Mudhakar Srivatsa, Ling Liu and Arun Iyengar
Presented by Mounica Atluri
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Voice-over-IP
Attacks
Proposed solution
Experimental Evaluation
Conclusion
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Data transmission through Public switched
telephone network
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Uses Circuit switched networks
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Expensive
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We see people talking through Skype,
Vonage, instant messengers
Technology behind is called VoIP
Transmission of voice traffic over IP-based
networks
Sounds are recorded and compressed
Benefit of VoIP: Very economical
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Caller anonymity and QoS
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Existing approaches use Mix networks
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Mix networks route traffic through nodes with
random delays and random routes
For example, Onion routing
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Other examples are Tor, Freedom and Tarzan
Mix networks cannot accommodate the QoS
requirement
Low latency apps are vulnerable to timing
attacks
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Uses RTP for data transmission
Route Set Up protocol for call set up and
termination
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Operates in four steps
1. initSearch: initiates a route set up request
2. processSearch: processes a route set up request
3. processResult: processes the results of a route set
up request
4. finSearch: concludes the route set up procedure
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src initiates a request by broadcasting
dst
src
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If p receives a request from q, it checks if the
sipurl is the url of the client connected to p.
dst
p
src
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If p receives result (searchId, q), it searches for
<searchId, sipurl, prev>, adds <sipurl, q>
and forwards result to prev
dst
p
src
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If src receives result, it adds <dst, q> to
its routing table
dst
q
src
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Encryption with shared symmetric key
Exposes dst (through dst.sipurl)
dst adds a random delay
src or dst can be inferred if all of their
neighboring nodes are malicious
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Triangulation based timing attacks
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3 steps in triangulation based timing attacks
• Candidate caller detection: malicious nodes deduce
a list of potential callers
• Candidate caller ranking: malicious nodes associate
a score with every potential caller
• Triangulation: Colluding malicious nodes combine
their sets to obtain more accurate list of callers.
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Deterministic triangulation attack
Statistical triangulation attack
Differential triangulation attack
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2 assumptions
• Link latencies are deterministic
• All nodes are synchronized
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2 properties of route setup protocol
• Protocol establishes shortest route between the src
and dst
• Node p that receives route set up request originated
from src can estimate dist(src, p)
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Candidate caller detection
• Compute S(p) for all s ∈ S(p),
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Candidate caller ranking
• Compute the score
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Triangulation
• Compute the final score
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Link latencies are independently distributed
Length of a path P is given by
In candidate caller detection, p computes a
set of Pareto-optimal distances to all nodes v
A set of path lengths d1, d2.. dm is Paretooptimal if for all other path lengths d,
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A node v is marked as a candidate caller if
If link latencies follow Gaussian, the path
latencies follow Gaussian too
Score of v can be computed as
For other any other distribution, use
Chebyshev’s inequality to compute
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In Triangulation step, the aggregate score for
a candidate caller v is computed
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Eliminates time stamp ts from the route set
up request
Malicious nodes can estimate the difference
In candidate caller detection, malicious node
p computes statistical shortest distances to
every other node v as
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Statistical distance distpq[v] is given by
distp[v] – distq[v]
v is a candidate caller if
If the link latency distribution is Gaussian, the
score of v is given by
Finally, the average score for v is computed
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Network topology should be known for
Timing attacks
Achieved by ping and pong messages
pong(y´,x)
y´
ping(x,all)
x
y
pong(y, x)
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Experimental set up
• A synthetic network with 1024 nodes
• Topology was constructed using NS-2 topology
generator
• Node-to-node round trip times varies from 24ms150ms with a mean of 74ms
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Deterministic Triangulation
• Number of suspects varies with number of
malicious nodes
• Epsilon should not be too small or large
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Statistical Triangulation
• More effective than deterministic when there are
uncertainties in link latencies
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Differential Triangulation
• Statistical attack performs better if the clocks are
synchronized
• Differential triangulation can achieve a top-10
probability of 0.78 with only 10 malicious nodes
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Topology Discovery
• With m=20 and ttl=2, about 75% of the topology is
discovered
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Latency perturbation
• each node adds random delay
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Random Walk Search Algorithm
• Resilient to timing attacks but generates
suboptimal routes
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Hybrid route set up
• Trade off anonymity with QoS
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Sends a search request to a randomly chosen
neighbor
Two key properties
• Markovian property
• Random walker does not traverse the shortest path
between any two nodes
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Controlled Random Walk
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Combination of two protocols
γ limits the length of random walk
Starts with random walk search
Switches to broadcast search with probability 1-γ
q
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Multi-Agent Random Walk
• Similar to random walk
• Src sends ω random walkers (ω >1)
• Route is established when the first random walker
reaches dst
• Higher ω results in optimal route latency
• Vulnerable to triangulation based timing attack if
src sends out random walkers at time t=0
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Performed on 1024-node synthetic VoIP
network topology using NS-2
Algorithms implemented using Phex: an open
source Java based implementation of peerto-peer broadcast based route set up
protocol
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Performance
• Characterized by cost of messaging
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QoS guarantees
• Routes with latency<250ms satisfy QoS
requirements
• Larger route set up latency does not affect the
quality of voice conversation
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Optimal parameter settings
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Attack resilience
• 99% optimal parameter settings
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Topology discovery
• Only fraction of topology has been discovered
• Top-10 probability for marw was 42% less, crw was
33% less and broadcast was only 9% less
• Random walk protocols are more sensitive to
topology
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VoIP in becoming popular due to its
advantages in cost and convenience
It is a major concern to provide anonymity to
the clients
Threat models targeting callers’ anonymity
are efficient
Even if a small fraction of network is
malicious, the caller can be inferred
accurately
It is difficult to trade QoS with anonymity