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A closer look at the capabilities and risks of iPhone X face mapping

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On Friday Apple fans were queuing to get their hands on the newly released iPhone X: The flagship smartphone that Apple deemed a big enough update to skip a numeral. RIP iPhone 9.

The shiny new hardware includes a front-facing sensor module housed in the now infamous ‘notch’ which takes an unsightly but necessary bite out of the top of an otherwise (near) edge-to-edge display and thereby enables the smartphone to sense and map depth — including facial features.

So the iPhone X knows it’s your face looking at it and can act accordingly, e.g. by displaying the full content of notifications on the lock screen vs just a generic notice if someone else is looking. So hello contextual computing. And also hey there additional barriers to sharing a device.

Face ID has already generated a lot of excitement but the switch to a facial biometric does raise privacy concerns — given that the human face is naturally an expression-rich medium which, inevitably, communicates a lot of information about its owner without them necessarily realizing it.

You can’t argue that a face tells rather more stories over time than a mere digit can. So it pays to take a closer look at what Apple is (and isn’t doing here) as the iPhone X starts arriving in its first buyers’ hands…

Face ID

The core use for the iPhone X’s front-facing sensor module — aka the TrueDepth camera system, as Apple calls it — is to power a new authentication mechanism based on a facial biometric. Apple’s brand name for this is Face ID.

To use Face ID iPhone X owners register their facial biometric by tilting their face in front of the TrueDepth camera. (NB: The full Face ID enrollment process requires two scans so takes a little longer than the below GIF.)

The face biometric system replaces the Touch ID fingerprint biometric which is still in use on other iPhones (including on the new iPhone 8/8 Plus).

Only one face can be enrolled for Face ID per iPhone X — vs multiple fingerprints being allowed for Touch ID. Hence sharing a device being less easy, though you can still share your passcode.

As we’ve covered off in detail before Apple does not have access to the depth-mapped facial blueprints that users enroll when they register for Face ID. A mathematical model of the iPhone X user’s face is encrypted and stored locally on the device in a Secure Enclave.

Face ID also learns over time and some additional mathematical representations of the user’s face may also be created and stored in the Secure Enclave during day to day use — i.e. after a successful unlock — if the system deems them useful to “augment future matching”, as Apple’s white paper on Face ID puts it. This is so Face ID can adapt if you put on glasses, grow a beard, change your hair style, and so on.

The key point here is that Face ID data never leaves the user’s phone (or indeed the Secure Enclave). And any iOS app developers wanting to incorporate Face ID authentication into their apps do not gain access to it either. Rather authentication happens via a dedicated authentication API that only returns a positive or negative response after comparing the input signal with the Face ID data stored in the Secure Enclave.

Senator Al Franken wrote to Apple asking for reassurance on exactly these sorts of question. Apple’s response letter also confirmed that it does not generally retain face images during day-to-day unlocking of the device — beyond the sporadic Face ID augmentations noted above.

“Face images captured during normal unlock operations aren’t saved, but are instead immediately discarded once the mathematical representation is calculated for comparison to the enrolled Face ID data,” Apple told Franken.

Apple’s white paper further fleshes out how Face ID functions — noting, for example, that the TrueDepth camera’s dot projector module “projects and reads over 30,000 infrared dots to form a depth map of an attentive face” when someone tries to unlock the iPhone X (the system tracks gaze as well which means the user has to be actively looking at the face of the phone to activate Face ID), as well as grabbing a 2D infrared image (via the module’s infrared camera). This also allows Face ID to function in the dark.

“This data is used to create a sequence of 2D images and depth maps, which are digitally signed and sent to the Secure Enclave,” the white paper continues. “To counter both digital and physical spoofs, the TrueDepth camera randomizes the sequence of 2D images and depth map captures, and projects a device-specific random pattern. A portion of the A11 Bionic processor’s neural engine — protected within the Secure Enclave — transforms this data into a mathematical representation and compares that representation to the enrolled facial data. This enrolled facial data is itself a mathematical representation of your face captured across a variety of poses.”

