Method for securing data traffic between mobile radio network and IMS network

A method for user authentication and securing data traffic between a mobile radio network and an Internet protocol-based multimedia subsystem network, comprising:

• authenticating a mobile subscriber in the mobile radio network;
• authenticating the mobile subscriber in the Internet protocol-based multimedia subsystem network;
• determining whether a first identity of the mobile subscriber authenticated in the Internet protocol-based multimedia subsystem network corresponds to a second identity of the mobile subscriber authenticated in the mobile radio network;
• sending, if the first and second identities correspond, a confirmation message from the Internet protocol-based multimedia subsystem network to the mobile subscriber;
• and
• exchanging data between the mobile subscriber and the Internet protocol-based multimedia subsystem network using a security protocol protected by a common key derived from the confirmation message for the security protocol.