Patent ID: 7917766

Claim:
A decoder to decode a stegotext generated by modulating the log power spectrogram of a covertext signal with a key (K), multiples of the key having been added or subtracted in the log domain to the covertext power spectrogram in accordance with data of the watermark code with which the stegotext was generated, and returning the modulated power spectrogram into an original domain of the covertext, the decoder comprising: a Fast Fourier transformer circuit to carry 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; a key provider circuit which provides the key with which an original log power spectrogram of the covertext signal was encoded; a calculator circuit to subtract in the log power domain positive and negative multiples of the key from blocks of the log power spectrogram and evaluating a probability of the results of the subtractions representing an unmodified block of covertext in accordance with a predetermined statistical model; and an extractor circuit to recover the encoded data from an output of the calculator.