Transcript
Wireless Networks
Mahalingam Ramkumar
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.
How is wireless different? ●
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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
How is wireless different? ●
Mobility - brings even bigger challenges –
Trust in infrastructure ●
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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.
How is wireless different? ●
Mobility –
Trust in location ●
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Wired networks implicitly assume network address is equivalent to physical location (130.18.x.x MSU) In wireless networks physical location not tied to network address. Physical location may change transparent to end nodes.
Privacy of location ● ●
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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)
How is wireless different? ●
Processing power, memory & energy requirements – – –
Hand-held devices have stringent processing power, memory, and energy requirements Current security solutions require expensive processing power & memory Hand-held devices mandate inexpensive substitutes for ● ●
Crypto algorithms (AES instead of 3-DES) Authentication schemes –
Better one-time password schemes with feasible remote key updates
How is wireless different? ●
Network Topologies –
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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
Wireless LAN (WLAN) Technology ● ●
Based on 802.11b technology Uses Direct Sequence Spread Spectrum (DSSS) modulation at 2.4 Ghz –
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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
802.11 Components ●
Components of 802.11
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BSS (1)
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STA 1
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(AP)
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DS
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STA 2
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(AP)
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BSS (2)
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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) STAs that provide access to Distributed System Service (DSS) are Access Point (AP) 802.11 supports Ad-hoc networking Provides link level security
WLAN Basic Service Set ●
AP
Connects WLAN to/ extends wired network More units deliver higher speed and greater area coverage
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Client ●
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Many media (PCI, PCMCIA, etc.) Support for multiple operating systems
WLAN Extended Service Set ●
Two or more wired networks connected by wireless “bridge”
WLAN Ad Hoc (Peer-toPeer) Service Set ●
Two or more wireless-enabled devices create own wireless network without an AP
Wired Equivalent Privacy (WEP) ●
Wired equivalence privacy? – –
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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 –
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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
Wired Equivalent Privacy (WEP) ●
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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.
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Shared Key Authentication – provides access control to medium Encryption – provides confidentiality to data on network
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You can have confidentiality on an open system –
That is, you can encrypt all the traffic and not have access control to the medium!
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Also means a wily hacker can have all his traffic encrypted on our network so that no one “see” what he is doing!
Claimed Properties of WEP ●
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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
Current Security
SSID MAC address filtering WEP Unfortunately, most of these mechanisms are off by default
All of these mechanisms have problems
802.11b Network Security Issues ● ● ● ●
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“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
802.11b Issues (cont.) ●
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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
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
⊕
Decryption works the same way: p = c ⊕ b
Ciphertext data byte p
WEP crypto function IV init. vector
24
secret key
seed 64
WEP PRNG
key sequence
40
+
plaintext integrity algorithm
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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)
WEP Frame
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Key id (2)
IV 3
ICV 4
pad (6)
PDU >=1
IV 4
Key id is used to choose between four secret keys ICV is integrity check sum (CRC-32) Pad is zero. Unused.
RC4 algorithm ●
Proprietary algorithm belonging to RSADS Inc.
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Secret key stream cipher.
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Variable length key (up to 2048 bits).
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Fairly fast (1Mbyte/sec on 33MHz processor).
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Claimed to be very strong.
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Exportable outside the U.S.
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Algorithm leaked onto the Internet in 1994.
Properties of Vernam Ciphers What happens when p1 and p2 are encrypted under the same “random” byte b? c1 = p1 ⊕ b
c2 = p2 ⊕ b
Then: c1 ⊕ c2 = (p1 ⊕ b) ⊕ (p2 ⊕ b) = p1 ⊕ p2 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.
Attacks on WEP ●
Stream ciphers and keystream reuse – – –
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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 C1 = P1 + RC4(IV, key) C2 = P2 + RC4(IV, key) but… (C1+C2) = (P1+P2) and (P1+P2) can be easily cryptanalyzed
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Given two cipher texts with the same IV, we can remove the effects of XORing with the RC4 stream (for the same secret key)
Attacks on WEP ●
Two assumptions for this attack –
Availability of ciphertexts with same IV ● ● ● ● ●
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IV length is fixed 24 bits (224 = 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 ●
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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
Dictionary Attack ●
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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 (~224 entries) can be built – For each IV find the associated key stream Ci= Pi + RC4(IVi, 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. Cn = Pn + RC4(IV, key) we know RC4(IV, key) from the dictionary, we know Cn so we can find Pn! 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!
Key Management ●
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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.
Message Modification ● ● ●
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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)) it 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 100..000. And so on …
Message Injection ●
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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.
Attack on Access Control nonce+RC4(IV, key)
Request received
128 nonce nonce+RC4(IV, key) IV
Decrypt the packet and verify nonce
Request.Authentication nonce+RC4(IV, key)
128 nonce nonce+RC4(IV, key) IV
hacker ●
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Request received
Decrypt the packet and verify nonce
server
Hacker Using Data Obtained From Previous Session
client
Normal session
Request.Authentication
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.
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
Switch
Victim B
d an y p rd Co rwa Fo Victim A
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
802.11b
Attacker
Switch
Victim B
d an y p rd Co rwa Fo Victim A
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.
General authentication requirements for access to networks ●
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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 authenticationmechanisms
WEP2 – Some Proposed Modifications ● ● ●
Increases size of IV space to 128 bits 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
VPN-based Security
Enterprise Network
Contivity
Private Network - Unencrypted Public Network - Encrypted
802.11 APs
802.11 terminals
RC4 and IPSec Use IPSec Security
WEP Security
AP VPN Device
Wireless client
IEEE 802.1X ●
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Access solution (Layer 2) between client and AP Several available authenticationmechanisms (EAP-MD5, EAP-TLS, EAP-TTLS, PEAP) Standardised Also encrypts all data, using dynamic keys RADIUS back end: – –
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Scaleable Re-use existing Trust relationships
Client software necessary (OS-built in or third-party)
802.1X ≠ 802.11x ●
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802.11x is sometimes used to summarise all ethernet standards (i.e. 802.11a, 802.11b) 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.
