Nitrokey 3C NFC: WebAuthn and OpenPGP

Some time has passed since my last post about the Nitrokey.

I do own the newer model Nitrokey 3C NFC and want to share my experience with it, especially in regards of OpenPGP (which I use heavily) and WebAuthn, which I'd like to use more in the future.

1. OpenPGP and nitropy

nitropy is a utility to control your Nitrokey.

1.1. Install required packages

Check their official documentation please, but here are some commands which I have used.

sudo pacman -S python-pynitrokey
sudo wget --directory-prefix=/etc/udev/rules.d/ https://raw.githubusercontent.com/Nitrokey/nitrokey-udev-rules/main/41-nitrokey.rules

Reboot afterwards.

1.2. General commands

nitropy nk3 list
nitropy nk3 status
nitropy nk3 get-config opcard.use_se050_backend
nitropy fido2 list-credentials

1.3. Firmware update

nitropy nk3 update
# Optional:
nitropy nk3 factory-reset

Be careful of course, a factory reset will delete everything on the device.

The current firmware at the time of writing is 1.7.2.

1.4. Enable the secure element

By default, the secure element is not used to store your OpenPGP private key. Instead, it is stored (unencrypted?) in the flash memory of the controller.

Make sure you use the secure element by doing this:

nitropy nk3 set-config opcard.use_se050_backend true

See their FAQ.

1.5. Generate test secret key and move it to the Nitrokey

gpg --quick-generate-key "My Name (MyComment) <my@email.com>" rsa4096 cert,sign,auth,encrypt 30d
gpg --list-secret-keys --keyid-format long --with-keygrip --with-subkey-fingerprints

gpg --card-status
gpg --edit-key my@email.com keytocard

1.6. Testing a signature

echo test | gpg --clear-sign --local-user my@email.com

You should be prompted to insert the GPG card, if the Nitrokey is not already connected. Then, you will be prompted to enter the user PIN (default: 123456). The signature will be printed to the stdout on success.

You won't be able to sign messages without the Nitrokey now.

1.7. Android support via OpenKeychain

OpenKeychain is a great app to handle OpenPGP credentials on Android.

Their list of supported hardware tokens does not include my Nitrokey model, and there is an open issue for it.

What used to work and does not work any longer is NFC support, see here:

NK3A worked with OpenKeychain over NFC on alpha versions 1.3.0 when I tested it in March. I decided to try the same on 1.4.0, and it doesn't connect durin encryption, it says that I removed the token too early, although this is not the case. At the same time, a red light is on on the token.

The NFC chip gives not enough power for the additional storage and secure element use.

This is really, really sad and it seems like a closed-source Yubikey is your only option if you want to have OpenPGP via NFC on your Android phone. :(

I can at least use my Nitrokey via NFC on Android for WebAuthn, which brings us to the next topic.

2. WebAuthn and Passkeys

I don't fully understand passkeys yet. What I do know: WebAuthn Keys stay on a device and are never shared. I do like that approach.

Passkeys however are synced in the cloud to grant multi-device access. This made me think. ChatGPT told me the keys are indeed sent over the internet, but are end-to-end encrypted. See the full chat. I don't even understand why you would encrypt a key with another key. Inception.

I am oldschool in this regard: No matter how secure that process might be, I don't like the idea to share a private key over the internet.

2.1. WebAuthn: Resident vs. non-resident (a.k.a Discoverable credentials)

In both cases, the private key stays on the Nitrokey.

Resident keys can be used for username-less authentication. User details like an ID, username or similar is stored together with a relying party ID and the whole private key on the Nitrokey and therefore require storage. Storage is limited so are the number of resident keys you can store on a device.

Nitrokey does not publish how many keys can be stored.

For non-resident credentials, you can store an unlimited amount. They are not persisted, but I guess somehow derived from a single private key on the device. Not sure about that, though.

2.2. My preference

I prefer non-resident credentials. They are never shared, you can have unlimited of them on your device.

The only thing you need to be careful about: You should have a second device or another way to access a service in case the device gets lost or stolen.

Update: Here a Nitrokey employee states that:

Per default, FIDO2 credentials are non-discoverable. This means that the secret is stored on the server, not on the device (but encrypted with a key that is stored on the device). Only discoverable credentials (also called resident keys) are stored on the device and shown by list-credentials. These are typically used for password-less login, e. g. by Google or Microsoft.

And links a great post about WebAuthn details and Nitrokey:

FIDO2, WebAuthn, Passkeys in 2022 and 2023

2.3. Test WebAuthn

This website is great for testing WebAuthn: https://webauthn.io/.

A more detailed website with JSON debug output is https://webauthn.me/debugger.

2.4. WebAuthn and Android

I successfully tested the Nitrokey with NFC and the Firefox browser on the website above. I just had to change the option User Verification to Discouraged, otherwise I would see an error message.

For the more privacy-focused browsers like Mull or Fennec it did not work however, and for Mull they state WebAuthn is not supported, because of Google Play Services and proprietary blobs.

2.5. List Nitrokey FIDO2 / WebAuthn resident credentials

While the commands in nitropy are in the nitropy fido2 category, they also work with a Nitrokey 3C NFC.

In order to use FIDO2, you need to set an initial PIN of your choice:

nitropy fido2 set-pin

Note: After setting the PIN, you also need to provide it on you devices.

Next, you can list all credentials:

nitropy fido2 list-credentials

If no PIN has been set here, you will see the critial error message: Please set a pin in order to manage credentials

If the command worked, you will see all you credentials and also a guess how much storage is left: There is an estimated amount of 10 credential slots left

Another way to manage the key is to use the fido2-token cli command:

sudo pacman -S libfido2

# List devices
fido2-token -L

# Print information about a device
fido2-token -I /dev/hidraw2

# Print resident credential metadata
fido2-token -I -c /dev/hidraw2

3. TOTP

TOTP and passwords have a different PIN on the Nitrokey. Set it via:

nitropy nk3 secrets set-pin

Go to https://totp.danhersam.com/ to generate a test TOTP. Add it via:

nitropy nk3 secrets add-otp --kind TOTP [--hash SHA1 --touch-button --protect-with-pin] testtotp JBSWY3DPEHPK3PXP

SHA1 still seems to be the default for TOTP, even though SHA256 is supported by the Nitrokey as well.

You need to provide the secret directly on the command line. This is bad practice and there is already an issue for it: https://github.com/Nitrokey/pynitrokey/issues/401

Now, you can retrieve a code anytime by using this command:

nitropy nk3 secrets get-otp testtotp

I couldn't find a good documentation on this, so I just thought I write the commands down here in my blog.