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,
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) WAN Wide 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.”
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Wireless Wide Area Networks (WAN)
Started with cell phones – many technologies & standards.
Progressed through multiple generations:
Analog voice phones,
Digital voice phones, and
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 Point Palm VII Blackberry Digital Phone CDPD Modem WAN Wireless Device na 500 500 125 500 Purchase Cost ($) From fees 240 400 360 480 Network Service ($/device-yr) Network Bandwidth (kbps) Key Features 6 6 19 19 carrier owned no Outllook, coverage Outlook, coverage voice, CDPD-WAP good coverage
Blackberry Handheld Devices – Single Purpose Device Wireless e-mail using Microsoft Exchange
Blackberry Real-time Messaging Outlook/ Exchange Servers Your Office Colleague's Office Internet 1 2 3 4 6 You Blackberry Network 5 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. 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).
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
WAN Wide Area Network (National/Global) LAN Local Area Network (Campus/Building) Unlicensed, 900 Mhz, 2.4 Ghz, 5 Ghz Local Area Network (LAN)
You Wireless Access Point LAN Lab/Conference Room Your Office
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) 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 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 Data Link Layer 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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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
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
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 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.
RC4 – Operated in Output Feedback Mode E E -1 Plaintext p j Plaintext p j Ciphertext c j IV Key I j Key I j O j O j Leftmost r bits Leftmost r bits O j-1 O j-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: C 1 = P 1 RC4(IV 1 , K 1 ) & C 2 = P 2 RC4(IV 1 , K 1 ) C 1 C 2 = (P 1 RC4(IV 1 , K 1 )) (P 2 RC4(IV 1 , K 1 )) 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:
Unauthenticated & unassociated.
Authenticated and unassociated.
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 the appropriate 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?
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.
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 Ciphertext Message Integrity Check Algorithm Combine Secret Key IV IV ICV Plaintext Message Transmitted Message Keystream
WEP Decryption The WEP decryption model: RC4 EXOR Ciphertext Message Integrity Check Algorithm Combine IV Plaintext Message Received Message Secret Key Keystream Compute IVC (CRC-32) on plaintext message + attached IVC. If remainder is 0, Pass, Else fail.
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 2 24 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
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.
C 1 = P 1 RC4(IV 1 , K 1 ) & C 2 = P 2 RC4(IV 1 , K 1 )
C 1 C 2 = (P 1 RC4(IV 1 , K 1 )) (P 2 RC4(IV 1 , K 1 ))
C 1 , C 2 , P 1 , and certainty that the same IV & key were used.
Then C 1 C 2 P 1 = P 2
Encryption Breaks - Recap Test: Does C 1 C 2 P 1 = P 2 ? Assume P 1 = 0010, P 2 = 0100; Keystream for IV, K = 1100, then: C 1 = 0010 1100 = 1110 C 2 = 0100 1100 = 1000 C 1 C 2 P 1 = 1110 1000 0010 = 0100 --- QED.
Integrity Assurance The standard uses the following format: Message CRC 32 Keystream = Ciphertext IV Transmitted Data Stream Exclusive OR IV Input
802.11 Frame Formats Frame Control Duration Dest. Address Source Address BSSID Seq. 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 authentication Using 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 Wireless Access Point (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 is encrypted. 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.