1. SIA: Secure Information Aggregation in Sensor Networks B. Przydatek, D. Song, and A. Perrig. In Proc. of ACM SenSys 2003 Natalia Stakhanova cs610

2. Sensor networks  wireless network consisting of large number of small sensor devices Main objective: data collection  Sensors are severely constrained: Low memory Low power Limited bandwidth  Transfer of raw information is expensive (often individual readings are not needed) Solution: aggregate data and transfer the result only!

3. Data aggregation  Selected nodes are aggregators responsible for  data collection  computation of aggregated result  result transfer to the user ( home server )  Security concern: compromised aggregator compromised sensors

4. Proposed approach  Previous works assumed honest sensors  This work’s focus : stealthy attacks If user accepts the aggregation result, then there isa high probability that the reported result is “close” to the true result value

5. Considered model  single home server (user)  single aggregator more powerful than sensor has information about size and topology of the network  sensors have unique ids share a key with server and aggregator Both home server & aggregator have master key and able to compute key for each sensor aggregator home server sensors a1 a2 a3 A= ? A= (a1, a2, a3)

6. Aggregate-Commit-Prove approach  aggregate aggregator collect the data from sensors compute aggregated result  commit aggregator commits to the data  guarantee that result is computed using sensors’ data  prove aggregator send the aggregate result and commitment to home server home server  checks if commitment is good representation of the sensor data  aggregation result is close to the committed data values

7. Commit phase  Merkle hash tree – to commit to the data  a 1 a 2 a 3... a n - sensors’ data placed at the leaves  each internal node is hash of its children  root value is a commitment

8. Considered …  Most commonly used aggregation operations: Compute median Compute min, max Counting distinct elements

9. Computing median  Securely compute median of a 1 a 2 a 3... a n  Aggregate phase: take median of a random sample of sensor values commits to a sorted sequence using a Merkle hash tree  Prove phase: home server receives the commitment and computed median a med home server performs 2 tests:  requests a n/2 and compares it with a med  picks an element from a random position Checks if elements picked from left half is < than median Checks if elements picked from right half is > than median

10. Computing min/max  Securely compute min of a 1 a 2 a 3... a n  Assumption – sensors will not provide fake values  Computing min/max by sensors MinRootedTree protocol  construct minimum spanning tree rooted at the minimum value  each round node broadcast (min, id) pair  fills the table by smallest received value Final state is authenticated and sent to aggregator p minid S1 3id1 S2 1id2 S3S21Id2 S1S2 S3 Reading: 3Reading: 1 Reading: 5 p – id of the current parent min – min value so far id – id of the node with min p minid S1 3id1 S2 1id2 S3 5id3 p minid S1S21id2 S2 1id2 S3S21Id2

11. Computing min/max  Aggregate phase:  aggregator commits to the list of the states  reports the root of the tree to the server  Prove phase:  home server randomly picks a node in the list  traverses the path from the node to the root  If unsuccessful - rejects

12. Counting distinct elements  Securely determine number of distinct values given a 1 a 2 a 3... a n  Basic protocol: Pick random hash function h Apply to all elements a i Keep v=min i=1 n h(a i ) Number of distinct elements can be estimated by 1/v  Protocol can be used for: computing the size of the network computing average value

13. Conclusion  Hierarchical aggregation for very large networks the proposed protocols need to be slightly modified  Consider forward secure authentication for past results querying sensor’s key is recomputed each time interval using one-way function past readings are secure in case sensor is compromised  This is the first work that allows existence of malicious sensors