pam_usb

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pam_usb 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_usbAUR 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: 

# pamusb-conf --add-device MyDevice
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.

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: 

# pamusb-conf --add-user root
      
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.

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). 

# pamusb-check root
* 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.

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. 

$ su
* 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.

Troubleshooting

su fails to use pam_usb

If you set: 

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

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.

See also