無線感測器網路(Wireless Sensor Network，WSN)是在特定區域部署大量低成本的感測器，
這些感測器會監測環境的變化且利用無線網路把資訊傳遞給基地台。
由於無線網路的公開特性，所以感測器在把資訊傳遞出去之前，必需先加密以免被竊聽而直接取得完整資訊。
在一般的無線網路中，比較常使用公開金鑰密碼學(Public Key Cryptography)來保護網路安全，但是無線感測器網路的資源限制使得公開金鑰密碼學不能直接應用。
反之，對稱式金鑰密碼學(Symmetric Key Cryptography)所需的資源比公開金鑰密碼學少很多，也更容易應用在無線感測器網路上。
但是，長時間使用單一固定金鑰，容易遭受到金鑰還原攻擊，使得網路的安全性受到威脅。
如果把叢集(Cluster)的概念拿到 WSN 來應用，我們將會有一些資源較豐富的感測器稱之為Cluster Head 。
因此，本篇論文利用 Cluster Head 來執行金鑰的變更，阻撓竊聽，金鑰還原攻擊與假封包的過濾。

WSN is composed of lot of cheap and resource-constrain nodes deployed in speci c
area to monitor nature event, such as sound, re, vibration, motion or temperature
and to report the event to the base station. Because of the public feature of the wire-
less communication, we need to prevent attacker from eavesdropping the sensitive
information. Therefore, all the data should be encryption before sending.
We in general apply the Public Key Cryptography in wireless network. Owing to
the limitation of resource-constrain, we cannot fully apply the Public Key Cryptog-
raphy in WSN. Instead, the Symmetric Key Cryptography is light-weight encryption
system which is suitable for WSN. Besides, we should not neglect that each sensor
node use the same key all the time. It is easy to su er from cryptanalytic attack
and directly threaten the network security.
Now we apply the concept of cluster to WSN, then we can use the nodes possessing
abundant resource to perform complex operation. We make good use of the concept
of cluster to accomplish key evolution, packet ltering and to thwart eavesdropping,
cryptanalytic attack.

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