1/13/03 Chapter 18 Wireless Networks

1/13/03 Chapter 18 Wireless Networks 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


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!


Wireless Networks - Fundamentals

Network

Destination Network (wired or wireless) Access Point

Wire Transmission Path

Radio Transmission Path

Mobile Devices

Cell Phone

Palm Pilot

Laptop


Wireless Wide Area Networks (WAN)

WANWide Area Network(National/Global)

Licensed, 800-900 Mhz, 1.8-1.9 Ghz

• 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


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.


Wireless Devices and Selected Characteristics

Access PointPalm VIIBlackberryDigital PhoneCDPD Modem

WA

N

WirelessDevice

na500500125500

PurchaseCost ($)

From fees240400360480

Network Service($/device-yr)

NetworkBandwidth

(kbps)Key Features

66

1919

carrier ownedno Outllook, coverage

Outlook, coveragevoice, CDPD-WAP

good coverage


Blackberry Handheld Devices – Single Purpose Device

Wireless e-mail using Microsoft Exchange


Blackberry Real-time Messaging

Outlook/ExchangeServers

YourOffice

Colleague'sOffice

Internet

1

2 3

4

6

You

Blackberry Network5

1. Colleague sends urgent message.

2. Sent to Exchangeservers.

3. Received at yourdesktop PC (if on).

4. Encrypted and sent through the Internet.

5. Transmitted by Blackberry network.

6. Blackberry receives and decrypts message.

Firewall


Blackberry Architecture

Blackberry Handheld

Wireless Network Access Point

Internet

Firewall

Exchange Server

Blackberry Server

User’s Desktop


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).


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.

Secured Path Unsecured Path

Blackberry User’s Desktop Another User


WANWide Area Network

(National/Global)

WANWide Area Network

(National/Global)

LANLocal Area Network(Campus/Building)

LANLocal Area Network(Campus/Building)

Unlicensed, 900 Mhz, 2.4 Ghz, 5 Ghz

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…

Wireless Local Area Network (LAN)


You

Wireless Access Point

LAN

Lab/Conference Room

YourOffice

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.

Wireless Local Area Network (LAN)


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).


IEEE 802.11 Standard

The standard describes the Medium Access Control (MAC) andPhysical Layer (PHY) specifications. 802.11 is one part of the 802Specification as shown below.

802.2 Logical Link Control

802.1 Bridging

802.3MeduimAccess

802.3Physical

802.4MeduimAccess

802.4Physical

802.5MeduimAccess

802.5Physical

802.6MeduimAccess

802.6Physical

802.9MeduimAccess

802.9Physical

802.11MeduimAccess

802.11Physical

802.12MeduimAccess

802.12Physical

PhysicalLayer

DataLinkLayer

Ethernet Token Token Dual Integrated Wireless Demand Bus Ring Bus Services Priority


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.



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.



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.


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 withcompetitors (e.g., cell phones, microwave ovens, Bluetooth).


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.



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.



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.


IEEE 802.11d,e,f,h,i,and j – Some Variations

These are not complete specifications, but rather enhancements of802.11a, 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.



802.11f: IEEE

Purpose: Improves the handover mechanism in 802.11 so users canmaintain 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.



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.


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).


Access Point

Access Point

Wired Network

Wireless Application Servers

Wireless Handheld (WinCE or Palm)

R

To additional Network Segments

High Level Architecture


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

Access point

Laptop modem


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.


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.


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!


Eavesdropping Solution - Encrypt 802.11 Transmissions

Eavesdropping is mitigated if signals are not intelligible – 802.11 encryptstransmissions 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.


RC4 – Operated in Output Feedback Mode

E E-1

Plaintext pj Plaintext pjCiphertext cj

IV

Key

Ij

Key

Ij

Oj Oj

Leftmostr bits

Leftmostr bits

Oj-1Oj-1

IV


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.


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.


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:

1. Unauthenticated & unassociated.2. Authenticated and unassociated.3. Authenticated and associated.

Involves messages between a mobile station and an access point.


Authentication Messages

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).

Access Points send beacon messages, then:


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 theappropriate ones to implement for protecting the information.

