多媒體安全：加密，浮水印，及播送技術與應用

摘 要

隨著無線通訊與網路技術的進步，多媒體資料在網路上的傳輸機會與日俱增。由於多媒體資料的豐富性和多樣性，以及無線網路普及率節節升高，多媒體資料在不同使用者之間的分享與交換行為日益普遍。然而，多媒體資料在傳輸的過程中，若沒有加以保護，也就是所謂的多媒體安全技術若沒有適當發展，多媒體資料的內容將被任意存取與分享，而侵犯多媒體擁有者的著作權。在本論文中，多媒體安全的各種技術與應用，將被廣泛的討論，尤其是多媒體加密，多媒體浮水印，以及多媒體播送技術與應用。論文中我們將提及多媒體安全中目前極具挑戰性的問題，並且針對這些問題提出我們的看法與解決方案。

在本論文中，我們將從多媒體安全的第一道防線-多媒體加密，開始討論起。因為加密過後的多媒體資料在傳輸的過程當中可能遇到具有錯誤特性的網路環境，我們提出了一套以媒體赫序(Media Hash)為基礎的視訊錯誤回復機制。接著，我們會著墨於多媒體安全的第二道防線，多媒體浮水印。我們提出了一套強健性的多媒體浮水印技術，能夠抵抗幾何攻擊以及浮水印預估攻擊。此外，我們還提出了一套抵抗時間軸非同步攻擊的視訊浮水印同步技術，用來增加多媒體浮水印技術的強韌性。當多媒體安全的兩道防線都建立了，一個合適的多媒體散佈架構是相當需要的。在本論文中，我們提出了與AACS(advanced access control system)相容的結合式加密與多媒體指紋嵌入架構技術。與該架構相關的安全議題，挑戰，與相對應的解決方案也在此一並討論。

多媒體加密技術在安全的存取多媒體資料技術中扮演了第一道防線。傳統的加密技術能夠提供安全性的保障，但是連任何一個位元錯誤都無法容忍，這在多媒體傳輸應用中並不實用。為了達到強健性的多媒體加密技術，在多媒體傳輸中必須能夠具備錯誤回復能力。本論文中提出了一套利用嵌入於殘餘資料之以多媒體赫序為基礎的結合式視訊加密及傳輸機制，期望達到對於遺失視訊封包的移動向量預測及補償，同時能夠保持格式相容和密碼學上的安全性。有趣的是，既然視訊區塊赫序能夠保存精華的視訊內容，利用搜尋類似的區塊，移動向量預測隱含了強健性媒體赫序比對精神，這就是本方法的獨特性。

多媒體保護的第二道防線是浮水印技術。現存浮水印技術最主要的缺點是對於廣泛的幾何攻擊抵抗能力。此外，我們發現現存文獻若能夠抵抗預估浮水印攻擊，便降低了抵抗幾何攻擊的能力。基於以上的看法，在本論文中，我們提出了同時能夠抵抗幾何攻擊及浮水印預估攻擊的技術。本技術主要包含三個部份：(1)強健的碎塊產生和以碎塊為基礎的浮水印技術以抵抗幾何攻擊，(2)建立以媒體赫序和內容為基礎的浮水印以抵抗浮水印預估性攻擊，(3)提出一套以錯誤機率為導向的浮水印偵測技術，用來決策浮水印是否存在，以達到錯誤正向及錯誤負向判斷的取捨。

此外，同步問題在視訊浮水印技術中是一個具有挑戰性的問題。我們提出了一套基以訊框輪廓統計特性為基礎的時軸同步技術。訊框輪廓在x和y方向上以平均值和變異數代表一個訊框的特性，如此得到的統計值可以當作一個訊框的特徵。一連串的視訊訊框可以在解碼端進行訊框特徵比對，以達到再同步效應。在接收端，諸如：訊框置換，遺失，和插入等時軸非同步攻擊，藉由本技術提出的訊框統計特比對，可以達到再同步效應，以利於最後的浮水印偵測。

一旦多媒體安全的第一和第二道防線都經過適當的設計，一個安全的多媒體播送架構是必要的。本論文中提出了一套結合式加密和多媒體指紋嵌入的方法，並且可以和AACS(advanced access control system)相容。由於目前AACS是對多媒體存取控制的領導技術，並且是多家多媒體播送公司所共同制定的標準，因此我們發展的技術期望與這樣的標準在概念上相容。在本技術中，我們探討了許多可能的攻擊點以及抵抗措施，此外，我們提出了多重攻擊點所產生的串謀攻擊以及合適的抵抗對策。在本論文中，(1)我們提出了在不同點之間的多媒體加密技術，(2)我們提出了可複寫式的多媒體指紋嵌入技術，用以抵抗多重攻擊點所產生的串謀攻擊，(3)我們設計了SPSM(Spectral Perceptual Security Metric)來評估多重加密所能夠達到的安全程度。

