Scramble or not, that is the question a study of the security and usability of scramble keypad for PIN unlock on smartphones

PIN unlock is a popular screen locking mechanism used for protecting the sensitive private information on smart-phones. However, it is susceptible to a number of attacks such as guessing attacks, shoulder surfing attacks, smudge attacks, and side-channel attacks. Scramble keypad changes the keypad layout in each PIN-entry process to improve the security of PIN unlock. In this paper, the security and usability of scramble keypad for PIN unlock are studied. Our security analysis shows that scramble keypad can defend smudge attacks perfectly and greatly reduce the threats of side-channel attacks. A user study is conducted to demonstrate that scramble keypad has a better chance to defend shoulder surfing attacks than standard keypad. We also investigate how the usability of scramble keypad is compromised for improved security through a user study.