So…how does the standard and its implementation stack up?

TERRIBLE!!!!!!!!!!!


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?


Authentication BreaksUsing the 4 message exchange, the break works like this:

1. Frame 1 is sent in the clear to request authentication – that’s Ok.2. 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).3. 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.4. 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.


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.


WEP Encryption

The WEP encryption model:

RC4 EXOR CiphertextMessage

IntegrityCheck

Algorithm

Combine

Secret Key

IV

IV

ICV

PlaintextMessage

Transmitted Message

Keystream


WEP Decryption

The WEP decryption model:

RC4

EXORCiphertextMessage

IntegrityCheck

Algorithm

CombineIV

PlaintextMessage

ReceivedMessage

Secret Key

KeystreamCompute IVC (CRC-32)on plaintext message +attached IVC. If remainderis 0, Pass, Else fail.


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).


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.


Encryption Breaks - Recap

1. 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:a. Original plaintext/ciphertext pair encrypted with the secret key.b. 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


Encryption Breaks - Recap

Test: Does C1 C2 P1 = P2?

Assume P1= 0010, P2 = 0100; Keystream for IV, K = 1100, then:

C1 = 0010 1100 = 1110C2 = 0100 1100 = 1000

C1 C2 P1 = 1110 1000 0010 = 0100 --- QED.


Integrity Assurance

The standard uses the following format:

Message CRC 32

Keystream

=

CiphertextIV

Transmitted Data Stream

Exclusive OR

IVInput


802.11 Frame Formats

FrameControl

Duration Dest.Address

SourceAddress

BSSIDSeq.No. Frame Body FCS

Frame Control: Version #; Frame type (control,data, management); sub-type; and numerous flags

Duration:

Destination Address:

Source Address:

BSSID:

Sequence Number:

Frame Body:

FCS:

Octets: 2 2 6 6 6 2 0 – 2312 4


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 authenticationUsing the Point-to-Point Protocol PPP (see RFC 2284).

It is also know as port authentication.


Wired & Wireless Access Authentication

Consider the following wired and wireless network connections:

HUB

Port

WirelessAccessPoint

(WAP)Port

Wireless Link

Wired Link

Hub: Ethernet hub/switch with wired connections to desktop machines.

WAP: Wireless Access Point with wireless connections to wireless-equipped Devices (e.g., laptops, PDAS, etc.).

Authentication: Provided by the port device or by a service called by the port device.


Wired & Wireless Authentication

In an Ethernet wired network and a Windows environment, a system enters the network at bootup, by sending a request to the local network segment domain controller (found in the system configuration files).

The domain controller prompts the system for authentication credentials (e.g., a username password pair). On success, the system is authenticated.

In 802.1 wireless, the system associates with an access point and the access point authenticates the wireless system and allows/denies entry.

As we have seen, the wireless method is easily defeated.

IEEE 802.1x provides a method to call a stronger authenticator and will work with either a wired or wireless network.


IEEE 802.1X Authentication

1

2

3

4

Step 1: Using EAP, the user requests authentication. The Hub or AP forwards the request to the AS.Step 2: The AS issues a request for an authenticator (e.g., password,etc.).Step 3: The user presents the authenticator.Step 4: The AS authenticates/denies the access request and sends the result back to the AP and the user.If authentication succeeds, the AP opens a port for the user. All traffic isencrypted. AS creates/distributes session keys used by the user and AP.

User Hub or Access Point (AP) Authentication Server (AS)


IEEE 802.1X Authentication

The Authentication Server is specified in the standard as a RADIUS server.

RADIUS = Remote Authentication Dial-In User Service

RADIUS is the subject of two RFCs, 2138 and 2865.

RFC 2865 is the current RFC and it describes the operations and protocols supported by a RADIUS server.


References

Wi-Fi forum at www.wi-fi.org

HiperLAN forum at www.hiperlan2.com

HomeRF working group at www.homerf.org

Dornan, A., “LANS with No Wires, but Strings Still Attached,” Network Magazine, February 2002, pp. 44-47.

1/13/03 Chapter 18 Wireless Networks

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