Exposure Notification

The Exposure Notifications System (ENS),[2][3][4] originally known as the Privacy-Preserving Contact Tracing Project,[5][6] is a framework and specification developed by Apple Inc. and Google to facilitate digital contact tracing during the 2019-20 COVID-19 pandemic. When used by health authorities, it augments more traditional contact tracing techniques by automatically logging encounters with other ENS users using their Android or iOS smartphone. Exposure Notification is a decentralized reporting based protocol built on a combination of Bluetooth Low Energy technology and privacy-preserving cryptography. It is used as an opt-in feature within COVID-19 apps developed and published by authorized health authorities.[7][8] Originally unveiled on April 10, 2020, it was first made available on iOS on May 20, 2020 as part of the iOS 13.5 update.[9]

Exposure Notifications System
Developed by
IntroducedApril 2020 (2020-04)
IndustryDigital contact tracing
Compatible hardwareAndroid & iOS smartphones
Physical range~10 m (33 ft)[1]

The Apple/Google protocol is heavily-influenced by the Decentralized Privacy-Preserving Proximity Tracing (DP-3T) and the Temporary Contact Number (TCN) protocol by Covid Watch, but is implemented at the operating system level, which allows for more efficient operation as a background process.[10][11][12][13][14] Protocols such as TCN, DP-3T and BlueTrace are constrained in how they operate as they have no special privilege over normal apps. This leads to issues, particularly on iOS devices where digital contact tracing apps running in the background experience significantly degraded performance.[15][16][17] The joint approach is also designed to maintain interoperability between Android and iOS devices, which constitute the sheer majority of the market.[8] EPFL Professor Edouard Bugnion played an important role in getting Apple and Google to work together [10][11]

The ACLU stated the approach "appears to mitigate the worst privacy and centralization risks, but there is still room for improvement".[18] In late April, Google and Apple shifted the emphasis of the naming of the system, describing it as an "exposure notification service", rather than "contact tracing" system.[19]

Technical specification

Typically digital contact tracing protocols have two major responsibilities: encounter logging and infection reporting.[16] Exposure Notification only defines encounter logging which is an decentralized architecture, with the majority of the infection reporting, currently it is centralized, being delegated to individual app implementations.[20]

To handle encounter logging, the system uses Bluetooth Low Energy to send tracking messages to nearby devices running the protocol to discover encounters with other people. The tracking messages contain unique identifiers that are encrypted with a secret daily key held by the sending device. These identifiers change every 15-20 minutes as well as Bluetooth MAC address in order to prevent tracking of clients by malicious third parties through observing static identifiers over time.[21]:02:51:10

The sender's daily encryption keys are generated using a random number generator.[22] Devices record received messages, retaining them locally for 14 days. If a user tests positive for infection, the last 14 days of their daily encryption keys are uploaded to a central server, where it is then broadcast to all devices on the network. The method through which daily encryption keys are transmitted to the central server and broadcast is defined by individual app developers. The received keys are then provided to the protocol, where each client individually searches for matches in their local encounter history. If a match meeting certain risk parameters is found, the app notifies the user of potential infection.[23] Google and Apple intend to use the received signal strength (RSSI) of the beacon messages as a source to infer proximity.[24] RSSI and other signal metadata will also be encrypted to resist deanonymization attacks.[22]

Version 1.0

To generate encounter identifiers, first a persistent 32-byte private Tracing Key () is generated by a client. From this a 16 byte Daily Tracing Key is derived using the algorithm , where is a HKDF function using SHA-256, and is the day number for the 24-hour window the broadcast is in starting from Unix Epoch Time. These generated keys are later sent to the central reporting server should a user become infected.[25]

From the daily tracing key a 16-byte temporary Rolling Proximity Identifier is generated every 10 minutes with the algorithm , where is a HMAC function using SHA-256, and is the time interval number, representing a unique index for every 10 minute period in a 24 hour day. The Truncate function returns the first 16 bytes of the HMAC value. When two clients come within proximity of each other they exchange and locally store the current as the encounter identifier.[25]

Once a registered health authority has confirmed the infection of a user, the user's Daily Tracing Key for the past 14 days is uploaded to the central reporting server. Clients then download this report and individually recalculate every Rolling Proximity Identifier used in the report period, matching it against the user's local encounter log. If a matching entry is found, then contact has been established and the app presents a notification to the user warning them of potential infection.[25]

Version 1.1

Unlike version 1.0 of the protocol, version 1.1 does not use a persistent tracing key, rather every day a new random 16-byte Temporary Exposure Key () is generated. This is analogous to the daily tracing key from version 1.0. Here denotes the time is discretized in 10 minute intervals starting from Unix Epoch Time. From this two 128-bit keys are calculated, the Rolling Proximity Identifier Key () and the Associated Encrypted Metadata Key (). is calculated with the algorithm , and using the algorithm.[26]

