Security in Wireless Sensor Networks

Adrian Perrig, John Stankovic, and David Wagner

Overview

• WSN security: Too many problems... A number of solutions... Enough?

• Survey Paper: outlines security issues, discusses some existing solutions, and suggests possible research directions

• Issues include: – key establishment– secrecy– authentication– privacy– denial-of-service attacks More info in a later set of

slides – secure routing More info in a later set of slides – node capture

• Also discuses some sample security services for wireless sensor networks

Problems Applying Traditional Network Security Techniques

• Sensor devices are limited in their energy, computation, and communication capabilities

• Sensor nodes are often deployed in open areas, thus allowing physical attack

• Sensor networks closely interact with theirphysical environments and with people, posing new security problems

Key Establishment and Trust

• Sensor devices have limited computational power, making public-key cryptographic primitives too expensive in terms of system overhead.

• Simplest solution is a network-wide shared key – problem: if even a single node were compromised, the

secret key would be revealed, and decryption of all network traffic would be possible

• Slightly better solution:

– use a single shared key to establish a set of link keys, one per pair of communicating nodes, then erase the network-wide key

– problem: does not allow addition of new nodes after initial deployment

Key Establishment (continued)

• Bootstrapping keys using a trusted base station– Each node needs to share only a single

key with the base station and set up keys with other nodes through the base station

– The base station becomes a single point of failure • Utilize tamper-resistant packaging for the

base station, reducing the threat of physical attack

• Most existing work assumes base station is safe – Good assumption???

Random-key pre-distribution protocols

• Large pool of symmetric keys is chosen• Random subset of the pool is distributed to each sensor

node• To communicate, two nodes search their pools for a

common key– If they find one, they use it to establish a session key– Not every pair of nodes shares a common key, but if the

key-establishment probability is sufficiently high, nodes can securely communicate with sufficiently many nodes to obtain a connected network

• No need to include a central trusted base station

• Disadvantage: Attackers who compromised sufficiently many nodes could also reconstruct the complete key pool and break the scheme

Secrecy and Authentication

• We need cryptography as protection against eavesdropping, injection, and modification of packets

• Trade-offs when incorporating cryptography into

sensor networks:– end-to-end cryptography achieves a high level of

security but requires that keys be set up among all end points and be incompatible with passive participation and local broadcast

– link-layer cryptography with a network-wide shared key simplifies key setup and supports passive participation and local broadcast, but intermediate nodes might eavesdrop or alter messages

Hardware vs. Software Cryptography

• Hardware solutions are generally more efficient, but also more costly ($)

• University of California, Berkeley, implementation of TinySec incurs only an additional 5%–10% performance overhead using software-only methods– Most of the overhead is due to increases in packet size– Cryptographic calculations have little effect on latency

or throughput, since they can overlap with data transfer – Hardware reduces only the computational costs, not

packet size

• Thus, software-only techniques are sufficient (or reasonable to be more careful)

Privacy

• Issues– Employers might spy on their employees– Shop owners might spy on customers– Neighbours might spy on each other– Law enforcement agencies might spy on

public places• Technological improvements will only

worsen the problem – Devices will get smaller and easier to

conceal– Devices will get cheaper, thus

surveillance will be more affordable

Privacy (continued)

• Sensor networks raise new threats that are qualitatively different from what private citizens worldwide faced before– Sensor networks allow data collection, coordinated

analysis, and automated event correlation– Networked systems of sensors can enable routine

tracking of people and vehicles over long periods of time– EZ Pass + OnStar == Big Brother?

• Suggested ways of approaching solution include a mix of:– Societal norms– New laws– Technological responses

Robustness to Denial of Service

• Simple form: Radio jamming• Sophisticated form: Transmit while a

neighbor is also transmitting or continuously generating a request-to-send signal

• Possible solution (when the jamming affects only a portion of the network):– Detect the jamming– Map the affected region– Route around the jammed area

Secure Routing

• Proper routing and forwarding are essential for communication in sensor networks

• Injection attacks– Transmit malicious routing information into the network

resulting in routing inconsistencies– Authentication might guard against injection attacks, but

some routing protocols are vulnerable to replay by the attacker of legitimate routing messages

• Sensor network routing protocols are particularly susceptible to node-capture attacks– Compromise of a single node could be enough to take

over the entire network or prevent any communication within it

Resilience to Node Capture

• In traditional computing, physical security is often taken for granted

• Sensor nodes, by contrast, are likely to be placed in open locations– Attacker might capture sensor nodes– Extract cryptographic secrets– Modify programs/Replace them with malicious nodes

• Tamper-resistant packaging may be one defense, but it’s expensive

Algorithmic Solutionsto Node Capture

• Attempt to build networks that operate correctly even in the presence of nodes that might behave in an arbitrarily malicious way– Replicate state across the network and use

majority voting to detect inconsistencies– Gather redundant views of the environment

and crosscheck them for consistency

• Most challenging problems in sensor network security– We are far from a complete solution

Network Security Services

• So far, we’ve explored low-level security primitives for securing sensor networks.

