Shoulder Surf Resistant Screen Locking for Smartphones: A Review of Fifty Non-Biometric Methods

With a lot of personal information being stored in the smartphones nowadays, there is an urgent need to protect them from unauthorised access. The first line of defense of a smartphone is its screen lock. Thus, many screen locking methods have been designed with the aim of minimising, if not eliminating the chances of unauthorised access through attacks such as shoulder surfing. This paper provides an overview on the state of research on non-biometric shoulder surf resistant screen locking methods with the aim to identify the techniques used by these methods in defending against shoulder surf attacks. A systematic literature review performed found a total of 84 methods designed and developed to date, which covers the time span of ten years. A notably higher number of new methods was published each year during the last five years and the number continues to grow steadily, indicating the relevance and currentness of the research area. Non-biometric screen locking methods in particular was found to constitute the majority of the methods with 50 methods. During the analysis, these methods were first vertically classified into PIN-based, pattern-based and image-based. Then they were horizontally categorised into those that use fixed password and those that use one-time password to unlock the screen. At the end of the analysis, a total of ten shoulder surf resistant techniques used in these methods was extracted. These techniques were used either in isolation or in combination with one another. Findings from this study would be useful in providing the state-of-the-art summary of the non-biometric shoulder surf resistant screen locking methods.