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:
oathtoolincludes 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
oathtoolas 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
oathtoolfrom 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.)
oathtoolin 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
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
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
Since this is going to be a minimal environment in which we run
oathtoolfrom 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
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
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.
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:
You should then see the secret key as text:
Note that the length and format of the secret key may vary by service:
In your MFA AppVM, you can now use
oathtoolto 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 zy3xfrom 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.)
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 ##email@example.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.
Now, whenever Google prompts us for an authenticator code, all we have to do is this:
[user@mfa ~]$ ./google 640916
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.