Security in Wireless Networks

Mike Swift

CSE 802.11b

Summer 2003

Standard Preamble

• What is different about wireless?– No authentication of access port– Battery-operated devices– Frequent use of broadcast– Easy sniffing / packet injection– Jamming

What are the problems?

• Denial of service– Battery usage– Physical layer: jamming (not our problem)– MAC layer and up: injected messages

• Confidentiality / integrity - More or less solved

• Secure association• Routing -Preventing rogues from obtaining

routes

Power DOS attacks

• Turn off antenna to save power– Spoof “no messages” message when awakes– Spoof “message poll” so discarded before awakes– Spoof timer so desynchronizes

• Receiving / sending packets require power consumption– Attacker can forcer receiver to use use power

• Send many packets• Force it to resend packets

• Solution:– Power consumption management– Prioritize tasks when limited by power– Authenticate timer messages

MAC Layer DOS Attacks

• Problem– MAC layer message direct nodes when not to send

messages• RTS/CTS and NAV in 802.11 reserve channel

– MAC layer state machine directs nodes to ignore future messages

• Unauthenticated / unassociated state causes packets to be dropped silently

• States entered as result of unauthenticated messages

– Power requirements for DOS very low– Commercial MAC implementations allow sending of

arbitrary packets via. aux debug port

Solution to MAC layer DOS attacks

• Authenticate every messages– Prevents outsider from disassociating /

unauthenticating

• Verify messages– Verify channel in use after RTS/CTS– Verify no more messages after disassociation

General approaches

• Sign every packet – Prevent attackers from spoofing management

packets

• Authenticate then associate– Allows authentication of association

management packets– Prevents any communication before

authentication

Secure association

• How does my TV trust my remote?

• How does my laptop trust the printer in the airport?

• How do I get onto a wireless network?

Solutions for ad-hoc networks

• Location limited channels for key exchange– Physical contact– Direction-specific limited range (IR)– Demonstrative identification – easily visible– Pre-authentication: exchange keys before going

wireless

• Resurrected duckling– First association is binding– Removing binding reincarnates device (loses all state)

Solutions for Access Points

• Two-layer protocols– Application layer: key negotiation and

authentication– Link layer: message integrity and

confidentiality

• Access points allow only limited connectivity before association– Communication only for authentication /

address acquisition (DHCP)

Routing

• Routing works over unknown physical layout– Must infer topology / neighbors from messages

sent

• Attacks:– Corrupting routing updates– Forwarding messages inappropriately (wormhole)

• Result of attacks– Can force all traffic through a node– Can break reachability

Routing security solutions

• Solutions: – Cryptography to prevent forging route

messages (ask Ratul for details)• Ensure that route metrics can only be increased,

not decreased• Ensure that metrics received along two paths are

consistent– Ensure that packets received are physically sent (or

possibly physically sent) by in-range sender

Wormhole Attack

Geographic Leashes

• E computes distance = 408

• Distance too far!

• Requires GPS

A(400,150),t1A

B

C

D(50,10),t2

B

C

D

(400,150)

(10,30),t3

(50,10)

A(400,150),t1

A(400,150),t1

E

Temporal Leashes

• E computes t3-t1 > c * max distance : denied• E computes t3-t2 < c* max distance: accepted• Requires clocks synchronized to 183 ns• Requires RT OS/MAC to give deterministic packet

delivery/receipt times

A,t1A

B

C

D,t2

B

C

D

E:t3

A,t1

A,t1

E

General Principals

• Sign everything• Authenticate first• Use limited channels for initial

authentication• Trust, but verify

– sender confirms intent to disconnect• e.g. no more packets• associates to another AP

– sender in range