8.0 KiB
esp-ena
Implementation of contact tracing with the Covid-19 Exposure Notification API by Apple and Google on an ESP32 (with ESP-IDF). More information about the Covid-19 Exposure Notification at Apple and Google. This is fully compatible with the official API and is meant for people without smartphone or without access to Apples/Googles implementation.
The main source (the Exposure Notification API) is a separate branch in component.
This implementation fully covers the BLE part including the cryptography specifications needed and the exposure check.
The following acronyms will be used in code and comments:
- ENA Exposure Notification Api
- ENIN ENIntervalNumber - timestamp with 10 minutes resolution
- TEK Temporary Exposure Key - personal secret key changed every 24h, published when infected
- RPI Rolling Proximity Identifier - send and received identifer changed every 10 minutes
- AEM Associated Encrypted Metadata - send and received metadata
Features implemented
- send/receive BLE beacons as defined in Bluetooth® Specification (Apple/Google) and Cryptography Specification (Apple/Google)
- BLE privacy (change random MAC address in random interval)
- permanent storage on flash of last keys, beacons and exposures (storage is limited, see storage math for details)
- parsing of Exposure Key export binaries as defined in Exposure Key export file format and verification (big thanks to nanopb for making this easier than I thought!)
- calculating exposure risks/scores (after adding reported keys and storing exposure information) as defined in ENExposureConfiguration (Apple)
Additional features for full ENA device
- RTC support with DS3231 (for correct system time)
- display support with SSD1306
- interface with a 7 button control (joystick up,down,left,right,enter,cancel,ok) to
- show exposure status
- set time
- connect to wifi
- delete data
- set language
- enter tan for infection status (dummy for now)
Features in development
- automatically receive Exposure Key export from web (started with Corona Warn App)
- send infected status (will test Corona Warn App)
- battery support
- 3d print case
Limitations/Problems/Questions
- WiFi or other external connection needed for infections status (auto-connect to open WiFis?)
- obtaining accessibility
- all parameters (scanning time, thresholds etc.)
- memory (RAM) is really low with BLE and WiFi enabled, unzipping a Exposure Key export not possible for now, maybe disable BLE service for download.
- service UUID is send reversed, RPI and AEM also send in reverse? Don't know BLE specification enough
some storage math
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 |
|---|---|---|
| 50000 | 3571 | 24 |
| 70000 | 5000 | 34 |
| 76629 | 5473 | 38 |
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.
How to use
Hardware Required
For base functionality just an ESP32 is required. DS3231 RTC, SSD1306 Display and 7 buttons are required for a complete device.
Configure the project
idf.py menuconfig
required
- enable bluetooth (BLE)
- add partition-table for storage (currently hardcoded name "ena")
- mbedTLS enable HKDF
recommended
- BLE Scan Duplicate (By Device Address and Advertising Data)
debug options
- Log output set to Debug
- Exposure Notification API / Storage enable Dump storage
Build and Flash
May flash partition table:
idf.py partition_table-flash
Build the project and flash it to the board, then run monitor tool to view serial output:
idf.py -p PORT flash monitor
(Replace PORT with the name of the serial port to use.)
(To exit the serial monitor, type Ctrl-].)
Troubleshooting
Sometimes I get errors from BT-stack of ESP-IDF printed. Didn't affect functionality for now, but I also could not find out what it caused and what it means.
E (909164) BT_HCI: btu_hcif_hdl_command_complete opcode 0x2005 status 0xc
Structure
The project is divided in different components. The main.c just wrap up all components. The Exposure Notification API is in ena module
ena
The ena module contains the main functions of eps-ena with bluetooth scanning and adverting, storing data, handle beacons and check exposure.
- ena-beacons handles scanned data by storing temporary beacons, check for threshold and store beacons permanently
- ena-crypto covers cryptography part (key creation, encryption etc.)
- ena-storage storage part to store own TEKs and beacons
- ena-bluetooth-scan BLE scans for detecting other beacons
- ena-bluetooth-advertise BLE advertising to send own beacons
- ena-exposure compare exposed keys with stored beacons, calculate score and risk
- ena run all together and timing for scanning and advertising
ena-binary-export
Module to decode Exposure Key export.
ena-cwa
Connection to german Exposure App (Corona Warn App) for download Exposure Key export (and maybe later report infection).
interface
Adds interface functionality for control and setup.
display
General module for display and gfx.
rtc
General module for set/get time from RTC.
i2c-main
Just start I2C driver for display and RTC.
interface-input-buttons
Interface with 7 button input
rtc-ds3231
I2C driver for a DS3231 RTC, implementation of rtc module
display-ssd1306
I2C driver for a SSD1306 display, implementation of display module
nanopb
Nanopb for reading Protocol Buffers of Exposure Key export. Including already generated Headers from *.proto files.