Accelerometer-based biometric gait recognition for authentication on smartphones

The authentication via accelerometer-based biometric gait recognition offers a user-friendly alternative to common authentication methods on smartphones. It has the great advantage that the authentication can be performed without user interaction. When the user is walking, his walk-pattern can be extracted from the accelerations measured using the integrated sensors of the smartphone. This pattern can be used for authentication. A study showed that users often deactivate the authentication methods of their mobile devices because they consume too much time. Because all steps necessary to perform biometric gait recognition can be executed in the background, no user interaction is necessary for the presented technique. Performing a continuous authentication while the user is walking, an up-to-date authentication result is available at any point in time. During log-in, no calculations are necessary anymore, hence there is no delay. Only in cases where the user is not walking, an alternative authentication method has to be used. This is a great benefit for the user because he has the advantages of a phone which is protected by authentication but without the disadvantages common methods impose. This high user-friendliness is likely to increase the number of smartphones for which the screen-lock is linked with an authentication. Therefore, a higher security of the data stored in smartphones can be achieved. A misuse of the stored information by an unauthorized user can be prevented. Due to the growing distribution of powerful smartphones, the number of available applications is increasing as well. These applications result in a growing amount of data stored on the devices, which make the protection of the device necessary. These data comprise e.g. addresses, appointments or GPS-information. Additionally, some applications, e.g. of e-mail-providers or social networks, require the user to authenticate himself. Often these credentials are stored by the user on the phone, such that it is not necessary to enter them each time. In case an unauthorized person has access to such a phone he can use these services without restrictions and therefore substantially harm the user. The objective of this thesis was to develop methods for accelerometer-based biometric gait recognition which achieve sufficient low error rates, as well as to demonstrate that their computational effort is low and allows for an execution on current smartphones. Because the basis of existing methods is the extraction of gait cycles (i.e. two steps) from the accelerometer data, a cycle-based method was developed and evaluated in a scenario test. This method uses raw data of the gait cycles as feature vectors and accomplishes the classification using distance functions. In addition, a further approach was selected, which does not need the time-costly and error-prone gait cycle extraction. Instead, it is using overlapping segments of a fixed time length. Several features are extracted from these segments and combined to feature vectors. Machine learning algorithms are used for classification. A benchmark of the approaches on a challenging database showed that these methods yield low equal error rates between 6% and 7% and are outperforming the cycle-based methods. These error rates were achieved under the realistic conditions that training and probe data are not collected on the same day. It was shown that five minutes of gait data are sufficient to thoroughly train the models. It should be regarded that the training data contain the different walking velocities at which the user should be recognized later on. To obtain low false rejection rates, the classification should be based on around three minutes walk data. Two of the developed methods were implemented on a smartphone. It was shown that both methods are able to perform the classification fast enough to allow for an authentication without delay for the user.