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@ -29,21 +29,24 @@ We assume a close association of a mobile phone and its user and, thus, equate t
## INTRODUCTION
To reduce the spread of [COVID-19](https://www.ecdc.europa.eu/en/covid-19-pandemic), it is necessary to inform people about their close proximity to positively tested individuals. So far, health departments and affected individuals identified these in personal dialogs based on the individual's memory. This led to a high number of unknown connections, e.g. when using public transport.
To reduce the spread of [COVID-19](https://www.ecdc.europa.eu/en/covid-19-pandemic), it is necessary to inform people about their close proximity to positively tested individuals. So far, health departments and affected individuals identified these in personal dialogs based on the individuals' memory. This led to a high number of unknown connections, e.g. when using public transport.
![Figure 1: High-level architecture overview](images/solution_architecture/figure_1.svg "Figure 1: High-level architecture overview")
The Corona-Warn-App (see [scoping document](https://github.com/corona-warn-app/cwa-documentation/blob/master/scoping_document.md )), shown centrally in *Figure 1*, enables individuals to trace their personal exposure risk via their mobile phones. The Corona-Warn-App uses a new framework provided by Apple and Google called [Exposure Notification Framework](https://www.apple.com/covid19/contacttracing). The framework employs [Bluetooth Low Energy (BLE)](https://en.wikipedia.org/wiki/Bluetooth_Low_Energy) mechanics. BLE lets the individual mobile phones act as beacons meaning that they constantly broadcast a temporary identifier called Rolling Proximity Identifier (RPI) that is remembered and, at the same time, lets the mobile phone scan for identifiers of other mobile phones. This is shown on the right side of *Figure 1*.
Identifiers are ID numbers sent out by the mobile phones. To ensure privacy and to prevent the tracking of movement patterns of the app user, those broadcasted identifiers are only temporary and change constantly. New identifiers are derived from a Temporary Exposure Key (TEK) that is substituted at midnight (UTC) every day through means of cryptography. For a more detailed explanation, see *Figure 10*. Once a TEK is linked to a positive test result, it remains technically the same, but is then called a Diagnosis Key.
The collected identifiers from other users as well as the own keys which can later be used to derive the identifiers are stored locally on the phone in the secure storage of the framework provided by Apple and Google. The application cannot access this secure storage directly, but only through the interfaces the Exposure Notification Framework provides. To prevent misuse, some of these interfaces are subjected to [rate limiting](https://developer.apple.com/documentation/exposurenotification/enmanager/3586331-detectexposures). If app users are tested positively for SARS-CoV-2, they can update their status in the app by providing a verification of their test and select an option to send their recent keys from up to 14 days back. On the Corona-Warn-App backend server, all keys of positively tested individuals are aggregated and are then made available to all mobile phones that have the app installed. Additionally, the configuration parameters for the framework are available for download, so that adjustments to the risk score calculation can be made, see the *Risk Scores* section.
Once the keys and the exposure detection configuration have been downloaded, the data is handed over to the Exposure Notification framework, which analyzes whether one of the identifiers collected by the mobile phone matches to those of a positively tested individual. Additionally, the metadata that has been broadcasted together with the identifiers such as the transmit power can now be decrypted and used. Based on the collected data, the Exposure Notification Framework provided by Apple and Google calculates a risk score for each individual exposure as well as for the overall situation. Exposures are defined as an aggregation of all encounters with another individual on a single calendar day (UTC timezone). For privacy reasons, it is not possible to track encounters with other individuals across multiple days.
The collected identifiers from other users as well as the own keys, which can later be used to derive the identifiers, are stored locally on the phone in the secure storage of the framework provided by Apple and Google. The application cannot access this secure storage directly, but only through the interfaces the Exposure Notification Framework provides. To prevent misuse, some of these interfaces are subjected to [rate limiting](https://developer.apple.com/documentation/exposurenotification/enmanager/3586331-detectexposures). If app users are tested positively for SARS-CoV-2, they can update their status in the app by providing a verification of their test and select an option to send their recent keys from up to 14 days back. On the Corona-Warn-App back-end server, all keys of positively tested individuals are aggregated and are then made available to all mobile phones that have the app installed. Additionally, the configuration parameters for the framework are available for download, so that adjustments to the risk score calculation can be made, see the *Risk Scores* section.
