Want to unlock your phone with your face? Here's how Apple says Face ID works on iPhone X

Apple's new iPhone X won't be available until later this year, but the company is revealing a bit more information about how the facial recognition features of the new phone will work. 

The company published a white paper Wednesday on Face ID security, alongside an update to its website's privacy section. The paper answers a handful of the most pressing privacy and security related questions that people have had in the weeks following the iPhone X reveal.

So, uh, remind me — how does this thing work?

Every time you wake the iPhone X, it looks for your face. Next, it tries to confirm that your eyes are open and looking at the device — what Apple calls "intent to unlock." Once that intent has been established, the phone's TrueDepth camera system attempts to authenticate your identity by creating a model of your face.

First, a dot projector projects over 30,000 invisible dots of infrared light onto your face, which is read by an infrared camera and used to create a 3D image, or depth map. Next, the camera captures a separate 2D infrared image with the help of something called a flood illuminator, which is what helps the phone see your face in the dark.

These images are combined to create what Apple calls a "mathematical representation" of your face, which it compares to the model that was created when you first setup Face ID — after which the images are discarded.

What if someone makes a really good mask?

Apple says its put a handful of safeguards in place to prevent an unauthorized user from accessing your phone with a photo or mask. By matching against the 3D image of your face, for example, Apple says you can't simply spoof the system with a 2D photograph, which doesn't have any depth. And the company also says it's trained a neural network dedicated to spotting spoofs — such as masks — though no additional information is provided.

And "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," the white paper reads.

As a result of these and other measures, Apple claims that there's a one in 1,000,000 chance that a random person could unlock your phone with their face (though Apple says this probability is "different" for twins and siblings, and doesn't specify what that probability is). 

What sort of data does Face ID collect?

All of the Face ID data is encrypted and stored in what Apple calls the Secure Enclave, a separate chip within recent iPhones that acts as a vault for highly system sensitive data. The Secure Enclave stores three things: the infrared images and mathematical models of your face that are captured when you setup Face ID, as well as new models that are calculated and saved in the future to help improve matching over time time (more on that in a second).

"This data never leaves the device. It is not sent to Apple, nor is it included in device backups," the white paper reads. Apple says that even it can't access what the Secure Enclave contains. 

What if I start wearing glasses, or put on a hat?

Good question! Seasons change. Empires rise and fall. Maybe you forgot to shave for a few days, or decided to cut off all your hair. To compensate, Face ID creates a new model of your face each time you unlock your phone. If the model is good enough, it may keep using it for an unspecified "finite number of additional unlocks before that data is discarded." 

But what if your face fails to unlock your phone, and you're forced to enter your passcode instead?  If it's a close enough match, Face ID may still save and use this model too. According to Apple, "these augmentation processes allow Face ID to keep up with dramatic changes in your facial hair or makeup use, while minimizing false acceptance."

So Face ID never sends my face data to Apple?

That's what Apple says. According to the white paper, the only time Face ID data can leave your device is when you opt in to something called "Face ID Diagnostics." If you're having a problem with Face ID, you can opt in and have your iPhone save the images used to setup Face ID and subsequently unlock your phone (including the photos that fail to match). After a week, you can review the images that were recorded, and approve or reject the ones you want to actually share with Apple support.

Can I turn Face ID off and use my passcode?

Of course, if you don't want to use Face ID at all, you can always turn it off — just the way you can with Apple's fingerprint sensor Touch ID.