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Chapter 18: Wireless Networks Provide network access without wires – to reduce cost of wiring,

Chapter 18: Wireless Networks Provide network access without wires – to reduce cost of wiring, support inherently mobile devices – palms, laptops, PDAs. Use unconstrained media (e.g., radio) for transmission – no wires Three major system classes

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Chapter 18: Wireless Networks Provide network access without wires – to reduce cost of wiring,

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  1. Chapter 18: Wireless Networks • Provide network access without wires – to reduce cost of wiring, • support inherently mobile devices – palms, laptops, PDAs. • Use unconstrained media (e.g., radio) for transmission – no wires • Three major system classes • Wide Area Networks (WAN) – world–wide (global) in extent • Local Area Networks (LAN) – campus-wide in extent • Personal Area Networks (PAN) – office-wide in extent • Relatively new technologies –first developed in the mid-80s • Strongly support personal mobility – locally to globally • Many protocols, technologies, and implementations (new) • Standards relatively immature • Many security problems at all levels – theory, standards, • implementation Chapter 18 Wireless Networks

  2. Wireless Threats Denial of Service – radio frequency jamming or message flooding. Interception – eavesdrop since the signals are broadcast over the air. Manipulation – changing messages. Masquerading – posing as a legitimate user to enter a network. Awireless system should protect against these threats in the system design, implementation, and operational environment. Some do well, others are in bad shape! Chapter 18 Wireless Networks

  3. Wireless Networks - Fundamentals Mobile Devices Radio Transmission Path Cell Phone Network Palm Pilot Destination Network (wired or wireless) Access Point Wire Transmission Path Laptop Chapter 18 Wireless Networks

  4. Wireless Wide Area Networks (WAN) • Started with cellular phones (U.S.,1982) • 1G, 2G, 3G, 4G • Protocols - many • AMPS, TDMA, CDMA, GSM, CDPD, GPRS,UMT-2000,… • Rapid Growth – “By 2002, wireless phones worldwide will outnumber TVs and PCs combined.” • Strategic News Service WANWide Area Network(National/Global) Licensed, 800-900 Mhz, 1.8-1.9 Ghz Chapter 18 Wireless Networks

  5. Wireless Wide Area Networks (WAN) • Started with cell phones – many technologies & standards. • Progressed through multiple generations: • Analog voice phones, • Digital voice phones, and • Web-enabled phones. • Despite multiple generations, technology is still immature and • changing dynamically (e.g., web access from a cell phone). • Many providers – crowded market. • Interoperability a mixed bag – some good, some bad. • Some very differentiated products (voice-only, data only, mixed). • Don’t expect convergence anytime soon. Chapter 18 Wireless Networks

  6. Wireless Devices and Selected Characteristics NetworkBandwidth(kbps) PurchaseCost ($) Network Service($/device-yr) WirelessDevice Key Features Access PointPalm VIIBlackberryDigital PhoneCDPD Modem na500500125500 From fees240400360480 661919 carrier ownedno Outllook, coverageOutlook, coveragevoice, CDPD-WAPgood coverage WAN Chapter 18 Wireless Networks

  7. Blackberry Handheld Devices – Single Purpose Device Wireless e-mail using Microsoft Exchange Chapter 18 Wireless Networks

  8. Blackberry Real-time Messaging 1. Colleague sends urgent message. 2. Sent to Exchange servers. 3. Received at your desktop PC (if on). 4. Encrypted and sent through the Internet. 5. Transmitted by Blackberry network. 6. Blackberry receives and decrypts message. 4 5 Blackberry Network Internet 6 You 2 3 Colleague's Office YourOffice Firewall 1 Outlook/ExchangeServers Chapter 18 Wireless Networks

  9. Blackberry Architecture Blackberry Server Exchange Server User’s Desktop Internet Blackberry Handheld Firewall Wireless Network Access Point Chapter 18 Wireless Networks

