PSKA: Usable and Secure Key Agreement Scheme for Body Area Networks Authors: Krishna K. Venkatasubramanian, Ayan Banerjee, Sandeep K.S. Gupta Presenter:Francis Usher

Problem Domain: Body Area Networks (BANs) Sensors in BANs collect and disseminate sensitive health Security via cryptography requires key distribution

Cryptography (overview) Problem of sharing data securely Symmetric-key cryptography – Secret key k is used to obscure message m into cyphertext c – Given c, only k can be used to reveal m Advantage: provable that adversary can only break cryptosystem with negligible probability Problem: how do we communicate keys?

Key sharing (generic approaches) Pre-sharing – Manufacturer embeds long-term keys in device – Problem: not dynamic enough to handle key compromise situations Asymmetric crypto handshake – Asymmetric crypto doesn’t require shared secret – Problem: Usually requires contacting trusted identity authority

Physiological Signal-based key sharing (for BANs) Design goals: – Length & randomness – Low latency – Distinctiveness – Temporal variance Previous work: Inter-pulse-interval (IPI)-based, independent, mutual key generation Physiological Signal based Key Agreement (PSKA) – Use shared physiological signals to build & access “fuzzy vault” containing session key

IPI-based key generation Synchronized sensors measure IPI (EKG/PPG) Encode measurements as key 4 observations: – Meets randomness goal, however: – High-latency – Two keys generated tend to differ in half of bits – This distance tends not to vary much in time between generation or across different patients – No good tradeoff threshold between false positive/negative rates

Digression: Shamir’s Secret Sharing (precursor)

PSKA: Sharing keys using fuzzy vaults Different sensors measure phys. signals – “Loosely synchronized” Transform signals to create “features” Generate random polynomial representing key Map features under polynomial Obscure feature maps using “chaff” points Only similar feature set can infer polynomial from vault (features + chaff) Use MACs to affirm that key was shared correctly

Fuzzy Vault Security

Analysis of technique Long & random keys Low latency (only ~ 5-10 seconds of data) Distinctiveness (across subjects) Temporal variance (across time)

Feature generation Use peaks from frequency domain (FFTs) – Concatenate indexes & values across windows Features should demonstrate distinctiveness and temporal variance

Feasability: implementation Power, resource constraints Can technique be implemented as efficiently as competitors? – Classical, elliptic-curve Diffie Hellman Evaluation in VHDL (formal hardware specification language) Metrics: clock cycles, memory footprint

Possible attacks Fuzzy vault attacks – Some based on application to biometrics – One attack based on vault-construction artifact Early points (features) have more “free area” Dismissed but not thoroughly argued against

Idea for future work Use fuzzy vaults to communicate public keys – Use asymmetric crypto handshakes to establish session keys – Frequent update of public keys – Eliminates problem of contacting trusted authority since physiological signals good for authentication of body-area presence