## 3 Related Work  
Over the last two decades, several studies and reviews of continuous authentication technologies have been conducted, addressing two main topics: The first one involves the attitudes of users towards authentication, in general. Research [41] examined 175 users to investigate the adoption of continuous authentication and found that 85% of the participants believed that users should be aware of the ongoing monitoring in their devices. This revealed the importance of privacy regarding the implementation of an authentication process. The researchers also argued that the use of continuous monitoring is generally effective. Besides that, the above authors argued that some users are not comfortable with the transition to novel authentication techniques, such as continuous authentication. They also found that voice verification and fingerprint recognition were classified as the most suitable types of login authentication after passwords, with raw overall acceptability scores of 68% and 67%, respectively, indicating the growing, at the time, acceptance of biometrics. As smartphone use started to boost, a second, more focused, topic drew the interest of security researchers. This topic concerns the attitudes and perceptions of smartphone users towards biometrics-based continuous authentication, investigating at the same time the limiting and facilitating factors affecting the adoption of these technologies. Research [42] investigated the perspectives and attitudes towards continuous authentication versus common point-of-entry authentication techniques and found that users' perspectives and attitudes varied significantly. They argued that the main justification for the negative perspectives was the fear of uncomfortability generated by false rejections. Participants also demonstrated greater concern to privacy about biometric data processing and about being continuously monitored without being aware of it. A usability survey by [43] explored the perception of users regarding physiological and behavioral authentication methods, both of which involve active smartphones' interactivity. The results demonstrated that 87.3% of 331 attendees acknowledged that biometric authentication would be appropriate for the security of their smartphones. Also, 78.9% of the attendees agreed that they would store more private data on their devices if they were equipped with biometrics scanning features. Additionally, 66.2% of the attendees favored fingerprint as an authentication method, whereas 6.43% favored voice recognition. Research [44] found that face and hand recognition seem to be far more comfortable and lead to an enhanced perception of security in comparison to gesture and voice recognition. Additionally, research [45] found that the combined use of different modalities had an average accuracy of 88% while 15% EER was achieved. This is in alignment with the [46] study in which the authors argued that the development and use of biometric authentication, physical as well as behavioral, in smartphones enhance their performance, security, and ease of use without major hardware side-effects that can affect and increase the price of smartphones. Further to this, article [33] argued that behavioral biometrics decrease the need for legitimate authentication by 67% compared to knowledge-based techniques. Reference [47] argued that technologies that use behavioral biometrics to provide continuous authentication on smartphones are effective since their findings among 37 participants showed that 91% found the technology suitable while 81% of them considered the level of protection provided as sufficient. In fact, in [48], it is argued that users are amenable to adopt continuous authentication mechanisms for their smartphones. Finally, article [31] recommended BBCA as an appropriate security measure in the IoT era, since conventional methods of authentication suffer from major drawbacks, such as vulnerability and obtrusiveness.