Abstract: An input content signal—representing audio or video—is steganographically encoded to hide plural-bit auxiliary data therein. The encoding operation changes over time, e.g., by using different “key” data for successive portions of the content signal. In some embodiments, the key data used in one portion of the signal is a function of auxiliary data decoded from a preceding portion. Consequently, to successfully decode the auxiliary data, the decoder must have successfully decoded a previous portion. In essence, decoding instructions are sent “on the fly” in such embodiments. In some embodiments, the auxiliary data is changed in temporally adjoining portions of the content signal—e.g., in successive frames of video or blocks of audio. In other embodiments, the key is changed less frequently, e.g., on a daily basis. External events may also trigger changing of the key data.