  Recent research has shown the possibility of using smartphones' sensors and
accessories to extract some behavioral attributes such as touch dynamics,
keystroke dynamics and gait recognition. These attributes are known as
behavioral biometrics and could be used to verify or identify users implicitly
and continuously on smartphones. The authentication systems that have been
built based on these behavioral biometric traits are known as active or
continuous authentication systems.
  This paper provides a review of the active authentication systems. We present
the components and the operating process of the active authentication systems
in general, followed by an overview of the state-of-the-art behavioral
biometric traits that used to develop an active authentication systems and
their evaluation on smartphones. We discuss the issues, strengths and
limitations that associated with each behavioral biometric trait. Also, we
introduce a comparative summary between them. Finally, challenges and open
research problems are presented in this research field.
