影像與視訊處理軟體的普及讓數位資料內容的真實性遭到若干懷疑，近期許多研究試圖偵測多媒體資料是否曾被編輯或竄改，以及相對應的反偵測方式，藉由兩者的交互改進協助提昇多媒體資料的真實性。反偵測技術尋求現有偵測方式的弱點，將編輯後的多媒體資料內可能留下的某種特徵移除以讓偵測失敗。本論文提出利用畫面增刪攻擊而引入的兩種特徵，即異常編碼模式與H.264/AVC整數轉換中的量化係數分佈，實作視訊編輯竄改的反偵測方法。首先，根據連續畫面編碼模式之間的關係，在RDO (Rate Distortion Optimization)中限制不合理數量的畫面內預測模式。接著，使用編碼時的QP (Quantization Parameter)與位元率的關係，預測異常畫面內應有的整數轉換量化係數分佈，再將過多的非零係數逐步調整至預測的目標分佈，即藉由改變重建畫面內容移除可偵測特徵。完成以上步驟之後，將編碼模式與處理後的係數儲存起來，於編碼第二次時複製回去得到反偵測影片。實驗結果顯示，我們的方法成功地掩飾了視訊中的畫面增刪攻擊。論文最後也分別討論了兩個偵測方法，即使用去方塊濾波能量進行偵測，以及使用位元率控制應給定QP之偵測方法。雖然在這些方法中存在了某些限制條件，但是在合適的情況下仍具有值得研究的偵測效果。

Digital multimedia data can be edited easily by the powerful software these days. Therefore, many digital forensic techniques have been developed to authenticate multimedia content. Anti-forensic techniques are also proposed to remove editing traces. These anti-forensic methods study the weaknesses of existing detection algorithms to make editing undetectable. This thesis presents an anti-forensic method employing two features, abnormal coding modes and distribution of quantized transform coefficients, which are generated by the frame/scene adding or deletion. First, the coding modes are examined in the Rate Distortion Optimization (RDO) process to limit the use of intra coding blocks in certain frames. Then, the relationship between QP and rate are examined to predict the reasonable distribution of quantized coefficients. Next, we change the reconstruction content to erase the detection features by adjusting the quantized coefficients according to the predicted distribution. Following the above steps, we store the coding modes and the processed coefficients and then copy them back in the second encoding process. The experimental results show that our scheme can successfully eliminate the features. Finally, we discuss two possible detection methods, deblocking energy and examining QP values in the rate control. The former method detects the forgery by checking the deblocking intensity of the reconstruction frames, and the latter method uses a known rate control mechanism to determine whether a correct QP value is assigned. These methods are effective in certain appropriate conditions and deserve more discussions.

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