Desktop Login and Linux User Authentication

(Nitrokey FIDO2 - Linux)

Introduction

This guide will walk you through the configuration of Linux to use FIDO Universal 2nd Factor, i.e. FIDO U2F with libpam-u2f and Nitrokey FIDO2.

If you want to login to you computer using Nitrokey Pro 2, Nitrokey Storage 2 and Nitrokey Start you can visit the instructions for Windows available here, and for Linux here.

Requirements

  • Ubuntu 20.04 with Gnome Display Manager.
  • Nitrokey FIDO2 configured following these instructions.

Instructions

### Preparation

1. Set up <backup_user>

This step is not necessary for the setup, however it is recommended as fall-back if you are testing user-specific instructions.

GUI method

  1. In the lower left corner click on Show Applications and type settings in the search bar as following:

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  1. Scroll down in the right bar to Users

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  1. In the left corner click on Unlock and that would prompt for your password
  2. Select Administrator and enter the user name and password of your choice

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  1. Once you finish Step 4 you should be done

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CLI method

Create a backup user, and give it root privileges with these commands:

$ sudo adduser <backup_user>
$ sudo usermod -aG sudo <backup_user>

In case you prefer to setup U2F for a single user, and are locked out of your user session, you would still be able to login with the <backup_user>, and proceed with the maintenance.

Warning

The following guide can potentially lock you out of your computer. You should be aware of these risks, as it is recommended to first use the instructions below on a secondary computer, or after a full backup.

You might lose access to your data after configuring PAM modules.

2. Set up the rules to recognize the Nitrokey FIDO2

Under /etc/udev/rules.d download 41-nitrokey.rules

$ cd /etc/udev/rules.d/
$ sudo wget https://raw.githubusercontent.com/Nitrokey/libnitrokey/master/data/41-nitrokey.rules

And restart udev service

$ sudo systemctl restart udev

3. Install libpam-u2f

On Ubuntu 20.04 it is possible to download directly ibpam-u2f from the official repos

$ sudo apt install libpam-u2f

Note

Click for more options

  • Alternatively you can build libpam-u2f from Git.
  • To verify that the library is properly installed enter the following command:
$ file /lib/x86_64-linux-gnu/security/pam_u2f.so

The Output should be something like the following:

/lib/x86_64-linux-gnu/security/pam_u2f.so: \ ELF 64-bit LSB shared object, x86-64, version 1 (SYSV),\ dynamically linked, BuildID[sha1]=1d55e1b11a97be2038c6a139579f6c0d91caedb1, stripped

4. Prepare the Directory

Create .config/Nitrokey/ under your home directory

$ mkdir ~/.config/Nitrokey

And plug your Nitrokey FIDO2.

### Configuration

Once done with the preparation, we can start to configure the computer to use the Nitrokey FIDO2 for 2nd factor authentication at login and sudo. #### 5. Generate the U2F config file To generate the configuration file we will use the pamu2fcfg utility that comes with the libpam-u2f. For convenience, we will directly write the output of the utility to the u2f_keys file under .config/Nitrokey. First plug your Nitrokey FIDO2 (if you did not already), and enter the following command:

$ pamu2fcfg > ~/.config/Nitrokey/u2f_keys

Once you run the command above, you will need to touch the key while it flashes. Once done, pamu2fcfg will append its output the u2f_keys in the following format:

<username>:Zx...mw,04...0a

Note, the output will be much longer, but sensitive parts have been removed here. For better security, and once the config file generated, we will move the .config/Nitrokey directory under the etc/ directory with this command:

$ sudo mv ~/.config/Nitrokey /etc

Tip

  • The file under .config/Nitrokey must be named u2f_keys
  • It is recommended to first test the instructions with a single user. For this purpose the previous command takes the -u option, to specify a user, like in the example below:
$ pamu2fcfg -u <username> > ~/.config/Nitrokey/u2f_keys
  • For individual user configuration you should point to the home directory in the next step, or not include the authfile option in the PAM configuration.

