Opening your phone with your fingerprint or facial recognition is convenient. But in the United States and possibly most other countries, enabling Touch ID or Face ID gives law enforcement free access to your phone if you’re arrested. Thankfully iOS and Android let you temporarily turn off fingerprint or face recognition with various methods.
US courts have ruled that law enforcement can’t force you to enter your phone’s passcode, but they can force you to open your phone with your fingerprint. So any time you expect a brush with law enforcement, you should lock up your phone.
The easiest, least-detectable way to turn off Touch ID or Face ID is to hold down the power button and either of the volume buttons. (On iPhones 5S to 7, hit the power button five times.)
After a few seconds, your phone will vibrate and offer three options: Power off, Medical ID, and Emergency SOS. Whatever you select, even Cancel, your phone will ask for your passcode to unlock.
Once you unlock your phone, Touch ID will be enabled again. So you have to repeat this step every time you lock your phone.
By default, Medical ID shows whatever emergency contact info you’ve stored. Emergency SOS immediately calls 911. If you cancel that, it counts down 10 seconds before calling your emergency contacts. If you cancel that, it will do nothing, but your phone will still require your passcode to unlock.
On iOS, you can go into Settings > Touch ID & Passcode (or Face ID & Passcode) and toggle off iPhone Unlock. Next time you lock your phone, you’ll need to enter your passcode to get back in. Unlike the above methods, this change sticks around until you go back into Settings and turn Touch/Face ID back on. (Your phone still remembers your face or fingerprint, so you won’t have to teach it again.)
On Android Oreo:
Open Settings > Lock Screen and Security > Screen Lock Type and toggle off all the options under the Biometrics section.
On the beta Android P:
Open Settings > Security & location > Lock screen preferences. Toggle on Show lockdown option. Then any time you want to activate lockdown, hold down your power button and tap Enter lockdown.
If you have little time to react, you can always turn off your phone. Whenever you turn it back on, it will require your passcode to unlock.
If you know there is a likely encounter with law enforcement such as an organized protest, you can turn off fingerprint and facial recognition in advance and just use a passcode. (On Android you can also use a swipe pattern, but make sure it’s hard to guess.) Typing a passcode every time is a bit tedious, but at least it keeps the keys to your phone locked up in your head.