A key update method with respect to a member device of a group; the group comprising a varying number of group members, wherein each member of the group corresponds to a leaf node of a binary tree; wherein each of the members of the group has its own key, as well as the node key of all the nodes on a path from the leaf node to a root node of the binary tree; being the key of the root node shared by all the members of the group; wherein in the binary tree, a key corresponding to nodes, other than the root node, is used for updating a key of a parent node of a corresponding node; wherein each member of the group has an offline mode and an online mode; and wherein the member device of the group has switched from an offline mode to an online mode; the method comprising the steps of: storing, while the member device is still in an offline mode, data in a database of an update server, in response to a node key update for the group, the data having the encrypted key of all the nodes which have been updated as a result of the node key update for the group; wherein the encryption of the key of each of the updated nodes is performed using at least a child node key of each of the updated nodes; then; generating member update data for the member device which has switched from the offline mode to the online mode; wherein generating member updata data comprises first generating offline data by selecting from among the encrypted key of all the updated nodes comprised within the stored data, the encrypted key of the updated ancestor nodes of the member device which has switched from the offline mode to the online mode; and then selecting as member update data from among the offline data the encrypted key of the updated ancestor nodes, which has been encrypted with a key of a child node of each of the updated ancestor nodes which allows the correct decryption of the respective updated keys of the ancestor nodes, by the member device which has switched from the offline mode to the online mode; providing the generated member update data to the member device which has switched from the offline mode to the online mode, wherein, the provided member update data is decrypted by the member device which has switched from the offline mode to the online mode for calculating the shared root key and consequently achieving the key update with respect to the member device which has switched from the offline mode to the online mode; wherein, when a new member device joins the group while the member device is still in offline mode, a node key is established for a node corresponding to the new member device, the node key of all ancestor nodes of the node corresponding to the new member device is updated, and the data having the encrypted updated node key is generated by encrypting each updated ancestor node key corresponding to the ancestor nodes of the node corresponding to the new member device that joins the group, using at least a child node key of a child node of the updated ancestor node. storing, while the member device is still in an offline mode, data in a database of an update server, in response to a node key update for the group, the data having the encrypted key of all the nodes which have been updated as a result of the node key update for the group; wherein the encryption of the key of each of the updated nodes is performed using at least a child node key of each of the updated nodes; then; generating member update data for the member device which has switched from the offline mode to the online mode; wherein generating member updata data comprises first generating offline data by selecting from among the encrypted key of all the updated nodes comprised within the stored data, the encrypted key of the updated ancestor nodes of the member device which has switched from the offline mode to the online mode; and then selecting as member update data from among the offline data the encrypted key of the updated ancestor nodes, which has been encrypted with a key of a child node of each of the updated ancestor nodes which allows the correct decryption of the respective updated keys of the ancestor nodes, by the member device which has switched from the offline mode to the online mode; providing the generated member update data to the member device which has switched from the offline mode to the online mode, wherein, the provided member update data is decrypted by the member device which has switched from the offline mode to the online mode for calculating the shared root key and consequently achieving the key update with respect to the member device which has switched from the offline mode to the online mode; wherein, when a new member device joins the group while the member device is still in offline mode, a node key is established for a node corresponding to the new member device, the node key of all ancestor nodes of the node corresponding to the new member device is updated, and the data having the encrypted updated node key is generated by encrypting each updated ancestor node key corresponding to the ancestor nodes of the node corresponding to the new member device that joins the group, using at least a child node key of a child node of the updated ancestor node.