由於電腦的普及與網路的高速發展使電子媒體的傳輸趨於頻繁，連帶的引起媒體傳輸上的安全及著作權等問體的討論。而浮水印就是為了保護數位媒體(如影像、聲音等)在網路上的版權問題，藉由嵌入將來可作為識別之資料於數位媒體中且必須滿足不可察覺性(Imperceptibility)、強韌性(Robustness)等要求下，能夠使數位媒體在資源分享的網路上一旦被盜用可以證明其合法擁有權。
而浮水印的發展主要可分為加入空間域(Spatial domain)或加入頻率域(Frequency domain)兩種方式，各有其優缺點。為了對付目前各種攻擊方式，除了必須滿足上述的不可察覺性、強韌性之外，還必須能夠在多重浮水印的攻擊下充分的證實其來源可靠性。目前加密演算法及數位簽章已被廣泛應用在文件的保密上，為了使浮水印更趨於安全所以必須研究如何結合現有之技術來應用於本論文中。

所以本論文利用加密演算法及數位簽章的概念來找出位置及製作浮水印。在加密演算法方面，利用加密後的原始影像來找出加入浮水印位置，可增加影像合法性及減少浮水印被攻擊的機會;利用數位簽章的概念製造浮水印，可以使浮水印與影像間有依賴性，並且可結合CA來加入時戳以證明加入之時間以其抵抗各式攻擊。

With rapid growth of multimedia, that how to protect electronic data from attacking becomes an important issue to be considered. And digital watermarking is one scheme to hide copyright marks and information into digital media (such as audio, image and video) for copyright protection. And we need to be sure that the watermark still existed and could be recognized after extracted under compression and malicious attack.
In this paper, we implement the method that integrates encryption algorithm to find the positions where watermarks are going to be embedded into. This method is based on Block DWT (discrete wavelet transform) and we embed multi-watermark into Block DWT coefficients except the low frequency bands. And in this paper, we try to use the concept of digital signature to produce our watermark. Finally, we compare the results of experiments under different attacking.

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