OpenSK : Open-Source Implementation For Security Keys

OpenSK is an open-source implementation for security keys written in Rust that supports both FIDO U2F and FIDO2 standard.

This repository contains a Rust implementation of a FIDO2 authenticator.

We developed this as a Tock OS application and it has been successfully tested on the following boards:



Although we tested and implemented our firmware based on the published CTAP2.0 specifications, our implementation was not reviewed nor officially tested and doesn’t claim to be FIDO Certified.


We’re currently still in the process on making the ARMĀ® CryptoCell-310 embedded in the Nordic nRF52840 chip work to get hardware-accelerated cryptography. In the meantime we implemented the required cryptography algorithms (ECDSA, ECC secp256r1, HMAC-SHA256 and AES256) in Rust as a placeholder.

Those implementations are research-quality code and haven’t been reviewed. They don’t provide constant-time guarantees and are not designed to be resistant against side-channel attacks.


For a more detailed guide, please refer to our installation guide.

  • If you just cloned this repository, run the following script (Note: you only need to do this once):


  • If you have a Nordic development board that doesn’t already have Tock OS installed, you can install both TockOS and the OpenSK application by running the following command, depending on the board you have:

# Nordic nRF52840-DK board
board=nrf52840dk ./ os app
# Nordic nRF52840-Dongle
board=nrf52840_dongle ./ os app

  • If Tock OS is already installed and you want to install/update the OpenSK application on your board (Warning: it will erase the locally stored credentials), run:

./ app

  • On Linux, you may want to avoid the need for root privileges to interact with the key. For that purpose we provide a udev rule file that can be installed with the following command:

sudo cp rules.d/55-opensk.rules /etc/udev/rules.d/ &&
sudo udevadm control –reload


If you build your own security key, depending on the hardware you use, there are a few things you can personalize:

  • If you have multiple buttons, choose the buttons responsible for user presence in
  • Decide whether you want to use batch attestation. There is a boolean flag in ctap/ It is mandatory for U2F, and you can create your own self-signed certificate. The flag is used for FIDO2 and has some privacy implications. Please check WebAuthn for more information.
  • Decide whether you want to use signature counters. Currently, only global signature counters are implemented, as they are the default option for U2F. The flag in ctap/ only turns them off for FIDO2. The most privacy preserving solution is individual or no signature counters. Again, please check WebAuthn for documentation.
  • Depending on your available flash storage, choose an appropriate maximum number of supported residential keys and number of pages in ctap/

3D Printed Enclosure

To protect and carry your key, we partnered with a professional designer and we are providing a custom enclosure that can be printed on both professional 3D printers and hobbyist models.

All the required files can be downloaded from Thingiverse including the STEP file, allowing you to easily make the modifications you need to further customize it.


This project is proof-of-concept and a research platform. It’s still under development and as such comes with a few limitations: