1. A method for verifying an identity of a user accessing a user device, comprising: receiving a request to verify the user identity accessing the user device; receiving a plurality of multimedia identification signals from the user device; querying a deep-content-classification (DCC) system to find a match between at least one concept structure associated with the user and the plurality of received multimedia identification signals, wherein the concept structure is created during a sign-in procedure; and sending an authentication notification upon determination of a match between the at least one concept structure and the plurality of received multimedia identification signals.