Passwords get forgotten and are easily discovered, fingerprint recognition won't work if your hands are sweaty, and you might not want to get your hands out to unlock your phone in the cold.
So why not have another later of protection, and keep your data safe with a selfie password? For the uninitiated, that's a photographic self-portrait.
FaceCrypt is an app that will allow iPhone and iPad users to do just that. Other devices - like the Sony Xperia - offer something similar, allowing you to unlock your phone with your face.
That's not to say that such an app would be without problems. If you've got strong light overhead then it may be hard for your camera to distinguish any of your facial features.
But just in case those other security measures fail, FaceCrypt should allow you to protect your vital data.
Who knows, maybe Prime Minister David Cameron could be an early adopter.