Using Multi-factor Authentication with Qubes

(Note: This page concerns multi-factor authentication for logging into external services, not for logging into Qubes itself. For the latter, see here.)

Multi-factor authentication (MFA) today most commonly takes the form of a numerical code generated by a smartphone app or sent via SMS (text message) which the user must enter in addition to a password in order to log in to a website or other service.

One of the primary features of Qubes is that it allows us to create securely isolated VMs which can run arbitrary programs. (These VMs are securely isolated not only from each other but also, optionally, from the network.) This means that we can create a dedicated, network-isolated VM to function as a secure authenticator.

This guide will show you how to set up a VM which uses oathtool, an open-source one-time password tool, to generate authentication codes. This method presents several benefits over relying on a consumer smartphone app or SMS:

  • oathtool includes the time-based one-time password (TOTP) algorithm, which is the same algorithm used by Google Authenticator, one of the most commonly used authenticator apps. This means that we can use oathtool as a complete open-source replacement for Google Authenticator (which became propriety (closed-source) in May 2013 after version 2.21).

  • By keeping all of our authenticator data as plain text files in a dedicated VM, we have complete control over the secret keys used to generate our authentication tokens, and we can back up, copy, and transfer our authenticator data at will.

  • By creating a minimal environment in which to run oathtool from the command line, we can minimize our attack surface relative to most smartphone apps and SMS. Consumer smartphones are typically internet-facing devices which are increasingly targeted by malware. Most smartphones are bundled with proprietary software which allows service providers almost complete control over the device. Likewise, consumer SMS messages are often cleartext communications which can feasibly be intercepted and read by third parties. (In cases in which SMS messages are encrypted on the network by the service provider, the service provider itself still has full access, which means that the contents of such messages could be read by unscrupulous admins or turned over to government agencies.)

  • Using oathtool in a dedicated, network-isolated Qubes VM allows us to achieve a unique combination of security and convenience. The strong isolation Qubes provides allows us to reap the full security benefits of MFA, while virtualization frees us from having to worry about finding and handling a second physical device.

Optional Preparation Steps

  1. Start with a minimal template. In this example, we’ll use the minimal Fedora template. Get it if you haven’t already done so:

    [user@dom0 ~]$ sudo qubes-dom0-update qubes-template-fedora-24-minimal
    
  2. Since we’ll be making some modifications, you may want to clone the minimal template:

    [user@dom0 ~]$ qvm-clone fedora-24-minimal fedora-24-min-mfa
    
  3. Since this is going to be a minimal environment in which we run oathtool from the command line, we’ll install only a couple of packages:

    [user@fedora-24-min-mfa ~]$ su -
    [user@fedora-24-min-mfa ~]# yum install oathtool vim-minimal
    [user@fedora-24-min-mfa ~]$ poweroff
    
  4. Create an AppVM and set it to use the TemplateVM we just created:

    [user@dom0 ~]$ qvm-create -l black mfa
    [user@dom0 ~]$ qvm-prefs -s mfa template fedora-24-min-mfa
    
  5. Isolate the new AppVM from the network:

    [user@dom0 ~]$ qvm-prefs -s mfa netvm none
    

Using the MFA AppVM

Now that we have an AppVM set up to use oathtool securely, let’s use it with an external service. This process will vary slightly from service to service but is largely the same.

  1. Proceed with setting up multi-factor authentication as you normally would. If you are prompted to scan a QR code, instead select the option (if available) to view the secret key as text:

    Secret Key Example 0

    You should then see the secret key as text:

    Secret Key Example 1

    Note that the length and format of the secret key may vary by service:

    Secret Key Example 2

  2. In your MFA AppVM, you can now use oathtool to generate base32 TOTP authentication tokens just like Google Authenticator would. In this example, we’ll use the secret key xd2n mx5t ekg6 h6bi u74d 745k n4m7 zy3x from the second image above (substitute your own):

    [user@mfa ~]$ oathtool --base32 --totp "xd2n mx5t ekg6 h6bi u74d 745k n4m7 zy3x"
    279365
    

    The output is 279365. This is what you would enter when prompted for an authenticator code. (Note that this is a time-based one-time password, which means that your VM’s clock must be sufficiently accurate in order to generate a valid token. Qubes handles VM time syncing automatically, so you normally shouldn’t have to worry about this. As usual, the token will change after a short period of time.)

  3. To make this easier on ourselves in the future, we can create a simple shell script for each service we use. (The example service here is a Google account, using the example key from above. You’ll get a unique secret key from each service.) Create the script like so:

    [user@mfa ~]$ > google
    [user@mfa ~]$ vi google
    
    #!/bin/bash
    ##My Google Account
    ##me@gmail.com
    oathtool --base32 --totp "xd2n mx5t ekg6 h6bi u74d 745k n4m7 zy3x"
    
    [user@mfa ~]$ chmod +x google
    

    Since the secret key stored in the script never changes, we should never have to update this script, but we can easily do so if we ever want to.

  4. Now, whenever Google prompts us for an authenticator code, all we have to do is this:

    [user@mfa ~]$ ./google
    640916
    

    Done!

  5. Now you can create scripts for any other TOTP-supporting services you use, and enjoy the security and ease of quickly generating authentication tokens right from your Qubes VM command-line:

    [user@mfa ~]$ ./github
    495272
    [user@mfa ~]$ ./aws
    396732
    [user@mfa ~]$ ./facebook
    851956
    [user@mfa ~]$ ./dropbox
    294106
    [user@mfa ~]$ ./microsoft
    295592
    [user@mfa ~]$ ./slack
    501731
    [user@mfa ~]$ ./wordpress
    914625
    [user@mfa ~]$ ./tumblr
    701463
    

    For a more complete list of compatible services, see here.