Biometric User Authentication

One aspect provides a method of authenticating a user of a user device, the method comprising: receiving motion data captured using a motion sensor of the user device during an interval of motion of the user device induced by the user; processing the motion data to generate a device motion feature vector, inputting the device motion feature vector to a neural network, the neural network having been trained to distinguish between device motion feature vectors captured from different users; and authenticating the user of the user device, by using a resulting vector output of the neural network to determine whether the user-induced device motion matches an expected device motion pattern uniquely associated with an authorized user, the neural network having been trained based on device motion feature vectors captured from a group of training users, which does not include the authorized user.