Using YubiKey to Qubes authentication

You can use YubiKey to enhance Qubes user authentication, for example to mitigate risk of snooping the password. This can also slightly improve security when you have USB keyboard.

There (at least) two possible configurations: using OTP mode and using challenge-response mode.

OTP mode

This can be configured using app-linux-yubikey package. This package does not support sharing the same key slot with other applications (it will deny further authentications if you try).

Contrary to instruction there, currently there is no binary package in the Qubes repository and you need to compile it yourself. This might change in the future.

Challenge-response mode

In this mode, your YubiKey will generate a response based on the secret key, and random challenge (instead of counter). This means that it isn’t possible to generate a response in advance even if someone gets access to your YubiKey. This makes it reasonably safe to use the same YubiKey for other services (also in challenge-response mode).

Same as in the OTP case, you will need to set up your YubiKey, choose a separate password (other than your login password!) and apply the configuration.

To use this mode you need to:

  1. Configure your YubiKey for challenge-response HMAC-SHA1 mode, for example following this tutorial
  2. Install the ykpers package in template on which your USB VM is based.
  3. Create /usr/local/bin/yubikey-auth script in dom0:

    #!/bin/sh
    
    key="$1"
    
    if [ -z "$key" ]; then
        echo "Usage: $0 <AESKEY> [<PASSWORD-HASH>]"
        exit 1
    fi
    
    # if password has given, verify it
    if [ -n "$2" ]; then
        # PAM appends \0 at the end
        hash=`head -c -1 | openssl dgst -sha1 -r | cut -f1 -d ' '`
        if [ "x$2" != "x$hash" ]; then
            exit 1
        fi
    fi
    
    challenge=`head -c64 /dev/urandom | xxd -c 64 -ps`
    # You may need to adjust slot number and USB VM name here
    response=`qvm-run -u root --nogui -p sys-usb "ykchalresp -2 -x $challenge"`
    
    correct_response=`echo $challenge | xxd -r -ps | openssl dgst -sha1 -macopt hexkey:$key -mac HMAC -r | cut -f1 -d ' '`
    
    test "x$correct_response" = "x$response"
    exit $?
    
  4. Adjust USB VM name (sys-usb above), and possibly YubiKey slot number (2 above), then make the script executable.
  5. Edit /etc/pam.d/xscreensaver (or appropriate file if you are using other screen locker program). Add this line at the beginning:

    auth [success=done default=ignore] pam_exec.so expose_authtok quiet /usr/local/bin/yubikey-auth AESKEY PASSWORD-HASH
    

    Replace AESKEY with hex-encoded key configured in the first step, then replace PASSWORD-HASH with SHA1 hash for your YubiKey-linked password (other than your standard Qubes password). You can calculate it using this command:

    echo -n "PASSWORD" | openssl dgst -sha1
    

Usage

When you want to unlock your screen…

1) Plug YubiKey into USB slot. 2) Enter password associated with YubiKey. 3) Press Enter. 4) If you configured so, YubiKey will request confirmation by pressing button on it (it will blink).

When everything is ok, your screen will be unlocked.

In any case you can still use your login password, but do it in a secure location where no one can snoop your password.

Locking the screen when YubiKey is removed

You can setup your system to automatically lock the screen when you unplug your YubiKey. This will require creating a simple qrexec service which will expose the ability to lock the screen to your USB VM, and then adding a udev hook to actually call that service.

In dom0:

  1. First configure the qrexec service. Create /etc/qubes-rpc/custom.LockScreen with a simple command to lock the screen. In the case of xscreensaver (used in Xfce) it would be:

     DISPLAY=:0 xscreensaver-command -lock
    
  2. Allow your USB VM to call that service. Assuming that it’s named sys-usb it would require creating /etc/qubes-rpc/policy/custom.LockScreen with:

     sys-usb dom0 allow
    

In your USB VM:

  1. Create udev hook. Store it in /rw/config to have it persist across VM restarts. For example name the file /rw/config/yubikey.rules. Add the following line:

     ACTION=="remove", SUBSYSTEM=="usb", ENV{ID_SECURITY_TOKEN}=="1", RUN+="/usr/bin/qrexec-client-vm dom0 custom.LockScreen"
    
  2. Ensure that the udev hook is placed in the right place after VM restart. Append to /rw/config/rc.local:

     ln -s /rw/config/yubikey.rules /etc/udev/rules.d/
     udevadm control --reload
    
  3. Then make /rw/config/rc.local executable.

     sudo chmod +x /rw/config/rc.local
    
  4. For changes to take effect, you need to call this script manually for the first time.

     sudo /rw/config/rc.local
    

If you use KDE, the command(s) in first step would be different:

    # In the case of USB VM being autostarted, it will not have direct access to D-Bus
    # session bus, so find its address manually:
    kde_pid=`pidof kdeinit4`
    export `cat /proc/$kde_pid/environ|grep -ao 'DBUS_SESSION_BUS_ADDRESS=[[:graph:]]*'`
    qdbus org.freedesktop.ScreenSaver /ScreenSaver Lock