H.264/AVC視訊標準已被廣泛應用於多媒體資料傳輸。為了顧及資訊的保密性，以避免影像內容被未經授權的人觀看，提供H.264充分加密的技術變成十分重要。
在本篇論文中，我們提出一改良的加密方法。此法選擇性加密位元碼流上的移動向量差值(MVD)、量化參數(delta quantization parameter)以及適應性變動長度編碼(CAVLC)三部分。根據我們設計的方法，在接收端我們須使經加密的碼流於解密前能夠先被正確地解碼，因此我們採取完全相容於原本碼流結構的加密方法。除此之外，為了維持加密後與加密前具有相同的位元率，維持碼流長度不變是我們加密方法的特性之一。因為加密後不會增加位元數量，因此適合在異質網路上進行即時傳輸。我們以進階加密標準(AES)演算法產生加密所需的金鑰(key)，而金鑰號碼與每張畫面的序號之間擁有一致性。因此，安全性獲得更大提升，另一方面則確保解碼過程的同步性。
論文以四種具不同畫面特性的基準影片做實驗測試。在SSIM指標分析部分，所提方法得到的平均值為0.202，此值低於Shahid等人所提方法的0.233，因此我們提出的方法更能降低視覺品質。金鑰敏感度的測試部分，實驗結果顯示以改變1位元的金鑰解密得到的畫面依然保有良好的保密效果。在執行時間部分，我們的演算法對整體H.264增加的時間低於0.5%，因此可使用於即時傳輸。以上測試結果顯示本論文提出的分法適合於實際應用。

The H.264 video coding standard has been widely applied to multimedia data transmission. To prevent the contents of video from being recognized by someone unauthorized, providing sufficient security to H.264 has become more and more important.
In this thesis, an improved H.264/AVC video encryption method is proposed. As compared with the Shahid’s method, more data are encrypted. And this makes the outcome more desirable. In the proposed scheme, selective encryption (SE) is performed in the motion vector difference (MVD), delta quantization parameter (delta QP), and the context-based adaptive variable length coding (CAVLC) modules. Owing that the encrypted stream should be decoded correctly before decryption, a format-compliant method is presented. Furthermore, in order to keep the bitrate unchanged, keeping the same stream length is another important characteristic of our proposed scheme. Since there is no escalation in bitrate, the proposed encryption algorithm is suitable for real-time multimedia streaming over heterogeneous networks. The encrypting key is generated by using the advanced encryption standard (AES) algorithm, and every frame has its own unique key.
Four benchmark video sequences with different combinations of motion, texture and objects were used for experimental evaluation of the proposed scheme. The structural similarity (SSIM) index, a reference value to image quality, of the proposed scheme is 0.202 which is lower than 0.233 of the Shahid’s scheme. The key security is shown reliable under the test of only 1 bit change. The extra computation needed by our scheme is less than 0.5%, so it is feasible for real-time transmission. The above performance on security and sensitivity demonstrates that our proposed scheme is a good candidate for practical applications.

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