PUBLIC-KEY CRYPTOGRAPHIC APPARATUS HANDLING CIPHERTEXT BY PUBLIC-KEY

摘要

The invention provides a public-key cryptographicapparatus which does not leak information regarding aplaintext and can prevent an increase in block length.A quadratic residue calculation circuit calculates aresidue when the square of the lower n-1 bits of theplaintext of n bits is divided by public-key, and anexclusive OR circuit calculates an exclusive OR of theleast significant bit of a result of the calculationand the most significant bit of the plaintext. Then,public-key encipherment such as the RSA cryptosystem ora modified Rabin cryptosystem is performed twicerepetitively for totaling n bits of the output ofexclusive OR circuit and the lower n-1 bits ofplaintext by public-key enciphering circuits so as tomake it impossible to estimate the most significant bitof the plaintext from the ciphertext.