1. Field of the Invention
The present invention relates to a communication device and a communication control method.
2. Related Background Art
Typically, since control data and user data are transferred via radio bearer in mobile communication systems, a cipher process such as transmitting encrypted data such as encrypted control data and user data and then decrypting on the reception side is carried out. The cipher process prepares multiple key seeds for generation of keys to be used for encryption and decryption, and partially changes the key seed for each frame, thereby improving confidentiality. Parameters to be used for encryption such as key seeds and part thereof are called cipher parameters.
According to International Mobile Telecommunications-2000 (IMT-2000), since a key is changed for each frame when encrypting and decrypting transparent data, the frame number for each frame is used as a seed for generation of a key (e.g., see ‘6.6.3 Ciphering method, 6.6.4 Input parameters to the cipher algorithm’, 3GPP TS 33.102 V3.13.0, December, 2002.)
In addition, when newly starting a cipher process or when modifying and restarting the cipher process, a requested start time for starting the cipher process and a part of key seeds to be used after that requested start time are notified with a message (e.g., see ‘8.2 Radio Bearer control procedures, 8.6.4.3 RB information to setup’, 3GPP TS 25.331 V3.13.0, December, 2002).
The hyper frame number (HFN), which forms the upper bits of the frame number, is partially used for part of the key seeds. The requested start time is specified using a connection frame number (CFN), which forms the lower bits of the frame number. The CFN is incremented by ‘1’ per 10 milliseconds from an initial value of ‘0’, and is reset to ‘0’ when having reached ‘255’, which forms a CFN cycle. In other words, the CFN provides a cyclic timing of 256 frames (8 bits). The HFN is incremented by ‘2’ per CFN cycle.
When transmission of a message for notification of a part of the key seeds and a requested start time fails, that message is then retransmitted. As a result, the requested start time for a message transmitter may differ from a start time recognized by a message receiver based on that message. The HFN is incremented by ‘2’ per CFN cycle. Therefore, the start time difference between the transmitter and the receiver may lead to difference in key seeds therebetween, making it impossible to normally carry out the cipher process.
More specifically, as shown in FIG. 1, a radio network controller 230 transmits to a mobile station 210 a request message (RADIO BEARER SETUP) 201 to request the mobile station 210 to set a cipher parameter and a requested start time. The mobile station 210 sets ‘A’ to HFN as the cipher parameter, and ‘a’ to CFN as the requested start time. The mobile station 210 transmits to the radio network controller 230 a response message (RADIO BEARER SETUP COMPLETE) 202 including part of the set HFN and the requested start time.
In this manner, the mobile station 210 and the radio network controller 230 carry out an initial process before starting the cipher process. The requested start time ‘a’ for the mobile station 210 equals an actual time ‘n1’. The actual time means a one-way increasing actual time other than a cyclic time such as the CFN.
If transmission of a response message fails, the mobile station 210 then retransmits a response message a predetermined number of times. If multiple retransmissions are made taking a long time, the radio network controller 230 may receive a response message after the actual time ‘n1’. A single CFN cycle is 256 frames. Therefore, the radio network controller 230 erroneously recognizes that the requested start time ‘a’ represented by the CFN included in the message is an actual time ‘n2’, which is a single CFN cycle after the actual time ‘n1’.
As a result, since a single CFN cycle from ‘n1’ has elapsed, the mobile station 210 sets a one-incremented value of ‘A+1’ to the HFN at the time ‘n2’. On the other hand, the radio network controller 230 sets to the HFN, ‘A’ generated from a part of the HFN notified with the response message. Accordingly, at the time ‘n2’, the mobile station 210 and the radio network controller 230 start the cipher process using different cipher parameters.
As a result, the mobile station 210 and the radio network controller 230 cannot correctly decrypt encrypted data. Then received data results in abnormal. Such difference in cipher parameters is called ciphering shift. Such a ciphering shift may occur during a radio access link being kept between the mobile station 210 and the radio network controller 230, resulting in failure in effective data reception/transmission.