1. A user classification method using a keystroke pattern in a mobile device, the method comprising: receiving keystroke data from a user when the user performs authentication of an application included in the mobile device; extracting feature information including data for a posture performed by the user when the user performs keystroke and data for a keystroke pattern using the received keystroke data; classifying the posture of the user by comparing the feature information to posture feature information, the posture feature information having been generated based on postures performed by general users, the postures including a walking posture, a sitting posture, and a lying-down posture; selecting a keystroke profile that corresponds to the classified posture of the user; comparing the feature information to a pattern of the selected keystroke profile; determining whether the user is a pre-trained user based on a result of the comparing; and authenticating the user to use the application based on a result of the determining.