Would you rather unlock your smartphone with a plain four-digit PIN or with a smiley-face emoji? Would it be easier and more pleasant to remember emojis for example, or “2476”? Smartphone users commonly use emojis to express moods, emotions and nuances in emails and text messages – and even communicate entire messages only with emojis. In 2015, a British company tried using emoji passcodes in place of PINs at bank ATMs.

But there had been no formal study of how easy they were to use, or how secure they were in comparison to other methods, like PINs. To learn ...