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 signature or a gesture by a legitimate user; generating a user behavior model based on the plurality of training samples; acquiring a test sample corresponding to a second action performed on the touch screen, wherein the second action comprises an input of the signature or 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.