It’s usually more than a little risky to comment on hypothetical Apple products, but while I was out at Black Hat and DEF CON Apple accidentally released the firmware for their upcoming HomePod. Filled with references to other upcoming products and technologies, the firmware release makes it reasonably probable that Apple will release an updated iPhone without a Touch ID sensor, relying instead on facial recognition.

A reasonable probability is far from an absolute certainty, but this is an interesting enough change that I think it’s worth taking a few minutes to outline how I intend to evaluate any “Face ID”, should it actually appear.

They key is to look for equivalence, rather than exactness. I don’t care whether Face ID (we’ll roll with that name for now) works exactly like Touch ID – we just need it close enough, or even better.

Is it as secure?

There are three aspects to evaluate:

  • Does it cost as much to circumvent? Touch ID isn’t perfect – there are a variety of ways to create fake fingerprints which can spoof it. The financial cost is not prohibitive for a serious attacker, but the attacks are all time-consuming enough that the vast vast majority of iPhone users don’t need to worry about them. I am sure someone will come up with ways around Face ID, but if they need to take multiple photos from multiple angles, compute a 3D model, 3D print the model, then accurately surface it with additional facial feature details, I’ll call that a win for Apple. It will make an awesome DEF CON or CCC presentation though.
  • Does it have an equivalent false positive rate? From what I see, Touch ID has a false positive rate low enough to be effectively 0 in real-world use. As long as Face ID is about the same, we’ll be good to go.
  • Does it use a similarly secure hardware/software architecture? One of the most important aspects of Touch ID is how it ties into the Secure Enclave (and, by extension, the Secure Element). These are the links that embed anti-circumvention techniques in the hardware and iOS, enabling incredibly strong security; supporting use in payment systems, banking applications, etc. I would be shocked if Apple didn’t keep this model, but expect changes to support the different kind of processing and increased multi-purpose nature of the underlying hardware (general-purpose cameras, perhaps).

Is it as easy to use?

The genius of Touch ID was that it enabled consumers to use strong password, with the same convenience as no password at all (most of the time). Face ID will need to hit the same marks to be seen as successful.

  • Is it as fast? The first version of Touch ID was pretty darn fast, taking a second or less. The second (current) version is so fast that most of the time you barely notice it. Face ID doesn’t need to be exactly as fast, but close enough that the average user won’t notice a difference. If I need to hold my iPhone steady in front of my face while a little capture box pops up with a progress bar saying “Authenticating face…”, it will be a failure. But we all know that isn’t going to happen.
  • Does it work in as many different situations (at night, walking, etc.)? Touch ID is far from perfect. I work out a ton and, awesome athlete that I am, I sweat like Moist from Dr. Horrible’s Sing-Along Blog. Touch ID isn’t a fan. Face ID doesn’t need to work in exactly the same situations, but in an equivalent number of real-world situations. For example I use Touch ID to unlock my phone sitting on a table to pass off to one of the kids, or while lying sideways in bed with my face mushed into a pillow. Face ID will probably require me to pick the phone up and look at it. In exchange, I’ll probably be able to use it with wet hands in the kitchen. Tradeoffs are fine – so long as they are net neutral, positive, or insignificant.
  • Does it offer an equivalent set of features? My wife and I actually trust each other and share access to all our devices. With Touch ID we enroll each other’s fingerprints. Touch ID also (supposedly) improves over time. Ideally Face ID will work similarly.

Is it as reliable?

The key phrase here is false negative rate. Even second-generation Touch ID can be fiddly at times, as in my workout example above. With Face ID we’ll look more at things like changing facial hair, lighting conditions, moving/walking, etc. These tie into ease of use, but in those cases it’s more about number of situations where it works. This question comes down to Is Face ID as reliable within its supported scenarios? This is one area where I could see some big improvements over Touch ID.

Conclusion

Plenty of articles will focus on all the differences if Face ID becomes a reality. Plenty of people will complain it doesn’t work exactly the same. Plenty of security researchers will find ways to circumvent it. But what really matters is whether it hits the same goal:

Allow a user to use a strong password with the convenience of no password at all… most of the time.

Face ID doesn’t need to be the same as Touch ID – it just needs to work reasonably equivalently in real-world use. I won’t bet on Face ID being real, but I will bet that if Apple ships it, they will make sure it’s just as good as Touch ID.

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