Public-key cryptographic apparatus handling ciphertext by public-key

摘要

The invention provides a public-key cryptographic apparatus which does not leak information regarding a plaintext and can prevent an increase in block length. A quadratic residue calculation circuit calculates a residue when the square of the lower n-1 bits of the plaintext of n bits is divided by public-key, and an exclusive OR circuit calculates an exclusive OR of the least significant bit of a result of the calculation and the most significant bit of the plaintext. Then, public-key encipherment such as the RSA cryptosystem or a modified Rabin cryptosystem is performed twice repetitively for totaling n bits of the output of exclusive OR circuit and the lower n-1 bits of plaintext by public-key enciphering circuits so as to make it impossible to estimate the most significant bit of the plaintext from the ciphertext.