  10. Blackberry Architecture – How It Works • Mail arrives at the user’s desktop in the usual way. • Software is installed on the user’s desktop and configured • according to user-specified filtering/forwarding rules. • Messages are compressed, encrypted,forwarded to server that • maintains an outbound connection to the Blackberry network. • Messages are forwarded and displayed on the Blackberry handheld. • Similarly, messages can be originated on the handheld, sent back to the • user’s desktop and sent out over the mail connection. • Can operate in two modes: • Wireless LAN mode - as described above. • Directly between two handheld devices (peer-to-peer). Chapter 18 Wireless Networks

  11. Blackberry Protection • Peer-to-peer mode is not secure (scrambled, but not encrypted). • Wireless network mode: • Symmetric encryption-key shared between desktop & handheld. • 3 DES encryption, key exchange while handheld is docked. • Server behind firewall only supports outbound connections, • followed by out-bound/in-bound communications. Unsecured Path Secured Path Blackberry User’s Desktop Another User Chapter 18 Wireless Networks

  12. Wireless Local Area Network (LAN) WANWide Area Network(National/Global) Local Area Network (LAN) • IEEE Standards • 802.11, 1998 (2 Mbps) • 802.11b, 1999 (11 Mbps) • 802.11a, 1999 (54 Mbps) • 802.11g, 2000 (54 Mbps) • Interface Prices • $500 – 1997 (2 Mbps) • $160 – 2000 (11 Mbps) • Wireless Options Today • Laptops - Apple, Dell, Gateway, IBM, Compaq, Acer… LANLocal Area Network(Campus/Building) Unlicensed, 900 Mhz, 2.4 Ghz, 5 Ghz Chapter 18 Wireless Networks

  13. Wireless Local Area Network (LAN) • Work un-tethered. • Improve productivity by saving time (use idle time, minimize meeting prep time). • Have real-time access for urgent messages and key information. Lab/Conference Room You Wireless Access Point YourOffice LAN Chapter 18 Wireless Networks

  14. Local Area Networks (LAN) • Function – wireless equivalent to Ethernet Local Area Network. • Based on IEEE standard 802.11 series. • 802.11 – 1997, data rates to 2 Mb/s (outdated). • 802.11b - 1999, data rates to 11 Mb/s (available now). • 802.11g - 2000, data rates to 22 Mb/s (available 2002-2003). • 802.11a - emerging, data rates to 54 Mb/s (available late 2001). • 802.11b is dominant technology being implemented. • Part of the specification is the Wired Equivalent Protocol (WEP) • designed to protect link layer (over-the-air) traffic from • eavesdropping and other attacks (according to IEEE specification). Chapter 18 Wireless Networks

  15. IEEE 802.11 Standard The standard describes the Medium Access Control (MAC) and Physical Layer (PHY) specifications. 802.11 is one part of the 802 Specification as shown below. 802.2 Logical Link Control 802.1 Bridging Data Link Layer 802.3 Meduim Access 802.3 Physical 802.4 Meduim Access 802.4 Physical 802.5 Meduim Access 802.5 Physical 802.6 Meduim Access 802.6 Physical 802.9 Meduim Access 802.9 Physical 802.11 Meduim Access 802.11 Physical 802.12 Meduim Access 802.12 Physical Physical Layer EthernetToken Token Dual Integrated Wireless Demand Bus Ring Bus Services Priority Chapter 18 Wireless Networks

  16. IEEE 802.11a,b,g – Alphabet Soup 802.11a Data Rate: 54Mbps physical channel–31Mbps actual(due to protocol overhead). Further reduced if there is interference/errors (common in radio). Error Rate Reduction: Reduced rates -(48/36/24/18/12/9/6 Mbps). Range: 80 Meters = ~ 263 feet (antenna design can increase). Modulation: Orthogonal Frequency Division Multiplexing (OFDM). Channel bandwidth: 25 MHz. Frequency band: 5GHz. Number of Channels: 12 (in the USA), 4 (Asia) 0 (EU). Quality of Service: No. Availability: Now. Chapter 18 Wireless Networks

  17. IEEE 802.11a,b,g – Alphabet Soup 802.11b Data Rate: 11Mbps physical channel – 6Mbps actual(due to protocol overhead). Also further reduced if there is interference/errors. Error Rate Reduction: Reduced rates - (5.5/2/1 Mbps). Range: 100 Meters = ~ 328 feet. Modulation: Direct Sequence Spread Spectrum (DSSS). Channel bandwidth: 25 MHz. Frequency band: 2.4GHz. Number of Channels: 3 (in the USA), 3 (Asia), 3 (EU). Quality of Service: No. Availability: Now. Chapter 18 Wireless Networks

