FIELD: ciphering information stream (voice signal or data) to be transmitted in mobile communication system. SUBSTANCE: mobile communication system transmits information in compliance with multiple- access mode with time- division of channels between network (MSC, BSS) and particular mobile station (MS) out of a number of mobile stations. Proposed ciphering methods are used when information (B1, B2) of particular user is to be transmitted in two or more time spaces (TS1, TS2) within one frame. According to proposed ciphering methods various parameters (Kc, FN, PS) used in the course of ciphering are modified depending on sequence number of each of frame time spaces being used. For example, ciphering key (Kc) and frame number (FN) are modified depending on sequence number of respective time space (TSn) and known ciphering algorithm (A5) is used to obtain modified ciphering sequence (PSm'). Proposed method makes it possible to avoid essential changes in signaling protocol of system and in hardware used. EFFECT: enhanced ciphering reliability. 6 cl, 4 dwg
