PUBLIC KEY CRYPTOGRAPH AND KEY SHARING METHOD

摘要

A cryptograph communication method using public key cryptograph in which a sender creates a cryptogram by using a public key of the receiver by means of a sender device (100) and transmits it to the receiver device (200) through a communication line (300), and the receiver decrypts the cryptogram by using a secret key, wherein a procedure for encryption and decryption is so established to provide the features of security both the Rabin cryptograph which is one-way against chosen-plaintext attacks on the condition of difficulty of the problem of fractionization into prime factors and the ElGamal cryptograph which is strongly secret against chosen plaintext attacks on the condition of difficulty of the problem of Diffie-Hellman determination. Further while keeping secret the true plaintext space, the size of the plaintext space is reduced in order to use the space for key delivery of common key cryptogram. Thus a public key encrypting method and a key sharing method using the same are provided in which it is possible to prove the security on the condition of the problem more difficult than conventional, and high efficiency processing in the calculation for encryption/decryption is possible.