Universal 2nd Factor

Universal 2nd Factor (U2F) is an open standard that strengthens and simplifies two-factor authentication (2FA) using specialized USB or NFC devices based on similar security technology found in smart cards.

While initially developed by Google and Yubico, with contribution from NXP Semiconductors, the standard is now hosted by the FIDO Alliance.

For all articles on U2F and U2F-devices see: Category:Universal 2nd Factor.

Authentication for websites

U2F is supported by major sites like Google, Facebook, Twitter, or GitHub. Check out twofactorauth.org or dongleauth.com to find other websites and links to setup documentation. For all browsers which support it, likely the only action required is to install libfido2. Yubico offers a demo page for testing.

Firefox

Firefox/Tweaks#Fido U2F authentication

Chromium/Chrome

Chromium/Tips and tricks#U2F authentication

Authentication for Arch Linux

Yubico, the company creating the YubiKey, develops an U2F PAM module. It can be used to act as a second factor during login or replace the need for a password entirely.

Note: Despite the mentions of Yubico/YubiKey in this document, pam-u2f should also support any u2f and/or fido2 compliant keys.

Installing the PAM module

The module is part of the package pam-u2f.

Adding a key

Note: The folder name below is the default location. It can be changed for single user implementations by moving the file to your preferred directory and appending authfile=/path/to/u2f_keys to the end of the line for pam_u2f.so. This is also useful if you wish to move u2f_keys to a protected part of the file-system. For multi-user implementations use a central mapping file as explained in the official documentation of pam-u2f.

Keys need to be added with the tool pamu2fcfg:

$ mkdir ~/.config/Yubico
$ pamu2fcfg -o pam://hostname -i pam://hostname > ~/.config/Yubico/u2f_keys

After entering your PIN, click the button of your U2F key to confirm the key.

Note: If the hostname of your system changes, e.g. because of DHCP in different networks, you would be unable to login. In order to prevent that, it is recommended to specify the above mentioned options and replace hostname with the actual hostname.

If you own multiple keys, append them with

$ pamu2fcfg -o pam://hostname -i pam://hostname -n >> ~/.config/Yubico/u2f_keys

Note: The -n in the above command is required. It will omit the username portion of the generated line as required in the spec for subsequent entries for the same user. Multiple lines with the same username will cause unpredictable behaviour in PAM.

Passwordless sudo

Warning: Before making any changes to your configuration, start a separate shell with root permissions (e.g. sudo -s). This way you can revert any changes if something goes wrong.

Open /etc/pam.d/sudo and add

auth            sufficient      pam_u2f.so cue origin=pam://hostname appid=pam://hostname

as the first line. Be sure to replace the hostname as mentioned above. Then create a new terminal and type sudo ls. Your key's LED should flash and after clicking it the command is executed. The option cue is set to provide indication of what to do, i.e. Please touch the device.

In order to make the token the only method of sudo (ie. no password fallback) you will need to comment out the other auth methods present. This is usually just the default system-auth include.

#auth           include         system-auth

You should also change sufficient to required in the above pam_u2f.so line.

Warning: This will mean that no user on the system will be able to sudo without a U2F token configured.

GDM login

Open /etc/pam.d/gdm-password and add

auth            required      pam_u2f.so nouserok origin=pam://hostname appid=pam://hostname

after the existing auth lines. Please note the use of the nouserok option which allows the rule to fail if the user did not configure a key. This way setups with multiple users where only some of them use a U2F key are supported.

Note: This method will not work with encrypted home partitions because the decryption is not done before the login process completed, so the u2f_keys file is unavailable. In this case use a central mapping file as explained in the official documentation of pam-u2f.

Other authentication methods

Enable the PAM module for other services like explained above. For example, to secure the screensaver of Cinnamon, edit /etc/pam.d/cinnamon-screensaver.

Troubleshooting

If you managed to lock yourself out of the system, boot into recovery mode or from a USB pen drive. Then revert the changes in the PAM configuration and reboot.

OpenSSH

OpenSSH ≥8.2 supports FIDO/U2F hardware tokens natively, see SSH keys#FIDO/U2F.

Data-at-rest encryption with LUKS

Since version 248, systemd can be use to unlock a LUKS partition using a FIDO2 key.

First, you will need to setup your /etc/crypttab file, or customize your initramfs if you wish to unlock your root partition. The full procedure is similar to the use of a TPM chip for unlocking. See Trusted Platform Module#systemd-cryptenroll.

To register the key, you will need to use the systemd-cryptenroll utility. First, run the following command to list your detected keys:

$ systemd-cryptenroll --fido2-device=list

Then you can register the key in a LUKS slot, specifying the path to the FIDO2 device, or using the auto value :

$ systemd-cryptenroll --fido2-device=/path/to/fido2_device /dev/sdX