| 摘要 |
PURPOSE:To enhance the secrecy of transmission contents and to facilitate operation of a system by using three-layered keys whose change periods are short, long and fixed. CONSTITUTION:In a broadcasting station 31, a program signal is ciphered in a scrambler 35. This ciphering is designated by a bit string (key Ks) from a code generator 37, and the key Ks is ciphered in a ciphering device 39 by a bit string (key K) from a code generator 41. Further, the key K is ciphered in a ciphering device 45 by an individual key KM of an individual key file 47. The key KM corresponds to each receiving station 33. A separating device 49 in the receiving station 33 separates said scrambled signal and the signal ciphered into the key Ks from each other. Algorithms having relations of reversed function are designated to corresponding ciphering devices and deciphering devices in both stations by the same key. Thus, the key Ks having a long period is ciphered by the common key K having a long period, and the key K is ciphered by the key KM which has a long period or is peculiar to the receiving station. |
