Both of today's issued patents address techniques for watermarking or steganography (from the Greek for "hidden writing"). Assigned to Activated Content Corporation, the first patent concerns a stegotext encoder and decoder. Assigned to Sarnoff Corporation, the second patent discloses techniques for secure robust high-fidelity watermarking.
7,298,841, "Stegotext encoder and decoder," assigned to Activated Content Corporation.
The invention comprises an encoder for encoding a stegotext and a decoder for decoding the encoded stegotext, the stegotext being generated by modulating the log power spectrogram of a covertext signal with at least one key, the or each key having been added or subtracted in the log domain to the covertext power spectrogram in accordance with the data of the watermark code with which the stegotext was generated, and the modulated power spectrogram having been returned into the original domain of the covertext. The decoder carries out Fast Fourier Transformation and rectangular polar conversion of the stegotext signal so as to transform the stegotext signal into the log power spectrogram domain; subtracts in the log power domain positive and negative multiples of the key or keys from blocks of the log power spectrogram and evaluates the probability of the results of such subtractions representing an unmodified block of covertext in accordance with a predetermined statistical model.
7,298,865, "Secure robust high-fidelity watermarking," assigned to Sarnoff Corporation.
For each small image region (in space and time), a measure of perceptual transparence of each of a set of possible watermark carrier modulations is used to choose a subset of such modulations, from which a secure random number generator selects, for each image region, a single carrier, modulations of which carry the watermark data.