Method for creating and distributing cryptographic keys in a mobile radio system and corresponding mobile radio system

1. A method for creating and distributing cryptographic keys in a mobile radio system, wherein the mobile radio system has at least one mobile radio terminal, a first computer, a computer of a home communications network and a second computer, the at least one mobile radio terminal and the computer of the home communications network including authentication key materials resulting from an authentication, the method comprising:

creating a first cryptographic key and a second cryptographic key by the at least one mobile radio terminal and the computer of the home communications network using the authentication key materials, wherein the first cryptographic key and the second cryptographic key are created such that no inference can be made from the first cryptographic key as to the second cryptographic key,

no inference can be made from the second cryptographic key as to the first cryptographic key, and

no inference can be made from the first cryptographic key or from the second cryptographic key as to the authentication key materials;

transmitting the first cryptographic key from one or both of the at least one mobile terminal and the computer of the home communications network to the first computer; and

transmitting the second cryptographic key from one or both of the at least one mobile radio terminal and the computer of the home communications network to the second computer, wherein the first computer is a first application server computer and the second computer is a second application server computer.

2. The method according to claim 1 , wherein the first computer is a computer of a visited communications network, and the at least one mobile radio terminal is situated in the visited communications network.

3. The method according to claim 1 , wherein the first cryptographic key and the second cryptographic key are created using at least one key derivation function.

4. The method according to claim 1 , wherein the authentication key materials have at least two cryptographic keys.

5. The method according to claim 1 , wherein the mobile radio system is designed as a mobile radio system based on a 3GPP standard.

6. The method according to claim 5 , wherein the mobile radio system has an IP multimedia subsystem.

7. The method according to claim 1 , wherein the authentication key materials have an integrity key and a transfer key.

8. The method according to claim 7 , wherein the first cryptographic key and the second cryptographic key are derived from the transfer key.

9. The method according to claim 1 , wherein additional cryptographic keys are created by the at least one mobile radio terminal and by the computer of the home communications network for additional application server computers using the authentication key materials and are transmitted to respective application server computers.

10. The method according to claim 1 , wherein a same key derivation function is used to create the first and second cryptographic keys.

11. The method according to claim 1 , wherein different additional input parameters are used for a key derivation function to create the first cryptographic key and the second cryptographic key.

12. The method according to claim 11 , wherein parameters created during authentication are used as additional input parameters for the key derivation function.

13. The method according to claim 12 , wherein at least one of previously created first and second cryptographic keys are used as additional input parameters for the key derivation functions.

14. A mobile radio system, comprising:

at least one mobile radio terminal situated in a visited communications network, in which authentication key material resulting from an authentication are stored;

a first computer of the visited communications network, wherein the first computer is a first application server computer;

a computer of a home communications network in which computer are stored the same authentication key materials, resulting form the authentication, as are stored in the at least one mobile radio terminal; and

at least one second computer,

wherein the at least one mobile radio terminal and the computer of the home communications network each have a crypto unit for creating a first cryptographic key and a second cryptographic key by using the authentication key materials,

wherein one or both of the at least one mobile radio terminal and the computer of the home communications network comprise a transmitter which transmits the first cryptographic key to the first computer, and wherein one or both of the at least one mobile radio terminal and the computer of the home communications network comprise a transmitter which transmits the second cryptographic key to the second computer,

wherein the first cryptographic key and the second cryptographic key are created such that

no inference can be made from the first cryptographic key as to the second cryptographic key,

no inference can be made from the second cryptographic key as to the first cryptographic key, and

no inference can be made from the first cryptographic key or from the second cryptographic key as to the authentication key materials;

wherein the first computer of the visited communications network comprises memory for storing the first cryptographic key, and

wherein the second computer is an application server computer comprising memory for storing the second cryptographic key.