1. A method for verifying an identity of a user accessing a user device, comprising: receiving a request to verify the identity of the user accessing the user device; receiving a current set of multimedia identification signals from the user device in accordance with the verification request; querying a deep-content-classification (DCC) system to find a match between at least one pre-stored concept structure associated with the user to be verified and the current set of received multimedia identification signals, wherein the at least one concept structure is created during a sign-in procedure and is based on a grouping of associated clusters, wherein each cluster includes a compressed set of similar signatures generated from an initial set of previously received multimedia identification signals received during the sign-in procedure; and sending an authentication notification to the user upon finding a match between the at least one concept structure and the current set of received multimedia identification signals.