COMPUTATION ON ENCRYPTED DATA USING SOMEWHAT FULLY HOMOMORPHIC ENCRYPTION WITHOUT BOOTSTRAPPING USING TRUSTED THIRD PARTY

An apparatus and method for performing an operation using encrypted data are provided. The method for performing an operation according to one embodiment includes performing a homomorphic operation using one or more ciphertexts that are homomorphically encrypted based on an encryption key; determining a count value for a ciphertext generated through the homomorphic operation based on count values for each of the one or more ciphertexts; requesting a key management apparatus, which holds the encryption key and a decryption key corresponding to the encryption key, to re-encrypt the generated ciphertext based on the determined count value; acquiring, from the key management apparatus, a ciphertext generated by re-encrypting the generated ciphertext through decryption based on the decryption key and encryption based on the encryption key; and determining a count value for the acquired ciphertext to be a preset initial value.