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

Abstract

A ciphertext conversion system () includes a conversion key generation device () and a conversion device (). The conversion key generation device () performs encryption of a public-key encryption scheme using a public key and a first decryption enabling condition, so as to generate a first converted public-key ciphertext and a key corresponding to the first converted public-key ciphertext, performs encryption of a symmetric-key encryption scheme using a second symmetric-key encryption secret key as a plaintext and the key corresponding to the first converted public-key ciphertext as a secret key, so as to generate a first partial conversion key, and calculates, as a second partial conversion key, an exclusive-OR of a result of performing encryption using a first symmetric-key encryption secret key and first auxiliary information and a result of performing encryption using the second symmetric-key encryption secret key and second auxiliary information. The conversion device () calculates, as at least part of a converted symmetric-key ciphertext, an exclusive-OR of a symmetric-key ciphertext and the second partial conversion key, where the symmetric-key ciphertext is an exclusive-OR of a plaintext and a result of performing encryption using the first symmetric-key encryption secret key and the first auxiliary information.