802.1x Framework
Pre-Authentication State
Post-Authentication State
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
MD5
TLS
TTLS
EAP
EAP
CHAP
PAP
Protocol Overview
PEAP
EAP 802.1X PPP
802.11
MS-CHAPv2
EAP Messages
LEAP ● ●
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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
Simplified Architecture of LEAP
d io a R
tr
s mi s an
n s io
Radius Server
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Wired Ethernet LAN
Access Point
User Machine (with client adapter)
Auth dB
LEAP / Cisco Solution 2
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Access point blocks all user requests to access LAN 5
Client associates with access point
RADIUS server delivers session key to access point
Auth dB
Access Point
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User performs network log-on (User ID and password)
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RADIUS server and client mutually authenticate and derive WEP session key
Client and access point activate WEP.
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Client and access point use WEP and key for protection of transmissions.
LEAP / Cisco Solution – Sequence of Events Step 1. A wireless client performs an 802.11b 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.
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.
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)
MD5 Authentication of User to Network – Conceptually Auth dB
User ID Password
Password Hash
Challenge
Key
Hash Key
Hash
Hash
Response =?
Determine Action
Secure Channels – Unique Keys per User per Session Keys are shared between AP and all users Radius Server User1
K1 Wired Ethernet LAN K2
K5 K3
K4
User2
User5 User3
User4
Auth dB
Anonymity within LEAP Radius Server
Wired Ethernet LAN
Access Point
User Machine (with client adapter)
User / client will disclose some information violating anonymity. Disclosed Parameters: MAC Address Programmed SSID User ID
Auth dB
Wireless Snooping of Responses on Password-based schemes
d io a R
tr
s mi s an
n s io
Radius Server
s
Wired Ethernet LAN
Access Point
User Machine (with client adapter)
Adversary can eavesdrop on wireless link Capture: User IDs Random challenges Responses (Hashed passwords)
Auth dB
MD5 Authentication of User to Network – Conceptually Auth dB
User ID Password
Password Challenge
Hash Key
Hash Key
Hash
Hash
Response =?
Stolen Parameters
Determine Action
Conceptual Dictionary Attack – Beating Low Entropy passwords Select User
User
User ID, challenges, responses Dictionaries
Index
Index database
Possible password Response
Challenge
Increment Index
Hash
Captured Response
Hash
password cracked
=?
Password located Masquerade
Computed Response
Yes
No
Check next database entry
EAP-TTLS
Auth dB
User Machine
Access Point
(with client adapter)
secure password authentication tunnel
secure data tunnel
TLS Server
Radius Server
EAP-TTLS Protocol Overview ●
Two phases – –
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Phase 1 is used to authenticate TTLS server to client (and optionally, vice versa) – –
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TLS handshake phase TLS tunnel phase
Results is activation of cipher suite Allows Phase 2 to proceed (using TLS record)
Phase 2 uses TLS record layer to
Simplified Architecture of EAPTTLS Radius Server
Auth dB
dio a R
m ns a tr
is
n sio
s
Wired Ethernet LAN
Access Point TTLS Server
User Machine (with client adapter)
What’s Right ● ●
Protection of the infrastructure Authentication mechanism can – –
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change as needed address flaws in existing wireless security
Lightweight –
No encapsulation, no per packet overhead… simply periodic authentication transactions
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 ●
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Remember – the loudest access point wins
Still no Authentication of management frames (assoc/deassoc/beacons/etc…)
What’s Wrong Technical ●
MITM – –
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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
What’s Wrong – Technical ●
RADIUS uses shared secret with the Authenticator –
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Authentication after association presents roaming problems –
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Same issue as WEP, but on a more reasonable scale
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
What’s Next ●
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Integration of existing solutions to “raise the bar” Limited 802.1x implementations 802.11i (Task Group I – Security) – – –
On track… the right track Mutual auth, per packet auth 802.1x a part of the solution
Temporal Key Integrity Protocol (TKIP) ●
Designed as a wrapper around WEP – – –
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Can be implemented in software Reuses existing WEP hardware Runs WEP as a sub-component
Fast Packet Keying Packet MAC Dynamic Re-keying Key distribution via 802.1x Still RC4 based to be backward compatible AES with 802.1x keying in the distant future
TKIP design challenges ●
Mask WEP’s weaknesses… – – – –
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Prevent Prevent Prevent Prevent
data forgery replay attacks encryption misuse 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
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 Intermediate key Base key
Transmit Address: 00A0-C9-BA-4D-5F Packet Sequence #
Phase 1 Mixer
4 msb
2 lsb
Per-packet key Phase 2 Mixer
Further Reading – WLAN Security ●
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802.1x http://standards.ieee.org/reading/ieee/std/lanman/802.1X-2001.pdf RFC’s: see http://www.ietf-editor.org EAP RFC 2284 EAP-MD5 RFC 1994, RFC 2284 EAP-TLS RFC 2716 EAP-TTLS http://www.funk.com/NIdx/draft-ietf-pppext-eap-ttls01.txt PEAP http://www.globecom.net/ietf/draft/draft-josefsson-pppexteap-tls-eap-02.html RADIUS RFC 2865, 2866, 2867, 2868, 2869 (I/w EAP) Overview of IEEE 802.11b Security, Sultan Weatherspoon Intercepting Mobile Communications: The Insecurity of 802.11, 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.
