隨著時代的進步，網際網路也逐漸的普及，網路上的交易更是日益繁盛，電子商務也蓬勃的發展，但隨著這些網路行為的發展，網路安全的問題慢慢受到一般大眾的重視，由於網際網路上的便利性，愈來愈多的人以網路上的交談取代了面對面的溝通，然而在公開的網際網路上，就會讓有心人有機可乘，一個惡意的攻擊者，可能擷取或篡改你的訊息內容，更可能直接偽冒你的身份，這些都是網路上的行為所會遭遇到的問題，於是便發展出了CA-PKI的架構，讓網路上的使用者有個可以信任的公開架構，在這個公開架構的憑藉下互相信任，每個合法的使用者皆須由第三公正單位，也就是CA，發給合法的使用者身份憑證，藉由大家都相信的第三公正單位，使得每個使用者都能有所憑藉而互相信任。
　　電子憑證是由CA簽章發行的文件，可代表一個人的身份，電子憑證又可分為身份憑證及屬性憑證，身份憑證用來證明一個人的身份，及其擁有之公開金鑰，屬性憑證則用來作為存取控制的授權憑證，證明使用者擁有存取的權利，在PKI架構下，透過這些電子憑證，來驗證一個使用者的身份和權力，本文分成三個部份來討論，第二章描述在ad hoc network中憑證的使用，第三章則探討憑證間的關聯，第四章再介紹憑證在其他環境中之應用。

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