Once the keys and the exposure detection configuration have been downloaded, the data is handed over to the Exposure Notification Framework, which analyzes whether one of the identifiers collected by the mobile phone matches those of a positively tested individual. Additionally, the metadata that has been broadcasted together with the identifiers such as the transmit power can now be decrypted and used. Based on the collected data, the Exposure Notification Framework provided by Apple and Google calculates a risk score for each individual exposure as well as for the overall situation. Exposures are defined as an aggregation of all encounters with another individual on a single calendar day (UTC timezone). For privacy reasons, it is not possible to track encounters with other individuals across multiple days.
It is important to note that the persons that have been exposed to a positively tested indivdual are **not informed by a central instance**, but the risk of an exposure is calculated locally on their phones. The information about the exposure remains on the user’s mobile phone and is not shared.
The Corona-Warn-App pursues two objectives: First, it supports individuals in finding out whether they have been exposed to a person that has later been tested positively, and, second, in receiving the result of a SARS-CoV-2 test on their mobile phone through an online system. This helps to reduce the period until necessary precautions, e.g. a contact reduction and testing, can be taken.
The Corona-Warn-App pursues two objectives:
1. It supports individuals in finding out whether they have been exposed to a person that has later been tested positively.
2. It receives the result of a SARS-CoV-2 test on a user's mobile phone through an online system. This helps reduce the time until necessary precautions, e.g. a contact reduction and testing, can be taken.
In order to prevent misuse, individuals need to provide proof that they have been tested positively before they can upload their keys. Through this integrated approach, the verification needed for the upload of the diagnosis keys does not require any further action from the users.
They only have to confirm in the app and for the Exposure Notification Framework that they agree to share their diagnosis keys. Manual verification is also possible, if the lab that performed the testing does not support the direct electronic transmission of test results to the users' mobile phone or if users have decided against the electronic transmission of their test results.
They only have to confirm in the app and for the Exposure Notification Framework that they agree to share their diagnosis keys. Manual verification is also possible if the lab that performed the testing does not support the direct electronic transmission of test results to the users' mobile phones or if users have decided against the electronic transmission of their test results.
### Retrieval of Lab Results and Verification Process
@ -56,17 +59,17 @@ There are two ways for receiving this verification:
*Figure 2* and *Figure 3* illustrate the verification process. *Figure 2* shows the interaction flow of the user and *Figure 3* the flow and storage of data. Additions to the preexisting 'conventional' process through the introduction of the Corona-Warn-App and the integrated test result retrieval are marked blue in *Figure 2*.
This preexisting process for the processing of lab results includes that the doctor requesting the test also receives the results, so patients can be informed in a timely manner. As required by law ([§9 IfSG](https://www.gesetze-im-internet.de/ifsg/__9.html)), the responsible health authority (“Gesundheitsamt”) is notified by the lab about the test results as well. The notifications in case of a positive test includes, amongst others, the name, address, and date of birth of the positively tested individuals, so that they can be contacted and can be informed about the implications of their positive test. This is also represented in step 3 of *Figure 2*.
This preexisting process for the processing of lab results includes that the doctor requesting the test also receives the results, so patients can be informed in a timely manner. As required by law ([§9 IfSG](https://www.gesetze-im-internet.de/ifsg/__9.html)), the responsible health authority (“Gesundheitsamt”) is notified by the lab about the test results as well. The notifications in case of a positive test includes, amongst others, the name, address, and date of birth of the positively tested individuals, so that they can be contacted and informed about the implications of their positive test. This is also represented in step 3 of *Figure 2*.
The flow for using the app is as follows, referencing the steps from *Figure 2*:
- **Step 1:** Users of the Corona-Warn-App (i.e. broadcasting and collecting RPIs)
- (1) When a test is conducted, they receive an information flyer with a custom QR code. The code is either created on-site or is already available as a stack of pre-printed QR codes. The QR code contains a globally unique identifier (GUID).
