To output a key sequence of arbitrary length, and to control adequate security level in accordance with the length of the key sequence to be output.
An encryption apparatus for stream encryption includes: an internal state renewing function which generates an initial key and an initial vector and renews the internal state when a key sequence is output; an internal state memory means which memorizes the internal state output from the internal state renewing function; a key sequence generating function which reads the internal state memorized in the internal state memory means and outputs the key sequence; and an exclusive OR computing element which outputs a ciphertext by performing an exclusive OR operation of the key sequence output from the key sequence generating function and an input plaintext. The key sequence generating function is constituted by linking the arbitrary number of preprocessing blocks and function blocks.
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