From these values a temporary Rolling Proximity Identifier () is generated every time the BLE MAC address changes, roughly every 15-20 minutes. The following algorithm is used: , where is an AES cryptography function with a 128-bit key, the data is one 16-byte block, denotes the Unix Epoch Time at the moment the roll occurs, and is the corresponding 10-minute interval number. Next, additional Associated Encrypted Metadata is encrypted. What the metadata represents is not specified, likely to allow the later expansion of the protocol. The following algorithm is used: , where denotes AES encryption with a 128-bit key in CTR mode. The Rolling Proximity Identifier and the Associated Encrypted Metadata are then combined and broadcast using BLE. Clients exchange and log these payloads.[26]

Once a registered health authority has confirmed the infection of a user, the user's Temporary Exposure Keys and their respective interval numbers for the past 14 days are uploaded to the central reporting server. Clients then download this report and individually recalculate every Rolling Proximity Identifier starting from interval number , matching it against the user's local encounter log. If a matching entry is found, then contact has been established and the app presents a notification to the user warning them of potential infection.[26]

Version 1.2

Version 1.2 of the protocol is identical to version 1.1, only introducing minor terminology changes.[26]

Adoption requirements

Modeling by researchers at Oxford University has suggested that 80% of all smartphone users in a city of one million people would have to use a tracking system to be effective against the coronavirus if no other measures against the spread of the virus were taken but contact tracing.[27] Since the two vendors effectively control the entire smartphone market (with Android having an 86.6% market share as of 2020, and iOS the remaining 13.4%),[28] the joint initiative between the companies puts them in a unique position compared to other potential actors in this field.

To address this, the Exposure Notification protocol is designed to be deployed and maintained via both platforms' respective application stores and update systems.[29] The APIs enabled through such updates will then be available for authorized applications from national health authorities.[30][31]

Privacy

Preservation of privacy was referred to as a major component of the protocol; it is designed so that no personally identifiable information can be obtained about the user or their device.[32][8][33][34] Apps implementing Exposure Notification are only allowed to collect personal information from users on a voluntary basis.[35] As an additional measure, the companies stated that it would sunset the protocol by-region once they determine that it is "no longer needed".[29]

The Electronic Frontier Foundation showed concerns the protocol was vulnerable to "linkage attacks", where sufficiently capable third parties who had recorded beacon traffic may retroactively be able to turn this information into tracking information, for only areas in which they had already recorded beacons, for a limited time segment and for only users who have disclosed their COVID-19 status, once a device's set of daily encryption keys have been revealed.[36]

Release schedule

Deployment plan

According to the joint announcement by Apple and Google, the system is intended to be rolled out in three stages:[37][38]

  • API specification and publication
  • rollout of tools to enable governments to create official privacy-preserving coronavirus tracing apps
  • integration of this functionality directly into iOS and Android

Apple have stated that the system is designed to work on all recent devices that can support iOS 13.[22]

The companies planned an API for development on April 28, 2020[39] and it was released to developers the following day.[40]

Release

The iOS 13.5 update released on May 20, 2020 introduced support for the Exposure Notification API.[9] Google stated that on Android, Exposure Notification will be serviced via Google Play Services (a system API component for Google services that is present on almost all Android devices outside of mainland China, and updated independently of Android itself via the Google Play store), ensuring compatibility with Android Marshmallow and later and not requiring them to be integrated into an Android firmware (which would hinder deployment).[41]

Regulatory scrutiny

On April 16, the European Union started the process of assessing the proposed system for compatibility with privacy and data protection laws, including the General Data Protection Regulation (GDPR).[42] On April 17, 2020, the UK's Information Commissioner's Office, a supervisory authority for data protection, published an opinion analyzing both Exposure Notification and the Decentralized Privacy-Preserving Proximity Tracing protocol, stating that the systems are "aligned with the principles of data protection by design and by default" (as mandated by the GDPR).[43]

GAEN Applications and Servers

Each participating Public Health Authority (PHA) is allowed to publish a single iOS application and a single Android application. The applications are configured to communicate with a single Exposure Notification Key Server and a single Verification Server.

Designated PHAs

Each U.S. state and territory is able to participate in GAEN, with a one PHA per state/territory. Elsewhere, GAEN is available to one PHA per country.

Verification Server

The Google Apple Exposure Notification (GAEN) solution relies on users receive a positive COVID-19 diagnoses self-reporting their infection. When a public health authority (PHA) notifies a user of a positive test result, the PHA provides a "confirmation code" the user can enter into their local exposure notification application. The app communicates with the PHA's verification server[44] to retrieve a cryptographically signed certificate that unlocks the user's ability to upload their anonymous exposure keys to the exposure notification key server. This process prevents undiagnosed users from falsely reporting infections.

