Abstract: An enciphering apparatus embodying this invention comprises chaos generation means for generating a real-valued sequence along a chaotic orbit in accordance with a predetermined number of first common keys and a predetermined nonlinear map, bit generation means for performing predetermined binarization on each real value in the generated real-valued sequence based on a predetermined number of second common keys to generate a binary sequence, and logic operation means for executing a predetermined logic operation on a binary sequence of an input plaintext and the generated keystream sequence bit by bit to generate a binary sequence of a ciphertext. For example, the first common keys are an initial seed of a nonlinear map and a value of a parameter k, and the bit generation means acquires plural pieces of binary data for each real value in the real-valued sequence using values indicated by a plurality of second keys as thresholds and obtains exclusive OR of those binary data.