A server for performing an operation on a homomorphic ciphertext is configured to: receive a first homomorphic ciphertext, a public key, and a first hierarchical Galois key set from a client device; in response to a request for generating a second hierarchical Galois key set for performing a rotation operation on the first homomorphic ciphertext of the client device, generate the second hierarchical Galois key set, based on the received public key and hierarchical Galois key set; and when a decomposition operation for a first Galois key included in the second hierarchical Galois key set overlaps with a decomposition operation for a second Galois key, first perform the decomposition operation for the first Galois key, and then substitute the decomposition operation for the second Galois key with a result of the decomposition operation for the first Galois key.