Each PHA is responsible for establishing and operating the verification process. The PHAs can develop their own server, or use a third party solution. Google has created an reference verification server that any PHA can leverage. The choice of verification solution does not affect whether GAEN works across state or national boundaries.

Exposure Keys Server

Exposure keys are uploaded to an Exposure Key server designated by each participating public health authority. The server aggregates all keys into a file that is downloaded daily by each participating application. The GAEN applications rely on iOS and Android to process the key files to determine if the user has been exposed to another person with a positive diagnosis.

PHAs are responsible for hosting an exposure key server. They can develop their own, though most are leveraging an [reference exposure notifications server published by Google. The Association of Public Health Laboratories announced it is partnering with Microsoft to host a National Key Server,[45] that can be shared by all states and territories. The shared key server reduces costs and complexity for states, and it assures that exposure notifications work when users are from different states.

Google has also created a federation capability that allows servers to share exposure notifications keys.

Adoption by country

As of May 21, at least 22 countries had received access to the protocol.[35] Switzerland and Austria were among the first to back the protocol.[46] On April 26, after initially backing PEPP-PT, Germany announced it would back Exposure Notification,[47] followed shortly after by Ireland [48] and Italy.[49] Despite already adopting the centralised BlueTrace protocol,[50] Australia's Department of Health and Digital Transformation Agency are investigating whether the protocol could be implemented to overcome limitations of its COVIDSafe app.[35] On May 25, Switzerland became the first country to launch an app leveraging the protocol, SwissCovid, beginning with a small pilot group.[51]

In England, the National Health Service (NHS) trialed both an in-house app on a centralized platform developed by its NHSX division, and a second app using Exposure Notification.[52] On June 18, the NHS announced that it would focus on using Exposure Notification to compliment manual contact tracing, citing tests on the Isle of Wight showing that it had better cross-device compatibility (and would also be compatible with other European approaches), but that its distance calculations were not as reliable as the centralised version of the app.[53] Later, it was stated that the app would be supplemented by QR codes at venues.[54]

Canada launched its COVID Alert app, co-developed in partnership with BlackBerry Limited and Shopify,[55] on July 31 in Ontario.[56] On August 9, Alberta announced plans to migrate to COVID Alert from its BlueTrace-based ABTraceTogether app.[57][58]

In August 2020, Google stated that at least 20 U.S. states had expressed interest in using the protocol. In Alabama, the Alabama Department of Public Health, University of Alabama at Birmingham, and the University of Alabama System deployed a GuideSafe app for university students returning to campus, which includes Exposure Notification features.[59][60] On August 5, the Virginia Department of Health released its COVIDWise app — making it the first U.S. state to release an Exposure Notification-based app for the general public.[61][62][63] On July 17, 2000 the Association of Public Health Laboratories announced it is partnering with Microsoft to host a National Key Server.[45]

Country/State Name Announced/Released Notes
 Brazil Coronavírus-SUS July 31, 2020 [64]
 Canada COVID Alert July 31, 2020 Available in Ontario.[56]
 Denmark Smittestop June 18, 2020 [65]
 Germany Corona-Warn-App June 16, 2020 [66]
 Ireland COVID Tracker Ireland July 7, 2020 [67][68]
 Italy Immuni June 1, 2020 [69]
 Japan COCOA June 19, 2020 [70]
 Latvia Apturi Covid, May 29, 2020 [71]
Northern Ireland StopCOVID NI July 30, 2020 Interoperates with COVID Tracker Ireland.[68]
 Poland ProteGO Safe June 9, 2020 Update to existing encounter logging app.[72]
 Spain Radar COVID June 30, 2020 (beta test) [73]
  Switzerland SwissCovid, May 26, 2020 [51]
United States, Alabama GuideSafe August 3, 2020 Targeting University of Alabama students as part of a larger program under the same name.[59]
United States, North Dakota Care19 Alert[74] August 13, 2020 Care19 Alert uses the U.S. National Key Server hosted by APHL
United States, Virginia COVIDWise August 5, 2020 [63]
United States, Wyoming Care19 Alert[75] August 14, 2020 Care19 Alert uses the U.S. National Key Server hosted by APHL
 Uruguay Coronavirus - UY June 15, 2020 [76]
 Finland Koronavilkku August 31, 2020 [77]

Non-adopters

Some countries, such as France, have pursued centralized approaches to digital contact tracing, in order to maintain records of personal information that can be used to assist in investigating cases.[33][78][79] The French government has asked Apple to allow apps to perform Bluetooth operations in the background, allowing the government to create its own system independent of Exposure Notification.[80] Australia launched a custom exposure notification app called COVIDSafe, which has been criticized for working only on iPhones and not reliably exchanging Bluetooth identifiers.[81]

In the United States, states such as New York, California and Massachusetts declined to use the technology, opting for manual contact tracing.[82]

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