A key scheduler, which may selectively generate an encryption round key and a decryption round key corresponding to an initial round key, which may have a variable key length. The key scheduler may include a key storage unit, a key calculating unit, and a key output unit. The key storage unit may receive and store calculation key data items or storage key data items as input key data items, and may output the stored input key data items as the storage key data items. The key calculating unit may output the calculation key data items as the calculation result. The key output unit may select units of the input key data items and the storage key data items in response to output control signals, and may output them as an encryption round key or a decryption round key.