  18. IEEE 802.11a,b,g – Alphabet Soup 802.11g Data Rate: 54Mbps physical channel – 31Mbps actual. Range: 150 Meters = ~ 492 feet. Modulation: Orthogonal Frequency Division Multiplexing (OFDM) and Discrete Sequence Spread Spectrum (DSSS). Channel bandwidth: 25 MHz. Frequency band: 2.4GHz. Number of Channels: 3 (in the USA), 3 (Asia), 4 (EU). Quality of Service: No. Availability: Late 2002 – early 2003. Chapter 18 Wireless Networks

  19. IEEE 802.11a,b,g – Some Comparisons Data Rate: a & g at 54Mbps win over 11 Mbs for b. Range: g @150m, b @ 100m, a @ 80m. Number of Channels: 12 for a, 3 for b & g. Interference: a @ 5Ghz has little competition, 2.5GHz is loaded with competitors (e.g., cell phones, microwave ovens, Bluetooth). Chapter 18 Wireless Networks

  20. IEEE 802.11a,b,g – Competing Technologies HomeRF2: Developer: HomeRF Working Group (~ 70 members). Data Rate: 10Mbps physical channel – 6Mbps actual. Range: 50 Meters = ~ 164 feet. Modulation: Frequency Hopping Spread Spectrum (FSSS). Channel bandwidth: 5 MHz. Frequency band: 2.4GHz. Number of Channels: 15 (in the USA), 15 (Asia), 0 (EU). Quality of Service: Yes. Availability: Now. Chapter 18 Wireless Networks

  21. IEEE 802.11a,b,g – Competing Technologies HiperLAN2: Developer: Euro. Telecommunication Standards Institute (ETSI). Data Rate: 54Mbps physical channel – 31Mbps actual. Range: 80 Meters = ~ 262 feet. Modulation: Orthogonal Frequency Division Multiplexing (OFDM). Channel bandwidth: 25 MHz. Frequency band: 5GHz. Number of Channels: 12 (in the USA), 4 (Asia), 15 (EU). Quality of Service: Yes. Availability: 2003. Chapter 18 Wireless Networks

  22. IEEE 802.11a,b,g – Competing Technologies 5-UP (5GHz Unified Protocol): Developer: Joint project of IEEE and ETSI. Data Rate: 108Mbps physical channel – 72Mbps actual. Range: 80 Meters = ~ 262 feet. Modulation: Orthogonal Frequency Division Multiplexing (OFDM). Channel bandwidth: 50MHz. Frequency band: 5GHz. Number of Channels: 6 (in the USA), 2 (Asia), 7 (EU). Quality of Service: Yes. Availability: 2003. Note: Merges 802.11a & HiperLAN2 into a single protocol. Chapter 18 Wireless Networks

  23. IEEE 802.11d,e,f,h,i,and j – Some Variations These are not complete specifications, but rather enhancements of 802.11a, b, and g. 802.11d: IEEE: Purpose: Versions of 802.11b that operate on other frequencies Suitable in parts of the world where 2.4 GHz is not available. Status: May not be required since the International Telecommunications Union (ITU) and most countries are freeing up the required spectrum. Chapter 18 Wireless Networks

  24. IEEE 802.11d,e,f,h,i,and j – Some Variations 802.11f: IEEE Purpose: Improves the handover mechanism in 802.11 so users can maintain a connection while moving between two different switched network segments or two different access points attached to two different networks. 802.11h: IEEE Purpose: Adds better control over transmission power and radio channel selection to 802.11a. This and 802.11e could make the standard acceptable to the EU. Chapter 18 Wireless Networks

  25. IEEE 802.11d,e,f,h,i,and j – Some Variations IEEE 802.11i: Purpose: Replaces WEP with a new standard based on the Advanced Encryption Standard (AES). Also deals with an authentication standard. IEEE 802.11j: Purpose: To make 802.11a and HiperLAN networks co-exist on the same frequencies bands. Chapter 18 Wireless Networks

