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
77 Claims
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1-52. -52. (canceled)
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53. A decoder for decoding a stegotext generated by modulating the log power spectrogram of a covertext signal with at least one key (K), multiples of 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 returning the modulated power spectrogram into the original domain of the covertext the decoder comprising:
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a Fast Fourier transformer for carrying 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 which provides the key or keys with which the original log power spectrogram of the covertext signal was encoded; a calculator for subtracting in the log power domain positive and negative multiples of the key or keys from blocks of the log power spectrogram and evaluating the probability of the results of such subtractions representing an unmodified block of covertext in accordance with a predetermined statistical model; and an extractor for recovering the encoded data from the output of the calculation means. - View Dependent Claims (54, 55, 56, 57, 58, 59, 60, 61, 62)
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63. A method of decoding a stegotext generated by modulating the power spectrogram of a covertext signal with at least one key (K), multiples of the key or keys 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 to the original domain of the covertext, the method comprising:
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carrying 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; providing the key or keys with which the log power spectrogram of the original covertext signal was encoded; subtracting in the log power domain positive and negative multiples of the key or keys from blocks of the log power spectrogram and evaluating the probability of the results of such subtractions representing an unmodified block of covertext in accordance with a predetermined statistical model; and recovering the encoded data from the output of the calculation means. - View Dependent Claims (64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76)
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77. A decoder for decoding a stegotext generated by modulating the log power spectrogram of a covertext signal with at least one key (K), multiples of the 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 returning the modulated power spectrogram into the original domain of the covertext, the decoder comprising:
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transformation means for carrying 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; means for providing the key or keys with which the original log power spectrogram of the covertext signal was encoded; calculation means for subtracting in the log power domain positive and negative multiples of the key or keys from blocks of the log power spectrogram and evaluating the probability of the results of such subtractions representing an unmodified block of covertext in accordance with a predetermined statistical model; and extraction means for recovering the encoded data from the output of the calculation means.
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Specification