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Apple publishes Face ID security white paper

Face ID Security — Apple — “To authorize an in-store payment with Face ID, you must first confirm intent to pay by double-clicking the side button. You then authenticate using Face ID before placing your iPhone X near the contactless payment reader. If you’d like to select a different Apple Pay payment method after Face ID authentication, you’ll need to reauthenticate, but you won’t have to double-click the side button again.”