  26. 802.11 Wireless Local Area Network (LAN) • Three (3) possible physical layers are specified: • Infared (short range – line of sight), • Frequency Hopping Spread spectrum (FHSS), and • Direct Sequence Spread Spectrum (DSSS). • Three frequency bands are used; 900 MHz, 2.4 GHz, and 5 GHz. • 802.11b uses DSSS and the 2.4 GHz frequency band. • This is the unregulated Industrial, Scientific, and Medical (ISM) band. • Range is a few 100 - 300 feet – multiple access points provide campus • coverage (like cell phones). • 802.11b data rate is 11 Mb/s, but performance varies as a function of • distance between the mobile device and the nearest access point. • The specified protocol is Carrier Sense Multiple Access with Collision • Avoidance (CSMA/CA). Chapter 18 Wireless Networks

  27. High Level Architecture Wireless Application Servers To additional Network Segments Wired Network R Access Point Access Point Wireless Handheld (WinCE or Palm) Chapter 18 Wireless Networks

  28. High Level Architecture – Text Mobile device (Personal Digital Assistant, laptop, Palm Pilot, etc.) requires a radio frequency transmitting/receiving modem and client software compatible with the IEEE standard. Access point is a bridge between the backside wired network and the frontside wireless network. It sends and receives wireless frames, does error control, authenticates and authorizes users, encrypts wireless traffic, interfaces to the wired network Laptop modem Access point Chapter 18 Wireless Networks

  29. Objectives of 802.11 Secuirty - WEP Reasonably strong security – not perfect, but adequate. Self-Synchronizing – Signal strength varies, so it must be able to synchronize. Computationally efficient – Important for small (cheap) mobile devices. Exportable – Must meet U. S. export control requirements (now eased). Optional – WEP is an optional requirement of the standard. Chapter 18 Wireless Networks

  30. Wired Equivalent Privacy (WEP) – 802.11 Security • According to the standard, particular attention was paid to: • Defeating an adversaries ability to eavesdrop on wireless transmissions in • order to preserve confidentiality by encrypting the channel traffic, • Providing integrity assurance that a message has not been modified in • transit, and • Authenticating users over an encrypted channel. • We will discuss each of these capabilities. Chapter 18 Wireless Networks

  31. Eavesdropping – 802.11 Security • The problem – in-air broadcast signals can be always be intercepted. • Methods are different depending on the physical layer. • Infared - interception is difficult because of line-of-sight and short • distance requirements. Line of sight interception is difficult, but not • impossible (location issue). • The difficulty of recovering Frequency Hopping Spread Spectrum (FHSS) • and Direct Sequence Spread Spectrum (DSSS) is attributed to the • psuedo-random nature of the signal spreading. • Reality - any device designed to receive/transmit 802.11 signals can • intercept signals. Requires only simple modifications to drivers and/or • flash memory to operate in promiscuous mode. Basic • assumption – adversaries have access to all signals transmitted! Chapter 18 Wireless Networks

  32. Eavesdropping Solution - Encrypt 802.11 Transmissions • Eavesdropping is mitigated if signals are not intelligible – 802.11 encrypts • transmissions using RC4 developed in 1987 by Ron Rivest at MIT. RC4 is • considered a secure cipher. Background on Ron’s Code # 4 (RC4): • RC4 was kept secret for the first 7 years, but was anonymously posted • to the Cypherpunks mailing list in 1994 and became public knowledge. • RC4 is a symmetric cipher and can use several different key lengths. The • 802.11 specification allows for 40 bit (export controlled) and longer • (typically 128 bit) lengths although specific lengths and implementations • vary by vendor. • RC4 is generally considered a strong cipher by cryptographers. The 802.11 • implementation operates in Output Feedback (OFB) mode. Chapter 18 Wireless Networks

  33. RC4 – Operated in Output Feedback Mode Ij Oj-1 Oj-1 Ij IV IV E E-1 Key Key Leftmost r bits Leftmost r bits Oj Oj   Plaintext pj Ciphertext cj Plaintext pj Chapter 18 Wireless Networks

