1. A method for behavior based authentication for touch screen devices, comprising: acquiring a plurality of training samples corresponding to a first action performed on a touch screen of a touch screen device, wherein the first action comprises an input of a gesture by a legitimate user; generating a user behavior model based on the plurality of training samples, what features of the training samples by multiple users, and how features of the training samples by distinct users, wherein the what features comprise a displacement magnitude, and wherein the how features comprise the displacement magnitude; acquiring a test sample corresponding to a second action performed on the touch screen, wherein the second action comprises an input of the gesture by a user; and classifying the test sample based on the user behavior model, wherein classifying the test sample comprises determining whether the user is the legitimate user or an imposter, including by automatically adjusting a time resolution for each of a plurality of subparts of an input of the first action until and in order to obtain consistent feature values for the subparts for different locations of the first action.