Arranging authentication and ciphering in mobile communication system

A telecommunication system comprising:

• a first mobile communication network configured to use a first cipher key for ciphering traffic between a mobile station and the first mobile communication network;
• a second mobile communication network configured to use a second cipher key for ciphering traffic between a mobile station and the second mobile communication network;
• and
• a mobile station configured to support said different first and second mobile communication networks,
• wherein the first mobile communication network is configured to calculate the second cipher key when the mobile station operates in the first mobile communication network, and the first mobile communication network is configured to transmit information necessary for calculating the second cipher key from the first mobile communication network to the mobile station when the mobile station operates in the first mobile communication network, and the mobile station is configured to calculate said second cipher key,
• wherein the first mobile communication network is configured to transmit the second cipher key to the second mobile communication network before a handover to the second mobile communication network,
• wherein the mobile station is configured to transmit said second cipher key calculated at the mobile station to a ciphering module of the mobile station in response to the first mobile communication network transmitting a request to the mobile station for handover to the second mobile communication network, and
• wherein the mobile station and the second mobile communication network are configured to cipher traffic after the handover using the second cipher key.

A mobile station configured to support a first mobile communication network and a second mobile communication network, wherein

• the mobile station is configured to cipher traffic between the mobile station and the first mobile communication network using a first cipher key when the mobile station operates in the first mobile communication network,
• the mobile station is configured to receive from the first mobile communication network information necessary for calculating a second cipher key to be used for ciphering traffic between the mobile station and the second mobile communication network,
• the mobile station is configured to calculate the second cipher key,
• the mobile station is configured to transmit the second cipher key calculated at the mobile station to a ciphering module of the mobile station in response to the first mobile communication network transmitting a request to the mobile station for handover to the second mobile communication network, and
• the mobile station is configured to cipher traffic between the mobile station and the second mobile communication network using the second cipher key if the mobile station is handed over from the first mobile communication network to the second mobile communication network.