Variable-length key cryptosystem

Abstract

A variable-length key cryptosystem is provided, in which the amount of parameters for generating a key to be shared is small, security is high, and calculation cost is small. The length of a plain text inputted from a data input partis detected by a data string length detection processing part. A variable-length key generation processing partgenerates an encryption key with a required arbitrary length, using an initial character string and a conversion rule, based on the length of a plain text. A conversion character string generation processing partgenerates a converted character string from an initial character string by applying a conversion rule of increasing the length of a character string. Conversion is conducted until the length of a plain text is reached. An encryption key is not used twice. Therefore, an unused conversion character string is searched for, and a stream code processing partgenerates an encrypted text by using a generated encrypted string. A receiver systemgenerates a converted character string from an initial character string by the same conversion, based on the length of an encrypted text, thereby obtaining the same encryption key.