網路電話一直是網路技術中的重要應用之一，隨著Internet普及化及寬頻化的發展，人們除了選擇傳統PSTN電信網路，也可選擇更優惠的VoIP網路電話。
RTP(Real-time Transfer Protocol)為VoIP產業的基礎技術，用來處理具即時特性的資料，但RTP並未考慮安全性，因此使用者間的對話極有可能被竊聽。目前多數網路電話僅在軟體電話(soft phone)中提供的語音加密機制，並未實現於硬體資源缺乏的嵌入式網路電話上。
本論文著重於實做RTP語音資料的快速加密技術於嵌入式SIP終端設備(SIP User Agent)上，對語音串流封包進行加密動作，防止不肖人士竊取或竄改語音封包，以提升通話品質並確保通話的保密性。

VoIP technology is one of the important applications of the network. In addition to using traditional PSTN telephone, people can choose favorable VoIP telephone because of the Internet popularization and High speed Internet.
RTP (Real-time Transfer Protocol) is the foundation of VoIP, and it is suitable for applications to manage real-time transmission of multimedia data. But RTP does not provide any confidentiality, the eavesdrop of VoIP is easy to realize.
In this paper, we discuss how to design and implement an embedded SIP user agent that provide an fast secure encoding of RTP voice streaming. We use limited resources to implement RTP encoding mechanism to prevent anyone from eavesdropping .

中文摘要 I
ABSTRACT II
目次 III
圖目錄 V
表目錄 VII
1. 序論 1
1.1. 研究動機與目的 1
1.2. 論文架構 1
2. SIP概論與介紹 2
2.1. SIP概論與元件 2
2.2. 客戶端 3
2.3. 伺服器端 4
2.3.1. 代理伺服器 4
2.3.2. 註冊伺服器 5
2.3.3. 重新導向伺服器 5
2.4. SIP訊息 6
2.4.1. SIP請求訊息 6
2.4.2. SIP回應訊息 7
2.5. SIP運作流程 9
2.6. 即時傳輸協定(REAL-TIME TRANSPORT PROTOCOL) 10
2.7. 語音編碼 14
2.8. 網路電話竊聽 16
3. 開發環境 18
3.1. 伺服器端 18
3.2. 客戶端 19
3.2.1. 硬體架構 19
3.2.2. 軟體架構 23
3.2.3. SIP UA軟體運作流程 26
4. 系統設計與實作 29
4.1. 系統設計之流程 29
4.2. 通話金鑰傳遞流程 32
4.3. 加密策略 37
4.3.1. 語音封包部份 37
4.3.2. 通話金鑰部份 41
4.3.3. 數位信封機制(Digital Envelope) 44
4.4. 金鑰紀錄功能 47
4.5. 實驗與效能分析 50
5. 結論 53
參考文獻 54

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