The key acquisition unit (411) acquires the decryption key ski of the conversion source pair and the public key pkj of the conversion destination pair among the plurality of pairs of the decryption key and the public key. The conversion key generation unit (412) encrypts the decryption key ski acquired by the key acquisition unit (411) with the public key pkj, and converts the ciphertext encrypted with the public key pki of the conversion source pair into the conversion destination. A conversion key rki → j for conversion into a post-conversion ciphertext that can be decrypted with the decryption key skj of the pair is generated. The output unit (413) outputs the conversion key rki → j generated by the conversion key generation unit (412).
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