- (2) Optionally, they can scan the QR code with the Corona-Warn-App (*Figure 3*, step 1). If users decide against using the test retrieval functionality of the Corona-Warn-App, they still receive their test results through the regular channels explained before.
- (3) When the code is scanned, a web service call (REST) is placed against the Verification Server (*Figure 3*, step 2), linking the mobile phone with the data from the QR code through a registration token which is generated on the server (*Figure 3*, step 3) and stored on the mobile phone (*Figure 3*, step 4).
- (3) When the code is scanned, a web service call (REST) is placed against the Verification Server (*Figure 3*, step 2), linking the mobile phone with the data from the QR code through a registration token, which is generated on the server (*Figure 3*, step 3) and stored on the mobile phone (*Figure 3*, step 4).
- **Step 2:** The samples are transported to the lab together with a “Probenbegleitschein” which has a machine-readable QR code on it as well as other barcodes (lab ID, sample IDs).
- **Step 3:** As soon as the test result is available meaning the samples have been processed, the software running locally in the lab (lab client) transmits the test result to the Laboratory Information System together with the GUID from the QR code. The Laboratory Information System hashes the GUID and posts it together with the test result to the Test Result Server through a REST interface (*Figure 3*, step A), which in turn makes it available to the verification server.
- **Step 4a:** After signing up for notifications in step 1, the user’s mobile phone regularly checks the Verification Server for available test results (polling, figure 3, steps 5-8). This way, no external push servers need to be used. If results are available, the user is informed that information is available and when the result is only displayed after opening the app including recommendations for further actions (see the scoping document for more details).
- **Step 4a:** After signing up for notifications in step 1, the user’s mobile phone regularly checks the Verification Server for available test results (polling, figure 3, steps 5-8). This way, no external push servers need to be used. If results are available, the user is informed that information is available. The result themselves as well as recommendations for further actions are only displayed after the user has opened the app (see the scoping document for more details).
- If the test returns a positive result, users are asked to upload their keys to allow others to find out that they were exposed. If the users agree, the app retrieves a short-lived token (TAN) from the Verification Server (see also *Figure 3*, steps 9-13). As the Verification Server does not persist the test result, it is fetched from the Test Result Server once more (*Figure 3*, steps 10-11). The TAN is uploaded together with the diagnosis keys of the last 14 days to the Corona-Warn-App Server (*Figure 3*, step 14).
- The Corona-Warn-App Server uses the TAN to verify the authenticity (*Figure 3*, steps 15-17) of the submission with the Verification Server.
- The TAN is consumed by the Verification Server and becomes invalid (*Figure 3*, step 16).
@ -94,11 +97,11 @@ According to the current version of the documentation from Apple and Google (1.3
![Figure 5: Upload schedule for Temporary Exposure Keys (Diagnosis Keys)](images/solution_architecture/figure_5.svg "Figure 5: Upload schedule for Temporary Exposure Keys (Diagnosis Keys)")
As users are not required to confirm negative test results, the functionality of uploading Diagnosis Keys on subsequent days remains theoretical. Each of those uploads could take place earliest at the end of each subsequent day (see (3) in *Figure 5*). It would require explicit consent of the user for each day and could take place until the person is not considered contagious anymore and also no longer, as it would lead to false positives.
As users are not required to confirm negative test results, the functionality of uploading Diagnosis Keys on subsequent days remains theoretical. Each of those uploads could take place earliest at the end of each subsequent day (see (3) in *Figure 5*). It would require explicit consent of the user for each day and could take place up to the time when the person is not considered contagious anymore (but not any longer, as this would lead to false positives).
As of now, two uploads are required from the same mobile phone (past diagnosis keys and from the current day). This means, the registration token may not be invalidated after the first upload, but must remain active. The TANs sent to the Corona-Warn-App Server are only valid for a single use. In case of the teleTAN, an additional registration token is created which then allows the app to retrieve TANs for individual uploads.
## Backend
## Back End
![Figure 6: Actors in the system, including external parties (blue components to be open-sourced)](images/solution_architecture/figure_6.svg "Figure 6: Actors in the system, including external parties (blue components to be open-sourced)")