Computer Science
Using Directional Antennas to Prevent
Wormhole Attacks
Stephen Thomas
Acknowledgement: Portions of this presentation have been donated by
Dr. David Evans
Situation: Ad Hoc Beacon Routing
1
0
2
3
Nodes select parents
based on minimum
hops to base station
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4
Wormhole Attack
B
A
C
D
S
Y
X
Attacker needs a transceivers at two locations in the network, connected by a low
latency link
Attacker replays (selectively) packets heard at one location at the other location
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Wormhole Attack on Beacon Routing
1
0
2
2
1
0
X
Wormhole attack disrupts
network without needing
to break any cryptography!
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Y
Possible Solutions
• Packet Leashes [Hu, Perrig, Johnson 2003]
– Requires tightly synchronized clocks (temporal
leashes) or precise location information
(geographic leashes)
– Signal is transmitted at speed of light
– Calculate if packet could travel distance between
nodes in packet lifetime
• Packet Arrival Direction
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Directional Antennas
3
2
4
1
5
Directional
Transmission
from Zone 4
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North
6
Aligned to
magnetic North,
so zone 1 always
faces East
Omnidirectional Transmission
General benefits: power saving, less collisions
Assumptions
• Legitimate nodes can establish secure node-node links
– All critical messages are encrypted
• Network is fairly dense
• Nodes are stationary
• Most links are bidirectional (unidirectional links
cannot be established)
• Transmissions are perfect wedges
• Nodes are aligned perfectly (relaxed in paper)
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Directional Neighbor Discovery
3
2
B
4
A
5
1. A  Region
2. B  A
3. A  B
1
6
zone (B, A) = 4
is the antenna
zone in which
B hears A
HELLO | IDA
Sent by all antenna elements (sweeping)
IDB | EKBA (IDA | R | zone (B, A))
Sent by zone (B, A) element, R is nonce
R
Checks zone is opposite, sent by zone (A, B)
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Not Detecting False Neighbors
3
2
4
1
5
A
X
zone (A, B [X]) = 1
Y
B
zone (B, A[Y]) = 4
Undetected False Neighbor:
zone (A, B) = opposite of zone (B, A)
Directional neighbor discovery prevents 1/6 of
false direct links…but doesn’t prevent disruption
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6
Verified Neighbor Discovery
V
A
1. A  Region
2. B A
3. A  B
5. IDV | EKBV (IDA | zone (V, B))
B 4. INQUIRY | IDB | IDA | zone (B, A)
Announcement, done through sequential sweeping
Include nonce and zone information in the message
Check zone information and send back the nonce
4. B  Region
5. V  B
6. B  A
Same as
before
Request for verifier to validate A
If V is a valid verifier, sends confirmation
Accept A as its neighbor and notify A
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Verifier Analysis
3
3v
2
1
4
X
A
5
6
Region 1
4
2
B
1
Y
5
6
Region 2
Wormhole cannot trick a valid verifier:
zone (V, A [Y]) = 5
Not opposites: verification fails
zone (A, V [X]) = 1
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Worawannotai Attack
v
3
3
2
2
B
4
5
1
X
A
6
Region 1
5
6
Region 2
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V hears
A and B directly
A and B hear
V directly
But, A and B
hear each other
only through
repeated X
Preventing Attack
1. zone (B, A)  zone (B, V)
2. zone (B, A)  zone (V, A)
3. zone (B, V) cannot be both adjacent to zone (B, A)
and adjacent to zone (V, A)
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Connectivity and Routing
10
Network with density = 10
9
Average Path Length
8
7
Strict Protocol
Verified protocol:
0.5% links are lost
no nodes disconnected
Strict protocol:
40% links are lost
0.03% nodes disconnected
20% path increase from verified
6
Verified Protocol
5
4
Trust All
3
2
1
0
4
6
8
10
12
14
16
Omnidirectional Node Density
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18
20
Vulnerabilities
• Attacker with multiple wormhole endpoints
– Can create packets coming from different
directions to appear neighborly
• Magnet Attacks
– Protocol depends on compass alignment of nodes
• Antenna, orientation inaccuracies
– Real transmissions are not perfect wedges
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Conclusion
• An attacker with few resources and no crypto
keys can substantially disrupt a network with a
wormhole attack
• If you know your neighbors, can detect
wormhole
• Need to cooperate with your neighbors to
know who your legitimate neighbors are
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Future Work
• Analysis of protocol vulnerabilities to other
attacks
– Magnet for disruption (not pertaining to wormhole
attacks)
– Flipping nodes to disorient north from south
• Expand protocol to handle dynamic ad hoc
networks
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Thank you!
Questions?
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