Implementation of contact tracing with the Covid-19 Exposure Notification API by Apple and Google on an ESP32 (with [ESP-IDF](https://docs.espressif.com/projects/esp-idf/en/latest/esp32/index.html)).
More information about the Covid-19 Exposure Notification at [Apple](https://www.apple.com/covid19/contacttracing/) and [Google](https://www.google.com/covid19/exposurenotifications/). This is fully compatible with the official API and is meant for people without smartphone or without access to Apple/Google's implementation.
The main source (the Exposure Notification API) is also available in a separate branch in [**component**](https://github.com/Lurkars/esp-ena/tree/component).
* send/receive BLE beacons as defined in [Bluetooth® Specification (Apple/Google)](https://blog.google/documents/70/Exposure_Notification_-_Bluetooth_Specification_v1.2.2.pdf) and [Cryptography Specification (Apple/Google)](https://blog.google/documents/69/Exposure_Notification_-_Cryptography_Specification_v1.2.1.pdf)
* BLE privacy (change random MAC address in random interval)
* calculating exposure risks/scores (after adding reported keys and storing exposure information) as defined in [ENExposureConfiguration (Apple v1)](https://developer.apple.com/documentation/exposurenotification/enexposureconfiguration/calculating_the_exposure_risk_value_in_exposurenotification_version_1)
* receive Exposure Key export from an ENA Exposure Key export proxy server [ena-eke-proxy module](#ena-eke-proxy), see [ena-eke-proxy server reference implemenation](https://github.com/Lurkars/ena-eke-proxy)
Due to limited storage, I made some calculations. I have fixed counting of TEKs (14 for two weeks), temporary beacons (1000, longest period for temp. storage is 20 minutes, so recognizing about 1000 different beacons in 20 minutes is possible) and exposure information (choose 500, this is like a limit of infected keys to be met). So the biggest limitation is to store beacons permanently after threshold of 5 minutes. That's what those calculations are for to check, if storage is enough for practical use.
overview of storage in bytes without permanent beacons:
| | size (B) | num | overall (B) |
| :-----------: | -------: | ---: | ----------: |
| TEK | 21 | 14 | 294 |
| Exposure Info | 20 | 500 | 10000 |
| temp. Beacon | 32 | 1000 | 32000 |
Additional 4 bytes counting for every type gives overall 42310B used without perm. beacons.
For now, a partition size of 2494464B will leave 2452154B free for met beacons which leads to a total storage of 76629
beacons. This gives the following table, where I added some lower boundaries to calculate with.
| total beacons | aver. per day | aver. for 10 minute window |
So on average it is possible to meet 38 (24 on a lower boundary) different devices inside of 10 minutes. I have no practical experience/numbers how many beacons are stored on average for a 14-days period in currently running ENA-Apps. But I think regarding the average is calculated for 24h (which is quite unpractical because of sleep and hours without meeting many people), the storage should be enough for the purpose of contact tracing.
For base functionality just an ESP32 is required, for full device M5StickC (PLUS) or for a custom device DS3231 RTC, SSD1306 Display and 7 buttons are required.
For *ena-eke-proxy* connection over SSL a valid certificate for used server under *components/ena-eke-proxy/certs/cert.pem* is required.
For my own proxy server, I have added a self signed cert for cwa-proxy.champonthis.de. For using, copy or rename *components/ena-eke-proxy/certs/cwa-proxy.champonthis.de.pem* to *components/ena-eke-proxy/certs/cert.pem*.
> copy valid cert to *components/ena-eke-proxy/certs/cert.pem*
This module is for connecting to an Exposure Key export proxy server. The server must provide daily (and could hourly) fetch of daily keys in binary blob batches with the following format
Connection to German Exposure App ([Corona Warn App](https://github.com/corona-warn-app)) for download Exposure Key export (and maybe later report infection). \[Deprecated through ena-eke-proxy module\]
[Nanopb](https://github.com/nanopb/nanopb) for reading Protocol Buffers of Exposure Key export. Including already generated Headers from *.proto files. \[Deprecated through ena-eke-proxy module\]