  34. RC4 – Text description RC4 uses three (3) inputs: a random initialing vector IV, a random secret key k, and the plaintext P. The IV is input to E, the RC4 encryption algorithm, along with the key. E produces a random keystream that is sent to the output box O. The output box shifts the keystream out a Byte at a time and each Byte is combined with a Byte of plaintext under the Exclusive OR function. The output of E (the keystream) is also fed back to the I stage where it is combined with the IV to produce a new input to E. This causes the keystream to vary as a complex function of IV, K, and E. Reversed at the receiver. Both IV and K must be known to the receiver. K is passed securely (e.g., manually), IV is passed in clear text. Chapter 18 Wireless Networks

  35. RC4 – more The secret key is initially distributed to the access point and the mobile device. The method is not specified in IEEE 802.11, but should be secret. The IV which changes for each session, is sent in the clear as part of the Initial handshake. Does not have to be secret since the strength of the encryption is derived from the algorithm and key secrecy, not IV secrecy. Integrity of the IV must be a maintained between the transmitter and receiver or encryption/decryption won’t work. Also, the IV should not be re-used with the same key schedule. Consider 2 messages: C1 = P1 RC4(IV1, K1) & C2 = P2  RC4(IV1, K1) C1  C2 = (P1  RC4(IV1, K1))  (P2  RC4(IV1, K1)) The EXOR of 2 ciphertexts produces the EXOR of the two plaintexts. C’s are known - If one of the plaintexts is known, the second is revealed. Chapter 18 Wireless Networks

  36. Authentication in 802.11 • Two basic levels of authentication: • Open System Authentication – the default that authenticates • any device requesting authentication – essentially “none” • Shared-Key Authentication – The mobile device is authenticated OR • both the mobile devices and the access point mutually authenticate • to each other. Authentication is a three-state process: • Unauthenticated & unassociated. • Authenticated and unassociated. • Authenticated and associated. • Involves messages between a mobile station and an access point. Chapter 18 Wireless Networks

  37. Authentication Messages Access Points send beacon messages, then: Initiator (STA) Responder (AP) Authentication Request – Sequence # 1 Authentication Challenge – Sequence # 2 Authentication Response – Sequence # 3 Authentication Result – Sequence # 4 Challenge is a psuedo-random number, must be re-played by the initiator. If successful, the process is repeated in reverse (i.e., mutual authentication). Chapter 18 Wireless Networks

  38. Integrity Assurance – No change in transit An integrity checksum is computed for each message exchanged between a station and an access point. It is re-computed and tested when received. If the computed checksum does not match the appended checksum as received, the packet is discarded and re-transmission requested. All of this sounds reasonable on the surface. Certainly the goals of authentication, integrity, and confidentiality are the appropriate ones to implement for protecting the information. So…how does the standard and its implementation stack up? TERRIBLE!!!!!!!!!!! Chapter 18 Wireless Networks

  39. The Problems – High Level There are many attacks that reveal the secret key. It is easy to mount a known plaintext attack to recover keys. Integrity is not cryptographically assured – messages can be modified without the modification being readily detected. Many wireless networks are being operated using open authentication (i.e., no authentication or encryption). They are optional parts of the standard, not mandatory. Only the weak checksum is mandatory. So….How do we break such a network? Chapter 18 Wireless Networks

  40. Authentication Breaks • Using the 4 message exchange, the break works like this: • Frame 1 is sent in the clear to request authentication – that’s Ok. • The challenge response is returned by the AP – the challenge is not • encrypted. The challenge is generated by combining a random number, • an IV, and the shared key and is sent in the clear (128B message). • The responding station, extracts the challenge, puts it into a response • frame, encrypts it with the shared key using a new IV (sent in the • clear) and sends it back. • The AP decrypts, checks integrity and compares the challenge to the • original – if same, authentication of the station is successful. • An adversary can capture the clear text challenge and the ciphertext • challenge response. Knowing the IV, the attacker can derive the • keystream. The adversary can now create a valid response to a • new challenge and join the network. Chapter 18 Wireless Networks

