Automated Exposure Notification Schemes Overview
Explore Automated Exposure Notification (AEN) schemes for contact tracing, including ReBabbler and CleverParrot, offering strong security and privacy features. Learn about decentralized proximity-based contact tracing using Bluetooth technology and the construction of these schemes for passive and diagnosed users.
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Presentation Transcript
Automated Exposure Automated Exposure Notification Schemes Notification Schemes Ran Canetti, Yael T. Kalai, Anna Lysyanskaya, Ronald L. Rivest, Adi Shamir, Emily Shen, Ari Trachtenberg, Mayank Varia, Daniel J. Weitzner
Our Contributions Formalize the notion of Automated Exposure Notation (AEN) schemes Syntax Security properties: game-based and UC Propose three schemes Implemented by Apple and Google, introduced independently by DP-3T, TCN, UW-PACT, Covid Watch, Trace Together, 1. ReBabbler 2. CleverParrot stronger security at a cost to efficiency (and simplicity) 3. Certified CleverParrot
Contact Tracing For passive and diagnosed users 1. Identifying an index case 2. Determining their contacts Privacy?? AEN 3. Notifying the contacts 4. Follow-up Important Properties: Decentralized Proximity-based (uses Bluetooth technology)
Overview of the Constructions 1. Each user has a secret seed 2. Use this seed to continuously broadcast (using BLE) random-looking chirps 3. Each chirp is a function of the seed and the time (and is changed periodically, synchronously with BLE MAC address rerandomization) contains all chirps sent and heard (including times) 4. Locally store the chirps heard and the corresponding times. 5. Diagnosed users upload a function of their state to a public database. 6. Regularly download the database and check for contacts. chirps sent vs chirps heard
ReBabbler ReBabbler: : Simple ? ??? = ?????? Efficient F is a pseudorandom function (PRF), ? is the seed PRF ensures that chirps are unlinkable Upload what you sent: Upload the seeds used in the last 14 days Privacy for users who are not diagnosed Privacy for diagnosed users?? Check contact with infected individuals: For every stored (? ???,????) check if In all upload what you sent schemes a curious user can find the index case ? ??? = ??(????) for any uploaded seed ?
Similar to Apple Find My CleverParrot CleverParrot: : Less efficient ? ??? = ? ????? Simple ? ???? in group ?, ? is assumed to have some property (DDH) Assuming DDH and ? is a random function (ROM) Upload what you heard: For every chirp heardin the last 14 days , upload ?(????)?,? ???? Privacy for users who are not diagnosed Check contact with infected individuals: For every uploaded ?(????)?,? ????, check if ? ??????= ? ????
Similar to Apple Find My CleverParrot CleverParrot: : Less efficient ? ??? = ? ????? Simple ? ???? in group ?, ? is assumed to have some property (DDH) Assuming DDH and ? is a random function (ROM) Upload what you heard: For every chirp heardin the last 14 days , upload ?(????)?,? ???? Privacy for users who are not diagnosed Privacy for diagnosed users?? Check contact with infected individuals: For every uploaded ?(????)?,? ????, check if ? ??????= ? ????
Similar to Apple Find My CleverParrot CleverParrot: : Less efficient ? ??? = ? ????? Simple ? ???? in group ?, ? is assumed to have some property (DDH) Assuming DDH and ? is a random function (ROM) Upload what you heard: For every chirp heardin the last 14 days , upload ?(????)?,? ???? Privacy for users who are not diagnosed Privacy for diagnosed users?? Check contact with infected individuals: For every uploaded ?(????)?,? ????, check if ? ??????= ? ???? Yes! Unless users change their seed too often
In every AEN scheme where users can change their seeds as often as they wish, it is impossible to protect diagnosed users!
Certified Certified CleverParrot CleverParrot Add a registration authority that certifies seeds (to ensure that users register only once) Only trusted not to collude with the adversary to launch a sybil attack. Privacy for users who are not diagnosed Each chirp contains 3-4 group elements (doesn t fit in a single BLE packet) Privacy for diagnosed users More complicated (relies on bilinear maps)
Security Analysis Security properties Privacy of undiagnosed users Privacy of diagnosed users Integrity UC security [Canetti2000] Holistic approach to security
