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

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.

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