  41. Authentication Breaks - More That is: The responding station has created CHECK THIS OUT BOB BUT, the bad guy still does not have the shared secret key. (s)he has only been authenticated, so this attack is not of great value. What is required to go further is to discover the value of the shared secret key. As we shall see this can also be accomplished relatively easily. Chapter 18 Wireless Networks

  42. WEP Encryption The WEP encryption model: IV IV RC4 Ciphertext Message Keystream EXOR Secret Key Plaintext Message Combine Integrity Check Algorithm ICV Transmitted Message Chapter 18 Wireless Networks

  43. WEP Decryption The WEP decryption model: Received Message Secret Key Combine RC4 IV Ciphertext Message Compute IVC (CRC-32) on plaintext message + attached IVC. If remainder is 0, Pass, Else fail. Keystream EXOR Integrity Check Algorithm Plaintext Message Chapter 18 Wireless Networks

  44. Encryption Breaks • One of the issues with Output FeedBack (OFB) mode stream encryption • is that encrypting two messages under the same IV and key can reveal • Information about both the IV and key. • The IV is transmitted in the clear, so it is available. If the IV is a good • random number and not re-used, it is protected. Trouble is the IV: • Is initialized to 0 in some implementations (no standard requirement) • It is only 24 bits long. If initialized to 0, then it wraps around mod 24. • Doing the math 224 x 2346 B/packet = ~40GB (~320 Gb). • The network has a capacity to do about 432 Gb per day • The adversary can send a message to the network (known plaintext) • and sniff the ciphertext since the network will encrypt it for him (her). Chapter 18 Wireless Networks

  45. Encryption Breaks - contd Then the adversary sniffs the network for another instance of the same IV used for the known plaintext message and recovers that ciphertext. Now the adversary has a known plaintext/ciphertext pair encrypted with the same secret key and can recover the key. Since the keys are shared and typically manually distributed, they don’t change very often. That in itself is a problem – multiple users with the same key and difficulty in manually distributing keys tend to influence long time key use. Chapter 18 Wireless Networks

  46. Encryption Breaks - Recap • Send a plaintext message to a user on the wireless network and sniff • the network for the message. Moderate difficulty, trivial with insider help. • 2. Capture the IV (sent in the clear) and ciphertext. • 3. Sniff the network for another instance of the same IV with the original • message. Not difficult, but may require significant storage space. • 4. On a hit, the adversary has: • Original plaintext/ciphertext pair encrypted with the secret key. • IV and new ciphertext encrypted with the same key. • C1 = P1 RC4(IV1, K1) & C2 = P2  RC4(IV1, K1) • C1  C2 = (P1  RC4(IV1, K1))  (P2  RC4(IV1, K1)) • C1, C2, P1, and certainty that the same IV & key were used. • Then C1  C2  P1 = P2 Chapter 18 Wireless Networks

  47. Encryption Breaks - Recap Test: Does C1  C2  P1 = P2? Assume P1= 0010, P2 = 0100; Keystream for IV, K = 1100, then: C1 = 0010  1100 = 1110 C2 = 0100  1100 = 1000 C1  C2  P1 = 1110  1000  0010 = 0100 --- QED. Chapter 18 Wireless Networks

  48. Integrity Assurance The standard uses the following format: Message CRC 32  Exclusive OR Keystream IV Input = IV Ciphertext Transmitted Data Stream Chapter 18 Wireless Networks

  49. 802.11 Frame Formats Octets: 2 2 6 6 6 2 0 – 2312 4 Seq. No. Frame Control Dest. Address Source Address FCS Duration BSSID Frame Body Frame Control: Version #; Frame type (control,data, management); sub-type; and numerous flags Duration: Destination Address: Source Address: BSSID: Sequence Number: Frame Body: FCS: Chapter 18 Wireless Networks

  50. Improving Wireless Security – IEEE 802.1x In 802.11 users authenticate to access points and this is subject to the flaws we have already discussed. IEEE 802.1x describes an authentication method that is much stronger. Even better, it applies to wired networks as well as wireless networks. The authentication method is called Extensible Authentication Protocol (EAP) Over LANs (EAP-OL). It is an extension of EAP that was originally defined for dial-up authentication Using the Point-to-Point Protocol PPP (see RFC 2284). It is also know as port authentication. Chapter 18 Wireless Networks

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