So as long as you have confidence in the calibre of Apple’s security and engineering, Face ID’s architecture should given you confidence that the core encrypted facial blueprint to unlock your device and authenticate your identity in all sorts of apps is never being shared anywhere.

But Face ID is really just the tip of the tech being enabled by the iPhone X’s TrueDepth camera module.

Face-tracking via ARKit

Apple is also intending the depth sensing module to enable flashy and infectious consumer experiences for iPhone X users by enabling developers to track their facial expressions, and especially for face-tracking augmented reality. AR generally being a huge new area of focus for Apple — which revealed its ARKit support framework for developers to build augmented reality apps at its WWDC event this summer.

And while ARKit is not limited to the iPhone X, ARKit for face-tracking via the front-facing camera is. So that’s a big new capability incoming to Apple’s new flagship smartphone.

“ARKit and iPhone X enable a revolutionary capability for robust face tracking in AR apps. See how your app can detect the position, topology, and expression of the user’s face, all with high accuracy and in real time,” writes Apple on its developer website, going on to flag up some potential uses for the API — such as for applying “live selfie effects” or having users’ facial expressions “drive a 3D character”.

The consumer showcase of what’s possible here is of course Apple’s new animoji. Aka the animated emoji characters which were demoed on stage when Apple announced the iPhone X and which enable users to virtually wear an emoji character as if was a mask, and then record themselves saying (and facially expressing) something.

So an iPhone X user can automagically ‘put on’ the alien emoji. Or the pig. The fox. Or indeed the 3D poop.

But again, that’s just the beginning. With the iPhone X developers can access ARKit for face-tracking to power their own face-augmenting experiences — such as the already showcased face-masks in the Snap app.

“This new ability enables robust face detection and positional tracking in six degrees of freedom. Facial expressions are also tracked in real-time, and your apps provided with a fitted triangle mesh and weighted parameters representing over 50 specific muscle movements of the detected face,” writes Apple.

Now it’s worth emphasizing that developers using this API are not getting access to every datapoint the TrueDepth camera system can capture. This is also not literally recreating the Face ID model that’s locked up in the Secure Enclave — and which Apple touts as being accurate enough to have a failure rate as small as one in one million times.

But developers are clearly being given access to some pretty detailed face maps. Enough for them to build powerful user experiences — such as Snap’s fancy face masks that really do seem to be stuck to people’s skin like facepaint…

And enough, potentially, for them to read some of what a person’s facial expressions are saying — about how they feel, what they like or don’t like.

(Another API on the iPhone X provides for AV capture via the TrueDepth camera — which Apple says “returns a capture device representing the full capabilities of the TrueDepth camera”, suggesting the API returns photo + video + depth data (though not, presumably, at the full resolution that Apple is using for Face ID) — likely aimed at supporting additional visual special effects, such as background blur for a photo app.)

Now here we get to the fine line around what Apple is doing. Yes it’s protecting the mathematical models of your face it uses the iPhone X’s depth-sensing hardware to generate and which — via Face ID — become the key to unlocking your smartphone and authenticating your identity.

But it is also normalizing and encouraging the use of face mapping and facial tracking for all sorts of other purposes.

Entertaining ones, sure, like animoji and selfie lenses. And even neat stuff like helping people virtually try on accessories (see: Warby Parker for a first mover there). Or accessibility-geared interfaces powered by facial gestures. (One iOS developer we spoke to, James Thomson — maker of calculator app PCalc — said he’s curious “whether you could use the face tracking as an accessibility tool, for people who might not have good (or no) motor control, as an alternative control method”, for example.)

Yet it doesn’t take much imagination to think what else certain companies and developers might really want to use real-time tracking of facial expressions for: Hyper sensitive expression-targeted advertising and thus even more granular user profiling for ads/marketing purposes. Which would of course be another tech-enabled blow to privacy.

