Ciphertext conversion system, ciphertext conversion method, and non-transitory computer readable medium

Abstract

A ciphertext conversion system includes a conversion device () including a conversion unit (). The conversion unit () performs encryption of a public-key encryption scheme using a public key and a decryption enabling condition, so as to generate a transformed public-key ciphertext and a key corresponding to the transformed public-key ciphertext, and calculates, as at least part of a transformed symmetric-key ciphertext, an exclusive-OR of a first ciphertext, a value calculated by performing encryption of a symmetric-key encryption scheme using a sub secret key and second auxiliary information, and a value calculated by performing encryption of the symmetric-key encryption scheme using the key corresponding to the transformed public-key ciphertext and third auxiliary information, where the first ciphertext is an exclusive-OR of a value calculated by performing encryption of the symmetric-key encryption scheme using first auxiliary information and a main secret key, a value calculated by performing encryption of the symmetric-key encryption scheme using the second auxiliary information and the sub secret key, and a plaintext.