Secure Spread Spectrum Watermarking based on Trellis Coding

• Main Authors: Cheng-Chung Sun, 孫正忠
• Other Authors: Kuen-Tsair Lay
• Format: Others
• Language: en_US
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/17443224180632437492

碩士 === 國立臺灣科技大學 === 電子工程系 === 91 === ABSTRACT Digital watermarking is a technique that aims at hiding a message signal in a multimedia signal (e.g. speech, image, video signal, etc.) for copyright claim, authentication, device control, and broadcast monitoring, etc. In this thesis, we focus on the topic of embedding watermarks into still images. We hope that the embedded watermark induces low impairment to the original image. We also hope that the embedded watermark possesses enough robustness against tampering (intentional or unintentional) su®ered when it is stored or transmitted. We propose to scramble the watermark with pseudo-noise (PN) or orthogonal codes before it is embedded into an image. In terms of the communication scenario, this operation has the e®ect of spectrum spreading, which turns the watermark signal into additive white Gaussian noise (AWGN). In the extraction of the watermark, the aforementioned PN codes are applied once more to the watermarked image, in the meantime spectrum-spreading the original image and thus turning it into AWGN. This original-image-turned AWGN makes some watermark bits been detected incorrectly. In fact, there are some things more that are turned into AWGN, in addition to the original image. The e®ects of tampering operations such as quantization, compression, filtering, or even jamming, are reduced to AWGN as well. It is natural to consider the possibility of incorporating error correction coding (ECC) into the watermarking scheme to facilitate reduction of the watermark bit error rate (BER). We expect that among the various ECC techniques, trellis based schemes (e.g. trellis-coded modulation (TCM) and turbo code) should be adopted. Due to the similarity between the PN/orthogonal coded watermarking and the spread-spectrum communication, it is natural that, following similar derivations regarding data BER in the latter, we derive certain explicit quantitative relationship regarding the trade-o® between the watermark BER and the distortion su®ered by the original image. Finally, we want to point out that the PN codes can be used as keys for secrecy purpose. If we design a scheme to properly set the accessibility of the PN codes to various users of the watermarked images, this thesis may lead to a work that provides the functions of secrecy and authentication, in addition to copyright claim. Keywords : watermark, pseudo-noise code, trellis-coded modulation, turbo code, spread spectrum