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ArchWiki / Pam usb

ja:Pam usb pam_usb (active fork of the original repo) provides hardware authentication for Linux using ordinary USB Flash Drives.

It works with any application supporting PAM, such as su and Display manager.

Installation

Install the pam_usb package.

Configuration

Setting up pam_usb requires the following, once pam_usb is installed:

  1. Set up devices and users

  2. Configuring PAM for system authentication

Setting up Devices and Users

Once you have connected your USB device to the computer, use pamusb-conf to add it to the configuration file: 

Please select the device you wish to add.
* Using "SanDisk Corp. Cruzer Titanium (SNDKXXXXXXXXXXXXXXXX)" (only option)
Which volume would you like to use for storing data ?
* Using "/dev/sda1 (UUID: <6F6B-42FC>)" (only option)
Name            : MyDevice
Vendor          : SanDisk Corp.
Model           : Cruzer Titanium
Serial          : SNDKXXXXXXXXXXXXXXXX
Volume UUID     : 6F6B-42FC (/dev/sda1)
Save to /etc/pamusb.conf ?
[Y/n] y
Done.
# pamusb-conf --add-device MyDevice

Note that MyDevice can be any arbitrary name you would like. Also, you can add as many devices as you want.

Next, configure users you want to be able to authenticate with pam_usb: 

Which device would you like to use for authentication ?
* Using "MyDevice" (only option)
User            : root
Device          : MyDevice
Save to /etc/pamusb.conf ?
[Y/n] y
Done.
# pamusb-conf --add-user root

Check the configuration

You can run pamusb-check anytime to check if everything is correctly worked. This tool will simulate an authentication request (requires your device to be connected, otherwise it will fail). 

* Authentication request for user "root" (pamusb-check)
* Device "MyDevice" is connected (good).
* Performing one time pad verification...
* Verification match, updating one time pads...
* Access granted.
# pamusb-check root

Setting up the PAM module

To add pam_usb into the system authentication process, we need to edit /etc/pam.d/system-auth

The default PAM configuration file should include the following line: 

auth    [success=2 default=ignore]        pam_unix.so try_first_pass nullok_secure

Change it to: 

auth    sufficient                    pam_usb.so
auth    [success=2 default=ignore]    pam_unix.so nullok_secure

The sufficient keyword means that if pam_usb allows the authentication, then no password will be asked. If the authentication fails, then the default password-based authentication will be used as fallback.

If you change it to required, it means that both the USB flash drive and the password will be required to grant access to the system.

Now you should be able to authenticate with the relevant USB device plugged-in. 

* pam_usb v.SVN
* Authentication request for user "root" (su)
* Device "MyDevice" is connected (good).
* Performing one time pad verification...
* Verification match, updating one time pads...
* Access granted.
$ su

Enabling events management

pam_usb provides support for lock and unlock events that can trigger a user-defined list of commands along with custom environment variables. For instance, it can be used to instruct pam_usb to automatically lock the current session upon removal of a configured USB device, via the lock event.

There are two pre-requisites for the configured events to be triggered upon a configured USB device insertion/removal:

  1. The lock and/or unlock events configuration must be added to the pam_usb configuration file;

  2. The pamusb-agent must be running.

Both topics above are fully descibed in the pam_usb's wiki.

Here is an example of events configuration for a given user. Note that it also takes care of passing the DISPLAY and dbus environment details to the commands to be executed upon events. Both <env> and <cmd> elements content should be changed according to the target environment:

...
<user id="auser">
    <device>MyUSBKey</device>
    
    <agent event="lock">
      <cmd>xfce4-screensaver-command --lock</cmd>
      <env>DISPLAY=:0.0</env>
      <env>DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus</env>
    </agent>
    <agent event="unlock">
      <cmd>xfce4-screensaver-command --deactivate</cmd>
      <env>DISPLAY=:0.0</env>
      <env>DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus</env>
    </agent>
</user>
...
/etc/security/pam_usb.conf abstract

Note that although the pam_usb package installs pamusb-agent, it does not configure the system to either manage or start it automatically, this has to be taken care of manually post-installation as a service or simply as a session program, e.g. via Xfce Session and Startup 's Application Autostart control panel.

Troubleshooting

su fails to use pam_usb

If you set: 

auth          sufficient   pam_usb.so
/etc/pam.d/system-auth

and su prompts for a password, and does not use pam_usb, add the same line at the beginning of /etc/pam.d/su. This may be required for other pam-aware applications as well.

pamusb-check only works with elevated privileges (sudo)

To confirm that this is the same problem, run pamusb-check --debug <non-root-user>. Check that the output contains the line Cannot open device file: Permission denied.

By default the device pad is only accessible to privileged users/processes. To use the device to elevate your privileges, the device pad needs to be accessible to you.

To solve this:

  1. Create a new group that will have access to the file sudo groupadd pamusb.

Add all users, who should be able to use the device, to the group with sudo usermod -aG pamusb <username> e.g.

  1. Your own user.

  2. Any system users which will authenticate you, like for instance sddm

  3. etc.

    Mount the device and navigate to it (The next steps assume the working directory is the root of the device).

Change the ownership of the .pamusb directory and the device pad file to the pamusb group.

  • sudo chown -R <user>:pamusb .pamusb

  • Any user added to the pamusb group should work.

Add permissions so that any one in the group can read, write and execute everything in the .pamusb directory.

  • sudo chmod -R u+rwx,g+rwx .pamusb

pamusb-check --debug <non-root-user> should now work without elevated privileges.

See also