Aggregation and Secure Aggregation. Learning Objectives Understand why we need aggregation in WSNs Understand aggregation protocols in WSNs Understand

Aggregation and Secure Aggregation

Learning Objectives

• Understand why we need aggregation in WSNs

• Understand aggregation protocols in WSNs• Understand secure aggregation protocols in

WSNs

Prerequisites

• Module 7• Basic concepts of computer networks• Basic concepts of network security

[Aggre_1] Section 1 4

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

What is Aggregation?

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Data Transmission

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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

adopted from slides from S. Madden

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Scenario: Count

Goal: Count the number of nodes in the network.

Number of children is unknown.

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1 2 3 4 51 - - - -

1 1 1 - -

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1 2 3 4 51 - - - -

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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

Data Aggregation Process

• Sensor nodes are organized into a tree hierarchy rooted at the Base Station

• Non-leaf nodes act as the aggregators

Example Aggregation

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

Tiny Aggregation

• Distribution phase– Aggregate queries are pushed down into the

network• Collection phase

– Aggregate values are continuously routed up from children to parents

Energy Consumption

Time v. Current Draw During Query Processing

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0 0.5 1 1.5 2 2.5 3Time (s)

Cur

rent

(mA

) Snoozing

Processing

Processingand Listening

Transmitting

Declarative Queries for Sensor Networks

• Examples:SELECT nodeid, lightFROM sensorsWHERE light > 400EPOCH DURATION 1s

1 EpochEpoch NodeidNodeid LightLight TempTemp AccelAccel SoundSound

0 1 455 x x x

0 2 389 x x x

1 1 422 x x x

1 2 405 x x x

Sensors

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

Aggregation Queries

SELECT roomNo, AVG(sound)FROM sensorsGROUP BY roomNoHAVING AVG(sound) > 200EPOCH DURATION 10s

Rooms w/ sound > 200

3

2SELECT AVG(sound)FROM sensorsEPOCH DURATION 10s

Epoch AVG(sound)0 4401 445

Epoch roomNo AVG(sound)0 1 3600 2 5201 1 3701 2 520

Section 4.1 of TAG

Illustration: Aggregation

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Slot 1SELECT COUNT(*) FROM sensors


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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?

Section 7.1 of [Aggre_1]

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

Secure Aggregation

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

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

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– P1|P2: 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

DBE

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E(PA | MAC(PA, KAH), KAH)

E(PB | MAC(PB, KBH), KBH)

E(AggrH | MAC(AggrH, KHJ), KHJ)

E(PB | MAC(PB, KCH), KCH)

E(PB | MAC(PB, KDH), KDH)

Integrity and Authenticity

• CBC: Cipher Block Chaining

Assignment

• 1. Why do we need aggregation in wireless sensor networks?

• 2. What is the basic idea of TAG?• 3. What is the basic idea of LCG-based secure

aggregation in wireless sensor networks?

Documents

Aggregation and Secure Aggregation. Learning Objectives Understand why we need aggregation in WSNs Understand aggregation protocols in WSNs Understand