Systems, methods, and data structures for transparently embedding non-compliant data in a data stream are described. One method includes embedding random encryption/decryption information into an MEPG multimedia, video, or audio stream transparently to an MPEG decoder in an ISO/MPEG 13818-1 compliant system to control access. The invention works for variable length data streams and involves a PES header. Spare bytes, stuffing bytes, or additional bytes as defined in the MPEG specification for PES headers are used to store key information by a computer capable of modifying the original data stream. There is no need for a parallel data stream for the key information because the key information is transparently inserted directly into the data stream. Additional information is embedded into the data stream that can be used for encryption/decryption without having to modify other components in a playback system.