Aggregation and Secure Aggregation

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Aggregation and Secure Aggregation
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Why do we need Aggregation?

• Sensor networks Event-based Systems
• Example Query
• What is the maximum temperature in area A between
  10am and 11am?
• Redundancy in the event data
• Individual sensor readings are of limit use
• Forwarding raw information too expensive
• Scarce energy
• Scarce bandwidth
• Solution
• Combine the data coming from different sources
• Eliminate redundancy
• Minimize the number of transmissions
• Aggregation Summary

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What is Aggregation?
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One Example of Aggregation - Count

• Example consider a query that counts the number
  of motes in a network of indeterminate size

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Sensor
1 2 3 4 5
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
Time
Goal Count the number of nodes in the
network. Number of children is unknown.
Scenario Count
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Sensor
1 2 3 4 5
1 - - - -
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
Time
Goal Count the number of nodes in the
network. Number of children is unknown.
Scenario Count
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Sensor
1 2 3 4 5
1 - - - -
1 1 1 - -
1 2 1 1 - -
- - - - -
- - - - -
- - - - -
- - - - -
Time
Goal Count the number of nodes in the
network. Number of children is unknown.
Scenario Count
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Sensor
1 2 3 4 5
1 - - - -
1 1 1 - -
1 2 1 1 1 -
1 2 1 ½ 1 ½ 1 -
- - - - -
- - - - -
- - - - -
Time
Goal Count the number of nodes in the
network. Number of children is unknown.
Scenario Count
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Sensor
1 2 3 4 5
1 - - - -
1 1 1 - -
1 2 1 1 1 -
1 2 1 ½ 1 ½ 1 1
13 1 ½ 1 ½ 11 1
- - - - -
- - - - -
Time
Goal Count the number of nodes in the
network. Number of children is unknown.
Scenario Count
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Sensor
1 2 3 4 5
1 - - - -
1 1 1 - -
1 2 1 1 1 -
1 2 1 ½ 1 ½ 1 1
13 1 ½ 1 ½ 11 1
13 12/2 12/2 11 1
- - - - -
Time
Goal Count the number of nodes in the
network. Number of children is unknown.
Scenario Count
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Sensor
1 2 3 4 5
1 - - - -
1 1 1 - -
1 2 1 1 1 -
1 2 1 ½ 1 ½ 1 1
13 1 ½ 1 ½ 11 1
13 12/2 12/2 11 1
14 12/2 12/2 11 1
Time
Goal Count the number of nodes in the
network. Number of children is unknown.
Scenario Count
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Count Example A Better Scheme

• Each leaf node in the tree reports a count of 1
  to their parents
• Interior nodes sum the count of their children,
  add 1 to it, and report that value to their parent

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Data Aggregation Process

• Sensor nodes are organized into a tree hierarchy
  rooted at the Base Station
• Non-leaf nodes act as the aggregators

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Example Aggregation

• Max, Min
• Count, Sum
• Average
• Median

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Tiny Aggregation

• Distribution phase
• Aggregate queries are pushed down into the
  network
• Collection phase
• Aggregate values are continuously routed up from
  children to parents

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Energy Consumption
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Declarative Queries for Sensor Networks
Sensors

• Examples
• SELECT nodeid, light
• FROM sensors
• WHERE light gt 400
• EPOCH DURATION 1s

Epoch Nodeid Light Temp Accel Sound
0 1 455 x x x
0 2 389 x x x
1 1 422 x x x
1 2 405 x x x
1

• Time is partitioned into epochs of duration iA
  single aggregate value is produced to combine the
  readings of all devices during the epoch

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Aggregation Queries
Epoch AVG(sound)
0 440
1 445
Epoch roomNo AVG(sound)
0 1 360
0 2 520
1 1 370
1 2 520
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Illustration Aggregation
SELECT COUNT() FROM sensors
Slot 1
Sensor
1 2 3 4 5
1 1
2
3
4
1
Slot
1
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Illustration Aggregation
SELECT COUNT() FROM sensors
Slot 2
Sensor
1 2 3 4 5
1 1
2 2
3
4
1
2
Slot
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Illustration Aggregation
SELECT COUNT() FROM sensors
Slot 3
Sensor
1 2 3 4 5
1 1
2 2
3 1 3
4
1
1
3
Slot
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Illustration Aggregation
SELECT COUNT() FROM sensors
Slot 4
5
Sensor
1 2 3 4 5
1 1
2 2
3 1 3
4 5
1
Slot
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Illustration Aggregation
SELECT COUNT() FROM sensors
Slot 1
Sensor
1 2 3 4 5
1 1
2 2
3 1 3
4 5
1 1
Slot
1
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Flow Up the tree during an epoch
How parents choose the duration of the interval
in which they will receive values?
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Topology Maintenance and Recovery

• How to address the unreliable nature of WSNs in
  TAG?
• Each node maintains a fixed size of its neighbors
  Select a better parent node
• If a node does not hear from its parent for some
  time, it assumes that its parent has failed

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Secure Aggregation
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Secure Aggregation

• It is challenging to design suitable security
  mechanisms for Wireless Sensor Networks (WSNs)
• Stringent resource constraints on energy,
  processing power, memory, bandwidth, etc.
• WSNs need lightweight secure mechanisms
• We introduce an LCG-based secure aggregation
  scheme
• Efficiency and simplicity

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Security Goals

• Security Goals
• Confidentiality
• Sensor data/readings cannot be disclosed to
  attackers
• Integrity
• If an adversary modifies a data message, the
  receiver should be able to detect this tampering
• Authenticity
• Ensures that data messages come from the intended
  sender
• Assumptions
• The existence of a key management scheme
• WSN nodes can negotiate the key and trust setup

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LCG-based Security Protocols

• Basic Hop by Hop Message Transmission

• Notations
• A, B, C Sensor Nodes
• E(P, K) Encryption of plaintext message P using
  key K
• P1P2 Concatenation of message P1 and P2

• MAC(K, P) Message Authentication Code (MAC) of
  message P using key K
• X0 seed of the LCG
• a, b, m Parameters of the LCG

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Integrity and Authenticity

• CBC Cipher Block Chaining