It’s clear that Apple is well aware of the potential risks here. Clauses in its App Store Review Guidelines specify that developers must have “secure user consent” for collecting “depth of facial mapping information”, and also expressly prohibit developers from using data gathered via the TrueDepth camera system for advertising or marketing purposes.

In clause 5.1.2 (iii) of the developer guidelines, Apple writes:

Data gathered from the HomeKit API or from depth and/or facial mapping tools (e.g. ARKit, Camera APIs, or Photo APIs) may not be used for advertising or other use-based data mining, including by third parties.

It also forbids developers from using the iPhone X’s depth sensing module to try to create user profiles for the purpose of identifying and tracking anonymous users of the phone — writing in 5.1.2 (i):

You may not attempt, facilitate, or encourage others to identify anonymous users or reconstruct user profiles based on data collected from depth and/or facial mapping tools (e.g. ARKit, Camera APIs, or Photo APIs), or data that you say has been collected in an “anonymized,” “aggregated,” or otherwise non-identifiable way.

While another clause (2.5.13) in the policy requires developers not to use the TrueDepth camera system’s facial mapping capabilities for account authentication purposes.

Rather developers are required to stick to using the dedicated API Apple provides for interfacing with Face ID (and/or other iOS authentication mechanisms). So basically, devs can’t use the iPhone X’s sensor hardware to try and build their own version of ‘Face ID’ and deploy it on the iPhone X (as you’d expect).

They’re also barred from letting kids younger than 13 authenticate using facial recognition.

Apps using facial recognition for account authentication must use LocalAuthentication (and not ARKit or other facial recognition technology), and must use an alternate authentication method for users under 13 years old.

The sensitivity of facial data hardly needs to be stated. So Apple is clearly aiming to set parameters that narrow (if not entirely defuse) concerns about potential misuse of the depth and face tracking tools that its flagship hardware now provides. Both by controlling access to the key sensor hardware (via APIs), and by policies that its developers must abide by or risk being shut out of its App Store and barred from being able to monetize their apps.

“Protecting user privacy is paramount in the Apple ecosystem, and you should use care when handling personal data to ensure you’ve complied with applicable laws and the terms of the Apple Developer Program License Agreement, not to mention customer expectations,” Apple writes in its developer guidelines.

The wider question is how well the tech giant will be able to police each and every iOS app developer to ensure they and their apps stick to its rules. (We asked Apple for an interview on this topic but at the time of writing it had not provided a spokesperson.)

Depth data being provided by Apple to iOS developers — which was only previously available to these devs in even lower resolution on the iPhone 7 Plus, thanks to that device’s dual cameras — arguably makes facial tracking applications a whole lot easier to build now, thanks to the additional sensor hardware in the iPhone X.

Though developers aren’t yet being widely incentivized by Apple on this front — as the depth sensing capabilities remain limited to a minority of iPhone models for now.

Although it’s also true that any iOS app granted access to iPhone camera hardware in the past could potentially have been using a video feed from the front-facing camera, say, to try to algorithmically track facial expressions (i.e by inferring depth).

So privacy risks around face data and iPhones aren’t entirely new, just maybe a little better defined thanks to the fancier hardware on tap via the iPhone X.

Questions over consent

On the consent front, it’s worth noting that users do also have to actively give a particular app access to the camera in order for it to be able to access iOS’ face mapping and/or depth data APIs.

“Your app description should let people know what types of access (e.g. location, contacts, calendar, etc.) are requested by your app, and what aspects of the app won’t work if the user doesn’t grant permission,” Apple instructs developers.

Apps also can’t pull data from the APIs in the background. So even after a user has consented for an app to access the camera, they have to be actively using the app for it to be able to pull facial mapping and/or depth data. So it should not be possible for apps to continuously facially track users — unless a user continues to use their app.

Although it’s also fair to say that users failing to read and/or properly understand T&Cs for digital services remains a perennial problem. (And Apple has sometimes granted additional permissions to certain apps — such as when it temporarily gave Uber the ability to record the iPhone user’s screen even when the app was in the background. But that is an exception, not the rule.)


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