關鍵字 ¾ Multimedia Security, Encryption,Watermarking, Distribution

Multimedia Security: Encryption, Watermarking, and Distribution Technologies and Applications

Abstract

With the development progress of wireless communications and networking technologies, the transmission opportunities of multimedia data become increasing. Because of the planarity and diversity of multimedia, and the increasing rate of wireless network popularity, the sharing and exchanging actions among different users are common. However, during the transmission process, if the multimedia data is not properly protected, i.e. if the multimedia security technologies are not properly developed, multimedia content would be accessed and shared without any limitation, forcing the copyright of the multimedia content owner. In this dissertation, the multimedia applications and technologies would be widely discussed, especially multimedia encryption, watermarking, and distribution applications and technologies. In this dissertation, the recent most challenging problems in multimedia security would be addressed, discussed, and properly dealt with.

In this dissertation, the multimedia security issue is discussed from the first defense line: multimedia encryption. Because the protected encrypted multimedia data would be distributed over the error-prone network environment, a media-hash based video error resilient technique is proposed. Next, the second defense line, multimedia watermarking, is addressed. A robust image watermarking technique against geometric distortions and watermark estimation attack (WEA) is proposed. In addition, a temporal re-synchronization technique for video watermarking is discussed to improve the robustness. While the two defense lines for multimedia security are well-developed, a proper secure multimedia distribution framework is necessary. An advanced access control system (AACS)-compatible joint encryption and fingerprinting technique is proposed to form a proper framework. Some security issues, challenges, and solutions are addressed.

Media encryption technologies actively play the first line of defense in securing the access of multimedia data. Traditional cryptographic encryption can achieve provable security but is unfortunately sensitive to a single bit error, which will cause an unreliable packet to be dropped creating packet loss. In order to achieve robust media encryption, the requirement of error resilience can be achieved with error-resilient media transmission. This dissertation proposes a video joint encryption and transmission (video JET) scheme by exploiting media hash-embedded residual data to achieve motion estimation and compensation for recovering lost packets, while maintaining format compliance and cryptographic provable security. Interestingly, since video block hash preserves the condensed content to facilitate search of similar blocks, motion estimation is implicitly performed through robust media hash matching -- which is the unique characteristic of our method.

The second defense line of multimedia protection is watermarking. The major disadvantage of existing watermarking methods is their limited resistance to extensive geometric attacks. In addition, we have found that the weakness of multiple watermark embedding methods that were initially designed to resist geometric attacks is their inability to withstand the watermark-estimation attacks (WEAs), leading to reduce resistance to geometric attacks. In view of these facts, this dissertation proposes a robust image watermarking scheme that can withstand geometric distortions and WEAs simultaneously. Our scheme is mainly composed of three components: (i) robust mesh generation and mesh-based watermarking to resist geometric distortions; (ii) construction of media hash-based content-dependent watermark (CDW) to resist WEAs; and (iii) a mechanism of false positive-oriented watermark detection, which can be used to determine the existence of a watermark so as to achieve a trade-off between correct detection and false detection.

In addition, synchronization for video watermarking is a challenging problem. We propose a novel temporal synchronization method for video watermarking by matching the profile statistics. The profile statistics, represented by the characteristic parameters such as position mean and variance in x- and y-directions, of a frame in a video sequence can easily be calculated and sent as the side information to the receiver. At the receiving end, temporal attacks such as transposition, dropping, and insertion can be detected by comparing the side information and the characteristic parameters calculated from the received video.

While the two defense lines for multimedia security are well-developed, a proper secure multimedia distribution framework is necessary. In this dissertation, a new multimedia joint encryption and fingerprinting (JEF) scheme embedded into the advanced access control system (AACS) is proposed. AACS is selected because it has been the leading technology in access control developed by many significant companies for multimedia distribution. In this AACS-compatible JEF system, many attack points exist and can be explored to defend it. Furthermore, multiple attack points can form multi-point collusion attacks, which also engage the proposed system. In this paper, (i) we propose multimedia encryption at different points to resist some attacks points; (ii) we propose rewritable fingerprint embedding (RFE) to deal with some multi-point collusion attacks; (iii) we design SPSM to evaluate the degree of security which multiple encryptions are applied.

Key words ¾ Multimedia Security, Encryption, Watermarking, Distribution