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Merge pull request #104 from marcopashkov/patch-1

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better phrasing
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solution_architecture.md
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@ -37,7 +37,7 @@ With the Corona-Warn-App (see [scoping document](https://github.com/corona-warn-
Identifiers are ID numbers sent out by the phones. To ensure privacy and prevent 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 substitutes daily at midnight (UTC) through means of cryptography, which will be explained in more detail in *Figure 10*. When the TEKs are linked to a positive test, they remain technically the same, but are called Diagnosis Keys.
The collected identifiers of other users, as well as the own keys (which can later be used to derive the identifiers) are stored locally on the phone within the secure storage of the framework provided by Apple and Google. This secure storage cannot be accessed directly by the application, but only through the interfaces provided by the Exposure Notification Framework. Some of those interfaces are subjected to [rate limiting](https://developer.apple.com/documentation/exposurenotification/enmanager/3586331-detectexposures), in order to prevent misuse. In case users were 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 of up to 14 days. Once the keys have been uploaded to the Corona-Warn-App backend server, all keys of positively tested people are aggregated. This list of keys is then made available to all mobile phones with the app installed. Additionally, configuration parameters for the framework are available for download, so that adjustments to the calulation of the risk score can be taken (see the section regarding *Risk Scores*).
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 phone matches to those of an infected person. Additionally, meta data that has been broadcasted together with the identifiers (e.g. sending power), can now be decrypted and used. Based on the collected data a risk score for each individual exposure as well as for the overall situation is calculated for every single user within the Exposure Notification framework by Apple and Google. Exposures are defined as an aggregation of all encounters with another person on a single calendar day (UTC timezone). For privacy reasons it is not possible to track encounters with other individuals across multiple days.
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 phone matches to those of an infected person. Additionally, meta data that has been broadcasted together with the identifiers (e.g. transmit power), can now be decrypted and used. Based on the collected data a risk score for each individual exposure as well as for the overall situation is calculated for every single user within the Exposure Notification framework by Apple and Google. Exposures are defined as an aggregation of all encounters with another person 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 notice at this point, that the people that have been exposed to a positively tested person 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 phone and is not shared.