A Survey of Keystroke Dynamics Biometrics <s> Genuine and Imposter Samples <s> The present invention provides apparatus for changing the direction of rotation of a rotating shaft without changing the direction of rotation of the motor driving the shaft. The present invention includes a drive gear which causes two driven bevel gears to rotate in opposite directions. A control shaft having cam surfaces selectively forces one of two sets of balls into engagement with one of the driven gears to couple that gear with the driven shaft to rotate the shaft in one direction. Rotation of the driven shaft is reversed by changing the position of the control shaft which uncouples one set of balls from the driven shaft and couples the remaining set of balls to the driven shaft. <s> BIB001 </s> A Survey of Keystroke Dynamics Biometrics <s> Genuine and Imposter Samples <s> Internet services are important part of daily activities for most of us. These services come with sophisticated authentication requirements which may not be handled by average Internet users. The management of secure passwords for example creates an extra overhead which is often neglected due to usability reasons. Furthermore, password-based approaches are applicable only for initial logins and do not protect against unlocked workstation attacks. In this paper, we provide a non-intrusive identity verification scheme based on behavior biometrics where keystroke dynamics based-on free-text is used continuously for verifying the identity of a user in real-time. We improved existing keystroke dynamics based verification schemes in four aspects. First, we improve the scalability where we use a constant number of users instead of whole user space to verify the identity of target user. Second, we provide an adaptive user model which enables our solution to take the change of user behavior into consideration in verification decision. Next, we identify a new distance measure which enables us to verify identity of a user with shorter text. Fourth, we decrease the number of false results. Our solution is evaluated on a data set which we have collected from users while they were interacting with their mail-boxes during their daily activities. <s> BIB002
. Data collected will eventually be used for performance evaluation. The most common way of performance measurement is the degree of accuracy of a system's ability to distinguish genuine and imposter. Imposter samples are usually obtained by either the same individual who contributes to the generation of genuine samples in database  or via another group of individuals attacking or simulating the genuine samples stored in the database BIB002 . The former imposes participants to provide more inputs and devote more time in the experiment. The lengthy process may deter volunteer participation. On the other hand, the latter required less participation effort by each user but a separate pool is required. Difficulty to secure large pool of users due to resource limitation may be the reason why only 38% of the experiments in the literature opt for this way as compared to the earlier at 46%, while the unknown stands 16%. An alternative that may resolve this issue is by partitioning user sample data into two subsets. The first subset is used as training while the remaining as testing sample  . Leave-one-out, cross validation, or random split can be used in this context BIB001 . Having this way, separate imposter data collection set is supplementary. Although it seems to be advantageous, this method is only applicable if every subject's input is identical.