Abstract: A security device and a head end of a conditional access system (CAS), and a method for controlling illegal use of the conditional access system, capable of minimizing the number of entitlement management messages (EMMs) to be distributed from a head end to a user are provided. Update keys encrypted with user keys and channel management keys encrypted with the update keys are utilized.

Abstract: An RFID tag used to purchase service related to the RFID tag containing purchase information, a service purchase system using the RFID tag, and a service purchase method thereof. The RFID tag provides content information, right information, and purchase information. Therefore, when purchasing service products such as contents, user convenience can be increased and reliability can be guaranteed.

Abstract: A security method using an electronic signature, which improves the performance of an electronic signature authentication by generating and verifying an electronic signature using a mediator, and acquires a forward security in an electronic signature generation and verification by adding a forward secure signature of semi-trusted party (SEM) to a partial signature value generated based on a secret key piece of the SEM. A public key and an optional secret key composed of two kinds of pieces are generated by a certificate authority in response to a request from a user terminal device. The secret key pieces are issued to the user terminal device and a semi-trusted party not to be overlapped with each other. A first signature piece generated from the issued pieces of the private key is transmitted to the user terminal device from the semi-trusted party when a certificate of the user terminal device is still valid.

Abstract: A tag generation method for generating tags used in data packets in a broadcast encryption system is provided. The method includes detecting at least one revoked leaf node; setting a node identification (node ID) assigned to at least one node among nodes assigned node IDs at a layer 0 and to which the at least one revoked leaf node is subordinate, to a node path identification (NPID) of the at least one revoked leaf node at the layer 0; generating a tag list in the layer 0 by combining the NPID of each of the at least one revoked leaf nodes at the layer 0 in order of increment of node IDs of the corresponding at least one revoked leaf nodes; and generating a tag list in a lowest layer by repeatedly performing the setting and generation operation down to the lowest layer.

Abstract: Disclosed is a method for authenticating a mobile node in a wireless local area network including at least two access points and an authentication server. When the mobile node associates with a first access point and performs initial authentication, the mobile node receives a first session key for secure communication from the authentication server by using a first private key generated with a secret previously shared with the authentication server, and the first access point receives the first session key from the authentication server by using a second private key previously shared with the authentication server.