Arranging authentication and ciphering in mobile communication system

A method to provide data protection in a telecommunication system comprising a first mobile communication network, which is used an encryption key to encrypt traffic between a mobile station and said first mobile communication network, a second mobile communications network in which a second encryption key used to encrypt traffic between the mobile station and said second mobile communication network, and a mobile station that supports the above mobile communication networks, characterized by said calculating second encryption key in the first mobile communication network when the mobile station operates in the first mobile communication network, transmitting the information necessary for calculating said second encryption key from the first mobile communication network to the mobile station when the station Mobile operates in the first mobile communication network, and calculating said second cipher key in the mobile station.