6. Backup

This step is optional, however it is advised to have a backup Nitrokey in the case of loss, theft or destruction of your Nitrokey FIDO.

To set up a backup key, repeat the procedure above, and use pamu2fcfg -n. This will omit the <username> field, and the output can be appended to the line with your <username> like this:

<username>:Zx...mw,04...0a:xB...fw,04...3f

7. Modify the Pluggable Authentication Module PAM

The final step is configure the PAM module files under /etc/pam.d/. In this guide we will modify the common-auth file as it handles the authentication settings which are common to all services, but other options are possible. You can modify the file with the following command:

$ cd /etc/pam.d
$ sudo $editor common-auth

And add the following lines:

#Nitrokey FIDO2 config
auth    sufficient pam_u2f.so authfile=/etc/Nitrokey/u2f_keys cue prompt

Tip

  • Since we are using Central Authentication Mapping, we need to tell pam_u2f the location of the file to use with the authfile option.
  • If you often forget to insert the key, prompt option make pam_u2f print Insert your U2F device, then press ENTER. and give you a chance to insert the Nitrokey.
  • If you would like to be prompted to touch the Nitrokey, cue option will make pam_u2f print Please touch the device. message.

Once we modified the common-auth, we can save and exit the file.

You can test the configuration by typing sudo ls in the terminal. You should be prompted the message Please touch the device. and have a similar output on the terminal:

bash nitrouser@nitrouser:~$ sudo ls $ sudo ls [sudo] password for <username>:  Please touch the device. You can also test your configuration by logging out of the user session and logging back. A similar screen should be displayed once you you unplug/replug yout Nitrokey FIDO2 and type your password:

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Usage

After the PAM module modification, you will be able to test your configuration right away, but it is recommended to reboot your computer, and unplug/replug the Nitrokey FIDO2.

Once you have properly tested the instructions in this guide (and set up a backup), it is recommended to use either the required or the requisite control flag instead of sufficient.

The flags required and requisite provide a tighter access control, and will make the Nitrokey FIDO2 necessary to login, and/or use the configured service.

If you need more information about Control Flags in the PAM configuration line, you may see the last section of this guide to understand the difference, and the implications of using each of them.

#### PAM modules

There are several PAM modules files that can be modified according to your needs:

  • By modifying /etc/pam.d/common-auth file, you will be able to use you Nitrokey FIDO for 2nd factor authentication for graphic login and sudo. Note: common-auth should be modified by adding the additional configuration line at the end of the file.
  • If you wish to use FIDO U2F authentication solely for Gnome’s graphic login, you might prefer to modify the/etc/pam.d/gdm-password
  • Alternatively you can just modify the /etc/pam.d/sudo file if you wish to use FIDO U2F when using the sudo command. #### Control flags In step 7 we have used the sufficient control flag to determine the behavior of the PAM module when the Nitrokey is plugged or not. However it is possible to change this behavior by using the following control flags:
  • required: This is the most critical flag. The module result must be successful for authentication to continue. This flag can lock you out of your computer if you do not have access to the Nitrokey.
  • requisite: Similar to required however, in the case where a specific module returns a failure, control is directly returned to the application, or to the superior PAM stack. This flag can also lock you out of your computer if you do not have access to the Nitrokey.
  • sufficient: The module result is ignored if it fails. The sufficient flag considered to be safe for testing purposes.
  • optional: The success or failure of this module is only important if it is the only module in the stack associated with this service+type. The optional flag is considered to be safe to use for testing purposes.

Warning

  • If required or requisite is set, the failure of U2F authentication will cause a failure of the overall authentication. Failure will occur when the configured Nitrokey FIDO is not plugged, lost or destroyed.
  • You will lose access to your computer if you mis-configured the PAM module and used the required or requisite flags.
  • You will also lose the ability to use sudo if you set up Central Authentication Mapping and used the required or requisite flags.