Using U2F keys in Debian

U2F is an open standard for hardware two-factor / multi-factor authentication (2FA). U2F tokens are available in a variety of form factors, including various versions of USB, Bluetooth, and NFC. Any U2F token should work under Debian, with no drivers or low level configuration required. Much of the online discussion refers to some of the more popular, better-known brands, such as Yubico, Thetis, and Titan (Google), but other models, such as Hypersecu's HyperFIDO, work under Debian as well.

Many hardware tokens offer a variety of security protocols in addition to U2F (e.g., OATH-TOTP, OATH-HOTP, PIV, OpenPGP; see here for a list of tokens and their support for OTP); these are often more expensive than U2F-only tokens. This article focuses exclusively on U2F.

PAM Setup

PAM integration is accomplished via Yubico's pam-u2f module.

Check for the USB dongle:

lsusb | grep U2F

Install the required libraries:

sudo apt-get install pamu2fcfg libpam-u2f

Create authorization maps (full documentation):

mkdir -p ~/.config/Yubico
pamu2fcfg >> ~/.config/Yubico/u2f_keys

PAM configuration

WARNING: it is possible to lock yourself out of your system while changing PAM configuration.

PAM can be configured via either /etc/pam.conf or (more commonly) individual appropriately named files under /etc/pam.d/. Using the latter method, add lines like the following to files such as /etc/pam.d/xscreensaver and /etc/pam.d/lightdm

To allow password-less login using just the U2F key, use lines like:

auth sufficient pam_u2f.so

To use the U2F key as a required second factor (2FA), use lines like:

auth required pam_u2f.so

Note that integration with the existing default PAM configuration files supplied by Debian is tricky, and the order of the lines in the PAM files is apparently important. When using sufficient, it seems that the line should be added before @include common-auth, to avoid being asked first for a password, whereas when using required, the line may be added afterward. See the documentation for more information, including some useful module options such as cue and debug

Note also that PAM configuration changes do not take effect for already authenticated users; to test, either log out and log back in, or attempt to authenticate as a different user.

Additional resources:

SSH

OpenSSH supports FIDO/U2F keys from version 8.2:

ssh-keygen -t ecdsa-sk -f ~/.ssh/id_ecdsa_sk

As per the documentation:

See the documentation and man ssh-keygen for more details.

Website 2FA

Many websites can use U2F tokens for 2FA; see here for a list. No client-side configuration is required; simply follow the site's directions to register the key with the site, and subsequently follow the site's directions to authenticate with the key.

See Also