FIDO2 USB+NFC token optimized for security, extensibility, and style
Switch branches/tags
Nothing to show
Clone or download
Latest commit eb106c9 Sep 19, 2018


Solo is an affordable security key that implements FIDO2/U2F and supports USB, NFC, and extensions. Extensions include SSH, GPG, and cryptocurrency. Solo is an upgrade to U2F Zero and is a work in progress.

The Solo FIDO2/U2F code base is designed to be easily ported to different embedded systems. Right now, it has been ported to the NRF52840 and EFM32J. Soon to be supported is the SAM L11.

No hardware is needed for development. You can run and extend the FIDO2 code base using just your PC.


Solo is based on the SAM L11 secure microcontroller. It offers the following security features.

  • True random number generation to guarantee random keys.
  • Side channel resistant RAM and AES for physically secure key derivation.
  • ARM TrustZone to provide security isolation for master key.
  • Scrambled key storage to prevent invasive flash readout methods.
  • Secure boot to ensure application integrity.

The SAM L11 is one of the best chips for this application in terms of security, when considering the NDA-free market.

Solo can be trusted to be running the open source code. The firmware can be readout using a debugger to verify that a Solo is running the code posted publicly. The secret information is of course inaccessible.

How do I get one?

We are still working on open sourcing an implementation that anyone can cheaply build and program, just like with U2F Zero. This will be released soon. It will be easy to solder :)

In the meantime, you can port the code to your favorite microcontroller, or support us by signing up for our Kickstarter. Our aim is to crowdfund enough to make an economic bulk order and provide open source security tokens for everyone that is interested. We will offer "hackable" tokens that come with USB bootloaders and are reprogrammable.

Sign up here!

Setting up

Clone and Compile CBOR library and FIDO 2 client library.

git clone
cd solo/
git submodule update --init

cd tinycbor && make
cd ..

cd python-fido2/
python install

Note that our python-fido2 fork will only connect to the software FIDO2 application, not a hardware authenticator. Install Yubico's fork to do that.

Open crypto/tiny-AES-c/aes.h in a text editor and make sure AES256 is selected as follows.

//#define AES128 1
//#define AES192 1
#define AES256 1

Now compile FIDO 2.0 and U2F authenticator.


Testing and development

The application is set up to send and recv USB HID messages over UDP to ease development and reduce need for hardware.

Testing can be done using our fork of Yubico's client software, python-fido2.
Our fork of python-fido2 has small changes to make it send USB HID over UDP to the authenticator application.

Run FIDO 2 / U2F application.


Run example client software. This runs through a registration and authentication.

python python-fido2/examples/

Run the FIDO2 tests.

python tools/

Follow specifications to really dig in.


Extensions can be added to FIDO2/U2F to support things like SSH, GPG, and cryptocurrency. Right now, an experimental cryptocurrency extension can be found in fido2/extensions and web/index.html. More documentation to come.

The main goal is to expose an extensible API on Solo, like the following:

  • Command to store private key
  • Command to sign arbitrary hash
  • Command to derive a public key
  • Commands for setting/changing/authenticating a pin code (like in FIDO2)
  • Command to expose entropy from TRNG.

Using these generic commands, various external programs can be implemented for the security key. Since FIDO2/U2F are implemented, these programs can potentially work in the browser on desktops and mobile devices, with no drivers needed to be installed.


The main code base is in fido2/. See targets/nrf52840, targets/efm32/src, and pc/ for examples of FIDO2/U2F ports. In essence, you just need to reimplement device.c. Optionally you can re-implement crypto.c to accelerate operations and/or add other security features.

More documentation to come.


Contributors are welcome. The ultimate goal is to have a FIDO 2 hardware token capable of USB, Bluetooth, and NFC interfaces. There could be multiple tokens for each interface.

Look at the issues to see what is currently being worked on. Feel free to add issues as well.

This is an upgrade to U2F Zero.


Everything in this repo is open source and licensed under the MIT License.