A method of distributing an encoding/decoding program and a symmetric key in a security domain environment, and a device divider and data injector therefor are provided. The method includes selecting, at a highest-level security domain, an encoding/decoding program and a symmetric key to be distributed to a plurality of lower-level security domains; dividing the selected encoding/decoding program and the symmetric key into pieces as many as the number of lower-level security domains; and distributing the divided encoding/decoding program pieces and the symmetric key pieces to devices belonging to the lower-level security domains.