# EchoIA: A Cloud-Based Implicit Authentication Leveraging User Feedback  
[PERSON]  
1 Computer Science Department, University of Illinois, Springfield, IL 62629, USA; [EMAIL_ADDRESS] (Y.Y. ); 1 [EMAIL_ADDRESS] (S.L. )2 Department of Electrical Engineering and Computer Science, University of Tennessee, Knoxville, TN 37996, USA; [EMAIL_ADDRESS] 2  
[PERSON]  
2 Department of Electrical Engineering and Computer Science, University of Tennessee, Knoxville, TN 37996, USA; [EMAIL_ADDRESS] 2  
[PERSON]  
1 Computer Science Department, University of Illinois, Springfield, IL 62629, USA; [EMAIL_ADDRESS] (S.L. )2 Department of Electrical Engineering and Computer Science, University of Tennessee, Knoxville, TN 37996, USA; [EMAIL_ADDRESS] 2  
[PERSON]  
3 Temple Department of Computer & Information Sciences, Temple University, Philadelphia, PA 19122, USA; [EMAIL_ADDRESS] 3  
[PERSON]  
Correspondence: [EMAIL_ADDRESS]; Tel. : +1-(865)-974-0426; Fax: +1-(865)-974-4404 2  
most of the calculations and storage are generally offloaded to the cloud due to the energy limitation of smart devices [6]. Furthermore, since users do not need to interact with the system in the authentication, IA can be seamlessly applied to various authentication systems, which adds another layer of protection to the smart device. If suspicious behaviors were detected during the usage, IA will lock the device and ask users to perform a multi-factor authentication, e.g., inputting passwords [7, 8]. From the users' aspect, due to IA's transparency, they will not notice IA until the device has been locked. In addition, implicit authentication does not require user-system interaction, which releases users from tedious password inputting and the burden of memorizing the passwords. In implicit authentication (IA), the features used for the authentication are predefined by the system, which will not be able to change during the usage [9, 10]. To achieve better coverage in user authentication, the existing approaches tend to use multiple features [11, 12, 13], such as location, touch, and acceleration. However, only a small number of the features (personal features) in the entire feature set are needed for a specific user. Therefore, irrelevant features not only reduce the system's efficiency, but decrease the authentication accuracy as well. Nevertheless, due to the high complexity of human behavior, it requires a massive calculation to derive personal features, which is infeasible in practice [14]. Hence, finding a suitable scheme that dynamically derives personal features is critical for IA implementation. This work introduces a human-centered implicit authentication (EchoIA), which utilizes user feedback to pinpoint personal features during the usage with a small amount of calculation. By comparing with current IA schemes, the experiment shows that our method can significantly improve the authentication accuracy of traditional IA. In addition, the proposed method is lightweight, which can be easily embedded into existing IA systems as an add-on to achieve efficient authentication. As far as we know, we are the first group that utilizes user feedback to improve authentication accuracy and energy efficiency in IA. The major advantage of implicit authentication (IA) is its transparency, releasing users from the tedious authentication process. However, it is challenging to gather user feedback in a transparent environment since directly asking users' input will break the transparency. Even though we could have various user feedback, pinpointing the best suitable features is also challenging. To this end, we propose a method that utilizes the correct rate of inputted passwords to implicitly collect user feedback and find personal features. In implicit authentication, the system will lock the device and deem current users illegitimate when their behavior mismatches legitimate users' historical behavior. Legitimate users may be locked out due to the misidentification caused by using unsuitable features, but they can input a correct password to unlock the device.