secret with the Authenticator Same issue as WEP, but on a more reasonable scale Authentication after association presents roaming problems Authentication takes a non-trivial amount of time can disrupt data in transit Failure of RADIUS server == failure of network Many AP implementations don t allow multiple RADIUS servers Most RADIUS server failover is non-transparent 4/29-31 Module 9 - Wireless 65 What s Next Integration of existing solutions to raise the bar Limited 802.1x implementations i (Task Group I Security) On track the right track Mutual auth, per packet auth 802.1x a part of 4/29-31 Module 9 - Wireless 66 33

34 Temporal Key Integrity Protocol (TKIP) Designed as a wrapper around WEP Can be implemented in software Reuses existing WEP hardware Runs WEP as a sub-component Fast Packet Keying Packet MAC Dynamic Rekeying Key distribution via 802.1x Still RC4 based to be backward compatible AES with 802.1x keying in the distant future 4/29-31 Module 9 - Wireless 67 TKIP design challenges Mask WEP s weaknesses Prevent data forgery Prevent replay attacks Prevent encryption misuse Prevent key reuse On existing AP hardware 33 or 25 MHz ARM7 or i486 already running at 90% CPU utilization before TKIP Utilize existing WEP off-load hardware Software/firmware upgrade only Don t unduly degrade performance 4/29-31 Module 9 - Wireless 68 34

35 TKIP Design Build a better per-packet encryption key by preventing weak-key attacks and decorrelating WEP IV and per-packet key must be efficient on existing hardware Base key Phase 1 Mixer Intermediate key Transmit Address: 00-A0-C9-BA-4D-5F Packet Sequence # 4 msb 2 lsb Phase 2 Mixer Per-packet key 4/29-31 Module 9 - Wireless 69 Further Reading WLAN Security 802.1x pdf RFC s: see EAP RFC 2284 EAP-MD5 RFC 1994, RFC 2284 EAP-TLS RFC 2716 EAP-TTLS PEAP RADIUS RFC 2865, 2866, 2867, 2868, 2869 (I/w EAP) Overview of IEEE b Security, Sultan Weatherspoon Intercepting Mobile Communications: The Insecurity of , Nikita Borisov, Ian Goldberg et al. Coping with Risk: Moving to Coping with Risk: Moving to Wireless Wireless Using the Fluhrer, Mantin, and Shamir Attack to Break WEP, Adam Stubblefield, John Ioannidis, et al. 4/29-31 Module 9 - Wireless 70 35