STRIX

Satellite True Random Information eXtractor

A portable wireless True Random Number Generator built on an Arduino MKR Vidor 4000.

Design

Entropy comes from FPGA ring oscillators whose sampled timing jitter is XOR-combined into output bits, with no pseudo-random post-processing. A SAMD21 serves the results over USB, over HTTP on its own WiFi access point (stationary mode, on a charger), or over Bluetooth LE (mobile mode, on battery).

• 4 independent generators × 90 ring oscillators. Each generator XORs its rings every 120 MHz clock (spatial) and then several consecutive bits (temporal, default 5): about 24 Mbps per generator.
• Separate stream per generator, plus a combined stream of all healthy generators
• On-device health tests per generator: NIST SP 800-90B repetition count and adaptive proportion, plus a dead-ring monitor
• Live statistics per trial: bias, z-score, random walk and lag-1 autocorrelation, with rolling and cumulative windows
• Configurable random-walk / majority-vote bias amplifier on a bias path and an autocorrelation path, with sub-trial voting
• Von Neumann extractor that can be toggled on the stream, statistics and amplifier paths
• Per-second trials in Global Consciousness Project format (200-bit XOR-masked sums per generator, plus full-rate sums), with Stouffer Z and network variance
• Raw ring captures and frequency measurement for characterization

The USB, HTTP and BLE API is documented in arduino/strix/API.md.

Layout

• fpga/rtl/: Verilog sources; fpga/sim/: testbenches; fpga/src/: Quartus project
• arduino/strix/: device firmware
• arduino/strix_capture/: characterization tool firmware, used by tools/jitter.py
• arduino/strix_nina_passthrough/: connects USB to the WiFi module's UART for flashing its firmware
• arduino/libraries/: StrixFPGA (FPGA boot, SPI access, embedded FPGA image), VidorBoot (vendored subset of Arduino's VidorPeripherals), and Arduino's WiFiNINA, Arduino_SpiNINA and ArduinoBLE, patched to drive the WiFi module through the FPGA's pin pass-through (see arduino/libraries/STRIX-NINA-LIBRARIES.md)
• tools/: FPGA image converter, jitter and statistics experiments, API check

Development (Linux)

The toolchain is managed by devbox: arduino-cli, Intel Quartus Prime Lite (Cyclone 10 LP only, through a Nix flake in nix/), Icarus Verilog, Rust, and uv for the Python tools (pyproject.toml, uv.lock).

1. devbox run setup: install the Arduino SAMD core (libraries are vendored in arduino/libraries/)
2. devbox run sim: run the FPGA testbenches
3. devbox run fpga: build the FPGA bitstream (about 10 minutes)
4. devbox run image: embed the bitstream in the StrixFPGA library
5. devbox run build, then devbox run upload: build and flash the firmware (PORT defaults to /dev/ttyACM0)
6. devbox run api-check: end-to-end check of the USB API

Serial access requires membership in the dialout group. If a firmware leaves the board unable to stay on USB, enter the bootloader: hold reset while plugging in, release, then tap reset once quickly. For characterization, flash the capture tool with devbox run upload-capture and use devbox run jitter <freq|rings|sweep|output|health|stats|amp|pairs>. devbox run onod-build builds the onod3000 test suite used by jitter output and api-check.

WiFi module firmware

BLE needs NINA firmware 3.0.0 or newer (the board shipped with 1.0.0). To flash Arduino's nina-fw NINA.bin:

1. Flash the passthrough: devbox run -- arduino-cli compile -u -p /dev/ttyACM0 --fqbn arduino:samd:mkrvidor4000 --libraries arduino/libraries arduino/strix_nina_passthrough
2. devbox run -- uv run esptool --chip esp32 --port /dev/ttyACM0 --baud 115200 write-flash 0 NINA.bin
3. Flash the STRIX firmware again (devbox run upload); wifi probe reports the module firmware version.

The passthrough forwards reliably at 115200 baud, not at 460800.

TODO

• WiFi credentials stored on the device instead of compile-time defaults
• Battery pack
• 3D case

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Development Notes