• Now, we consider high-level security mechanisms.– Secure group management– Intrusion detection– Secure data aggregation

Secure Group Management

• Protocols for group management are required to– securely admit new group members– support secure group communication

• Outcome of group computation must be authenticated to ensure it comes from a valid group

• Any solution must also be efficient in terms of time and energy

Intrusion detection

• In wired networks, traffic and computation are typically monitored and analyzed for anomalies at various concentration points– expensive in terms of the network’s memory and energy

consumption– hurts bandwidth constraints

• Wireless sensor networks require a solution that is fully distributed and inexpensive in terms of communication, energy, and memory requirements

• In order to look for anomalies, applications and typical threat models must be understood

• It is particularly important for researchers and practitioners to understand how cooperating adversaries might attack the system

• The use of secure groups may be a promising approach for decentralized intrusion detection

Secure Data Aggregation

• One benefit of a wireless sensor network is the fine-grain sensing that large and dense sets of nodes can provide

• The sensed values must be aggregated to avoid overwhelming amounts of traffic back to the base station

• Depending on the architecture of the network, aggregation may take place in many places – All aggregation locations must be secured

• If the application tolerates approximate answers, powerful techniques are available– Randomly sampling a small fraction of nodes and

checking that they have behaved properly supports detection of many different types of attacks

Conclusions

• Constraints and open environments of wireless sensor networks make security for these systems challenging.

• Several properties of sensor networks may provide solutions.– architect security into these systems from the

outset (they are still in their early design stages)– exploit redundancy, scale, and the physical

characteristics of the environment in the solutions

– build sensor networks so that they can detect and work around some fraction of their nodes which are compromised

Future Research Areas

• Securing wireless communication links against– Eavesdropping– Tampering– Traffic analysis– Denial of service

• Resource constraints• Asymmetric protocols

– Most of the computation done at base station• Public-key cryptographic systems

– How to make efficient on low-end devices?• Working around the lack of physical security

– redundancy– knowledge about the physical environment

Denial of Service inSensor Networks

Anthony D. Woodand John A. Stankovic

Why Security?

• Battlefield• Disasters

– Protect the location and status of casualties from unauthorized disclosure, particularly if the disaster relates to ongoing terrorist activities

• Public safety– False alarms about chemical, biological, or

environmental threats could cause panic or disregard for warning systems. An attack on the system’s availability could precede a real attack on the protected resource

• Home healthcare – Because protecting privacy is paramount, only

authorized users can query or monitor the network. These networks can also form critical pieces of an accident-notification chain, thus they must be protected from failure

DENIAL OF SERVICE THREAT

• A DoS attack is any event that diminishes or eliminates a network’s capacity to perform its expected function

• Hardware failures, software bugs, resource exhaustion, environmental conditions, or their combination

• Intentional Attack

Adversary Capability

• Physically damaged or manipulated node – May be less powerful than a normally

functioning node

• Subverted nodes (or added ones)– Interact with the network only through

software– As powerful as other nodes

• Immensely more powerful adversaries– Existing wired network with virtually unlimited

computational and energy resources possible

Attacks on Physical Layer

• Jamming– Defenses

• Spread-spectrum• Region mapping: Less expensive

• Tampering– Defenses: Tamper-proofing, hiding

Link Layer Attacks

• Collision – Use error-correcting codes

• Exhaustion – Rate limitation

• Unfairness – Small frames

Network and Routing Attacks

• Neglect and greed – Redundancy, probing

• Traffic analysis – Encryption: enough? Maybe not

• Misdirection – Egress filtering, authorization, monitoring

• Black holes – Authorization, monitoring, probing,

redundancy

Neglect and Greed

• Neglect– Drops packets arbitrarily

• Greed– Gives undue priority to it’s own

messages

• Use multiple paths and/or redundant messages to mitigate these effects.

Traffic Analysis

• Geographic forwarding allows attacker to figure out where important nodes are

• Encrypting headers as well as content might alleviate this issue

• Cryptographic means may not help when the communication pattern is many-to-one– Just watch traffic intensity– INSENS [ICDCS ‘03]

Misdirection

• Diverting traffic away from intended destination– Targets the sender

• Misdirecting many flows in one direction– Targets an arbitrary victim (receiver)

• Defense– Egress Filtering

• Verification of source addresses• Legitimately generated from below?

Black Holes

• Distance-vector-based protocol weakness• Nodes advertise zero-cost routes to every

other node.• Fixes:

– Authorization– Monitoring

• Watchdog the next hop transmission of your packets by neighbors [Mobicom ’00]

– Probing• Send periodic messages across topology to test for

blackout regions– Redundancy

Transport Layer DoS

• Flooding – Client puzzles

• Make the adversary commit resources• Only useful if the adversary has limited

resources

• Desynchronization – Authentication

PROTOCOL VULNERABILITIES to DoS

Analyzing these vulnerabilities helps show why developers should

consider DoS susceptibility at design time.

