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Opt for TOTP to Deal With MFA App Sprawl

| Sean Verity

Do you have a bunch of MFA apps on your phone that leave you feeling like you can’t put your arms down?

Or maybe all those MFA apps on your phone feel like a litter of English bulldog puppies. They follow you around, demand your attention, and give you security snuggles.

Either way, that’s a lot of figurative winter gear to take care of, a lot of mouths to feed. With all those apps to keep track of, consider the following:

  • What will you lose access to if you break or lose your phone?
  • Do all your MFA apps let you do backup / export?
  • For the apps that you know do backups, did you do a backup within the past month or so?
  • What’s the process look like for restoring those MFA apps on a new phone?
    • Have you tested this process to make sure there aren’t any surprises?
  • How about migrating to a different MFA app?
    • Assuming you can export your data, how hard is it going to be to convert that data into a format that your next MFA app can parse?

I’ll be the first to admit that I’ve been socially engineered by service providers to install their MFA apps on more than one occasion. Think about one of those times where you registered MFA with a new service provider. The procedure usually starts with the service provider saying something like, “Your organization is requiring you to do MFA. Start by installing our MFA app.”

What if I already have three MFA apps on my phone? Are none of them good enough? If you’re lucky, the service provider will at least suggest a couple alternative MFA apps that could work. Even after registration though, there’s a pretty good chance that you’ll be gently nudged to install the service provider’s app every time you log in, with phrases like, “Can’t access our MFA app right now?” Talk about presumptuous.

While each MFA app has its own “special way of doing MFA that’s way more secure than every other vendor,” there is a standards-based approach to MFA you’re probably familiar with that just about every MFA app supports. It’s the MFA flavor where you have 30 seconds to enter a 6-digit code and it goes by a couple of names:

  • Google Authenticator
  • Time-based One Time Password (TOTP)

I used to think that you MUST install the Google Authenticator app when I saw “Google Authenticator” listed as a supported MFA app. However, I’ve now come to interpret “Google Authenticator” as “an app that supports TOTP.” From the Google Authenticator Github Wiki:

The nice thing about service providers that support TOTP, is that there are a LOT of apps that support TOTP because it is standardized1 and pretty simple. Here’s an overview of how TOTP authentication usually2 works:

  • The user and the service provider have a shared secret.
  • The user’s TOTP app counts the number of time steps since the Unix epoch. The number of time steps is combined with the shared secret to calculate a 6-digit code with HMAC-SHA-1.
  • The user presents the 6-digit code to the service provider during authentication. Upon receipt of the 6-digit code, the service provider does the same calculation using its copy of the shared secret.
  • If the codes match, the authentication is successful.

Easy peasy, hey? As a matter of fact, TOTP is so simple that you could implement it in 20 lines of python or almost entirely in bash if you’re not into the whole graphical user interface thing.

So, this means that you have more flexibility with what MFA apps to use and how many to install when you opt for TOTP. For example:

  • You could choose to use a commercially supported, closed-source app or an open-source app that you can audit.
  • You could do offline local backups of MFA keys or use a solution that offers an online backup feature that you trust.
  • You can find a TOTP app that lets you register multiple devices.
  • If you’re into retro computing, you could turn that old Commodore 64 into a stand-alone TOTP generator.

How to Tell if a Service Provider Supports TOTP

When it comes to identifying if a service provider supports TOTP, you probably will not get a clear declaration. Here are a few hints that you’re more likely to see during MFA registration:

  • The service provider will state something along the lines of “Enter a code from your MFA app.” This is an acknowledgement that you may already have an MFA app. Since TOTP is the most common flavor of MFA, there’s a good chance that TOTP is supported.
  • Look for an option that says “Google Authenticator.” Oftentimes, a Google Authenticator logo will be next to it.
  • If neither of those other hints are showing up, then TOTP might be buried as a fallback option. That is, you might need to reject the initial offerings from the service provider. This is sometimes indicated by “Try another method.”

How to Shop for TOTP Apps

Obviously, the primary consideration is whether the app supports TOTP. As with MFA registration, app descriptions in the App Store or Google Play generally don’t come out and say, “This app supports TOTP.” Most MFA apps do support TOTP, but to verify, search for something like “does XYZ MFA app support TOTP.” This should yield a technical support page, blog, or (best of all) source code.

Here’s a few more questions to consider which might take a little legwork to figure out as you start to narrow down your choices:

  • MFA Code Backups
    • Is cloud backup OK or do you need local backups?
  • Number of Device Registrations
    • Can you get away with having only one device registered with a service provider? If you want to have a backup device at the ready, keep an eye out for features that let you transfer or export MFA codes to a second device.
  • Do you have a “de-Googled” Android phone and want to keep it that way?
    • Google services are typically necessary to install or launch an MFA app when the app supports push notifications, even if a service provider doesn’t use push notifications for MFA…
  • Independent Security Audits

Will there be cases where your chosen MFA app doesn’t work for a specific scenario?

But, hopefully you’ve seen how opting for TOTP provides you some flexibility in how you do MFA. At the very least, you won’t have as many MFA apps to keep track of if you opt for TOTP authentication.

Footnotes

[1] https://datatracker.ietf.org/doc/html/rfc6238

[2] SHA-1 is the suggested default and by far the most common implementation. The RFC mentions SHA-256 or SHA-512 variants though. TOTP time steps don’t have to be 30 seconds either. You could go crazy and generate TOTPs every 5 seconds if you wanted to.

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