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Introduction

Universal 2-Factor Authentication is an open authentication standard developed primarily by Yubico and Google. The primary intention of U2F is to enable the use of simple hardware devices to aid with 2-factor authentication on popular applications such as GitHub and Google Apps.

However, U2F is not limited to just these applications. It can also be used to enable 2-factor authentication within a custom application or even for use on a local operating system. In this guide, I will show you how to use a U2F YubiKey (many thanks to GitHub and Yubico for partnering together on this) with a Linux system to enable U2F for login and unlocking purposes.

Getting Started

I’m using Arch Linux in combination with GNOME 3.18. While this guide is likely to work with other systems (namely Fedora), be aware that some changes may be required.

Additionally, it is assumed you have a U2F YubiKey (only a select few support U2F, be sure to check this before purchasing or just use the GitHub offer linked above). Other devices implementing U2F may work, however, I have not tested this procedure with anything other than my own YubiKey.

Software

For Arch, there is only a single package that needs to be installed. However, this package is in the AUR, not the main repositories. It would be helpful to use an AUR helper, although this is not required. In this example, I will be using pacaur:

pacaur -S pam_u2f

When it prompts you to view the PKGBUILD, I encourage you to say yes. I admit that while this should be the practiced followed for every AUR package, I typically don’t do this. But as we are installing a module that directly interacts with the authentication sub-system on your machine, you should inspect the PKGBUILD to ensure nothing suspicious is occurring.

The above package will pull in a few dependencies such as libu2f-host and libu2f-server. These libraries implement the U2F protocol and are required for the PAM module to communicate with the hardware device.

At this point, you should plug in your device to verify that the kernel can talk to it. Verify this by executing something like:

dmesg | grep -i yubico

If you don’t see something like the following:

hid-generic 0003:1050:0120.0003: hiddev0,hidraw0: USB HID v1.10 Device [Yubico Security Key by Yubico] on usb-0000:00:14.0-2/input0

then try rebooting. It’s possible that the udev rules have not kicked in after package installation and rebooting will ensure they are loaded.

Configuration

Once the packages have been successfully installed, there are two steps that must be taken.

Generating the U2F Config

A useful utility is included as part of the pam_u2f package we installed earlier: pamu2fcfg. This utility is capable of producing a configuration file that links your username and your particular U2F device that is read by the PAM module.

Use it as follows:

pamu2fcfg > ~/.config/Yubico/u2f_keys

pam_u2f searches for this file in the authenticating user’s home directory by default (although this can be changed with module parameters).

Touch the button on the device when it is blinking to complete the configuration process.

Configuring PAM

PAM (short for Pluggable Authentication Modules) is a mechanism for configuring system authentication in Linux and other Unix/Unix-like operating systems.

/etc/pam.d is the directory where the various configurations live on a Linux system. In particular, we’re interested in the file /etc/pam.d/gdm-password. This is the file GDM (GNOME Display Manager) uses to determine authentication techniques.

Open the file for editing with sudo and add the following to the top of the file:

auth sufficient pam_u2f.so

There is something important to note with the second parameter of that configuration, sufficient. This keyword lets PAM know what is required for authentication to succeed. In this case, sufficient refers to the fact that authenticating with U2F is sufficient for authentication to succeed, but it is not required.

If you opt for using the sufficient level, you are not employing true 2-factor authentication. Instead, you must indicate required:

auth required pam_u2f.so

In this mode, the device will be required for authentication in addition to whatever else is listed in the configuration (typically a password, fingerprint, or smartcard, for example).

Read pam(8) and pam.conf(5) for more information on the specifics of PAM control values.

Finishing Up

At this point, you should have everything you need for U2F authentication with GDM. Reboot and cross your fingers that you didn’t muck up your PAM configuration and lock yourself out of your machine (although if that happens, there are a few tricks to recover from that).

Thanks for reading!

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Craig Cabrey


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