Adaptive Rate Control – MAC Protocol by Woo & Cull

• Give preference to route-through traffic – This preserves the network’s investment in

packets that may have already traversed many hops

• Makes flooding attacks more effective – High bandwidth packet streams that an

adversary generates will receive preference– Thus, the network gives preference to

malicious traffic

RAP

• Real-time communication architecture– Geographic forwarding – Velocity monotonic scheduling (VMS) policy

• Originator of message sets deadline and destination– VMS layer computes velocity based on

time to deadline and distance remaining

RAP Vulnerability

• Flood with high velocity packets– Set destination at long distance

• Possibly outside the network

• Intermediate node adversary could lower the velocity of route through traffic– Causes deadline misses

• If relying on a synchronized clock, attacking that mechanism could cause another node to always drop – Protecting clock synchronization is a challenging

yet important problem by itself

Secure Routing in Wireless Sensor Networks: Attacks and

Countermeasures

Chris Karlof and David Wagner

Key Contributions

• Secure routing issues in WSNs– Show how they are different from ad hoc

networks– Introduce two new classes of attacks

• Sinkhole attack• Hello flood attack

• Analyze security aspects of major routing protocols

• Discuss countermeasures & design considerations for secure routing in WSNs

WSNs vs. Ad Hoc Networks

• Multi-hop wireless communications• Ad hoc nets: communication between two

arbitrary nodes• WSNs

– Specialized communication patterns• Many-to-one• One-to-many• Local communication

– More resource constrained– More trust needed for in-network processing,

aggregation, duplicate elimination

Assumptions

• Insecure radio links• Malicious nodes can collude to attack

the WSN• Sensors are not tamper-resistant• Adversary can access all key

material, data & code• Aggregation points may not be

trustworthy• Base station is trustworthy

Threat Models

• Device capability– Mote class attacker– Laptop class attacker: more energy,

more powerful CPU, sensitive antenna, more radio power

• Attacker type– Outside attacker: External to the

network– Inside attacker: Authorized node in the

WSN is compromised or malicious

Security Goals

• Secure routing – Support integrity, authenticity,

availability of messages in presence of attack

– Data confidentiality

Potential Attacks

• Attacks on general WSN routing• Attacks on specific WSN protocols

Attacks on General WSN Routing Protocols

• Spoof, alter, or replay routing info.– Create loops, attack or repel network

traffic, partition the network, attract or repel network traffic, etc.

– Message authentication can partly handle these issues

• Selective forwarding– Malicious node selectively drops

incoming packets

Sinkhole attack

• Specific to WSNs– All packets are directed to base station– A malicious node advertises a high

quality link to the base station to attract a lot of packets

– Enable other attacks, e.g., selective forwarding or wormhole attack

Sybil attack

• A single node presents multiple ID’s to other nodes

• Affect geographic routing, distributed storage, multi-path routing, topology maintenance

Wormhole attack

• Two colluding nodes• A node at one end of the wormhole

advertises high quality link to the base station

• Another node at the other end receives the attracted packets

Hello flood attack

• Specific to WSNs– In some protocols, nodes have to periodically

broadcast “hello” to advertise themselves• Not authenticated!

– Laptop-class attacker can convince it’s a neighbor of distant nodes by sending high power hello messages

Acknowledge spoofing

• Adversary spoofs ACKs to convince the sender a weak/dead link support good link quality

Attacks on Specific Routing Protocols

• TinyOS beaconing– Construct a BFS rooted at the base

station– Beacons are not authenticated– Adversary can take over the whole WSN

by broadcasting beacons

Directed diffusion

• Replay interest• Selective forwarding & data

tampering• Inject false data

Geographic routing

• Adversary can provide false, possibly multiple, location info. – Create routing loop– GEAR considers energy in addition to

location• Laptop-class attacker can exploit it

Countermeasures

• Shared key & link layer encryption– Prevent outsider attacks, e.g., Sybil attacks, selective

forwarding, ACK spoofing– Cannot handle insider attacks

• Wormhole, Hello flood, TinyOS beaconing

• Sybil attack– Every node shares a unique secret key with the base station– Create pairwise shared key for msg authentication– Limit the number of neighbors for a node

• Hello flood attack– Verify link bidirectionality– Doesn’t work if adversary has very sensitive radio

Countermeasures

• Wormhole, sinkhole attack– Cryptography may not help directly – Good routing protocol design– Geographic routing

• Geographic routing– Location verification– Use fixed topology, e.g., grid structure

• Selective forwarding– Multi-path routing– Route messages over disjoint or Braided paths– Dynamically pick next hop from a set of candidates– Measure the trustworthiness of neighbors

Countermeasures

• Authenticated broadcast– uTESLA

• Base station floods blacklist– Should be authenticated– Adversaries must not be able to spoof

Conclusions

• WSN security is challenging, new area of research

• #Problems >> #Solutions• Any ideas to address a problem?