STEGOTEXT ENCODER AND DECODER
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Abstract
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.
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Citations
60 Claims
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1-52. -52. (canceled)
- 53. A watermark key generator for generating a key for watermarking covertexts, the generator comprising a generator for generating a two-dimensional noise pattern of predetermined height and width, and a filter for filtering the noise signal in one dimension with a cut-off frequency which varies with position in the pattern.
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57. A method of generating a key for watermarking covertexts, the method comprising generating a two-dimensional noise pattern of predetermined height and width, and filtering the noise signal in one dimension with a cut-off frequency which varies with position in the pattern, the variation with position being substantially inversely with the modulus of the coordinate in the said dimension of the position relative to a reference point in the pattern and the filtering acting as a low pass filter to filter the noise pattern so that it has no frequency components with time scales shorter than a threshold time scale τ
- , where τ
=C|t|, where C is a positive constant in the range of 0.05 to 0.4 pixels per cycle per pixel, and t is the coordinate in the said dimension relative to the reference point. - View Dependent Claims (58, 59, 60)
- , where τ
Specification