無線通訊系統安全技術研究

Abstract

在傳統的有線通訊中，由於用戶與傳送系統有一條固定的通信線路連結著，所以無需使用特定的鑑識協定(Authentication Protocol)來鑑識用戶與系統的合法性，而通訊的內容也都能保持適當的私密性(Privacy)。隨著通訊技術的提昇，人類對無線通訊的喜愛與依賴愈來愈深。最主要的原因是無線通訊可以給人提供一種無遠弗界的便利，在任何地方、環境、都能達到通訊的目的。但在無線通訊中用戶個別台(Personal Station)與基地台(Base Station)之間並不存在一條固定的通訊線路，用戶可以任意的在區域內移動，不再受限於某一固定點的通訊。電波在開放空間傳送，使得任何人都可輕易的分接到通訊的信號，這種方便反而給攻擊者(Attacker)製造了分接竊聽和非法使用的機會。因此，無線通訊的安全性及私密性成為不可或缺的需求。 為了確保無線通訊的安全性，必需有一套完備且有效的鑑識協定。在鑑識過程中，用戶與系統間必需使用密碼，使得此後的通訊在加密的情況下進行，免除被竊聽的顧慮。隨著用戶通訊範圍的擴大，以及用戶任意移動的需求，將使鑑識協定變得更為複雜。主要原因是這種大區域的無線通訊服務，都是由多個彼此互不相屬的區域系統所組成。而每一區域的服務，又可能是由不同的通訊公司負責。當一個戶用選定一個母系統註冊之後，用戶與母系統之間由於共同分享一密鑰，因此用戶與母系統可以互相識別身份。但是當用戶由母系統移動到客系統時，客系統與用戶之間並無任何分享密鑰。如何讓用戶與提供服務的客系統間能相互鑑識，成為無線通訊中的一大挑戰。此外，由於可移動通訊器材的硬體限制，及有限的電池蓄電能力，並不能支持太過於複雜的計算。而且通訊頻道的建立又不能過於耗時。這些都是我們的設計此一鑑識協定時所必需考慮的因素。本文中我們對無線通訊的鑑識問題，做一瞭解。並且針對目前通用的識別協定，分別提出討論及比較。 無線通訊為求達到體積小，重量輕，低耗電率，以及價格便宜等四項趨勢，未來可能只有用積體電路設計才能同時滿足以上的需求。一個完整的無線通訊系統主要可分為三部分：射頻模組(RF modules)、基頻信號處理器(beseband signal processors)及通信協定處理(protocols)，其中僅有射頻模組尚難以積體化。使用密碼的通信協定處理技術受限於複雜且耗時的運算，但可由積體化來解決。在目前發展趨勢下，如何結合傳統與現代密碼技術，發展出一多功能，高效率的無線通訊安全服務，是一個刻不容緩的研究課題。 使用傳統加密方法固然可以加速運算，但對密鑰的管理，以及提供無爭議的服務等方面，就不若使用近代明鑰密碼的技術來得有效。相對的，使用近代明鑰密碼的技術，可以提供更多功能及更有效的服務。因此就未來趨勢而言，除了運用具隱密性的傳輸技術如寬頻分碼多重接取（W-CDMA）之外，我們認為用戶安全資料分別儲存在用戶和網路母系統，安全服務將可由一個網路母系統金鑰印契中心（key escrow system center）執行，網路母系統金鑰印契中心未來也可能是一家公司。我們建議經由零資訊交換證明法及明鑰密碼系統的鑑識等協定，並利用單向雜湊函數（One Way Hash Function）縮減明文長度，可以接取不同系統以得到無線網路便利的服務，並保有足夠的商業安全。

In the wired communication, since there is a fixed switching between the users, we do not need special authentication protocol to authenticate them to be legal or not The communications can reserve privacy properly. But we prefer to and depend on wireless communication more and more oy improving the communication techanology. It is the main reason that wireless can provide the convenience of communication everywhere and in any environments. There exist no fixed links between the personal station of user and base station for wireless communication The users can move arbitrarily in some area without limits. However, radio propagate in the open space, anyone can tap the communication signals easily that give the attackers chances to steal and embezzle information. Therefore, it is indispensable to conserve proper security in wireless communication We need a complete and efficient authentication protocoln to make sure the security of the wireless communication. In the authentication process, the communication between user and system is encrypted to avoid being tapped. Because the services of wireless communication in wide area are always supplied by different operators, the protocol becomes more complicated. When a user registers in a home system, the user and home system share a secret key to authenticate each other. But, when users move into a visited system, there is no key shared between them. It is hard challenge how the visited system can authenticate users. The limit of hardware of mobile terminals and the power of battery can not support too complicated computation, furthermore, the time of channel setup of can not take too much. These are key issues when we design a authentication protocol. In the thesis we study the authentication protocol of wireless communication. To meet the requirements of wireless communication in respect of small size, light weight, low power, and cheap, it must only use IC to design. A complete wireless communication system is composed of RF modules, digital signal processors and communication protocol process and only RF modules have some problems to solve in IC. Encryption technology used in protocol result in complicated and long time computation, but it can be designed in IC to overcome. Now it is urgent to combine traditional and contemporary encryption technology to develop a multifunction security service. Traditional encryption technology is fast in computation, but it is not efficient in key distribution management and can not support non-repudiation service compared with contemporary encryption technology. Contemporary encryption technology, is a public key cryptosystem, can supply more uses and function in service. Therefore, in spite of using privacy transmission technology in the future, such as W-CDMA, we should put secure data into user terminal and home network system to execute the secure service, maybe supported by a key escrow system center. In the future a key escrow system center can be a company. We suggest adopting zero knowledge interactive proof method and public key cryptosystem model to construct authentication protocol, and utilize one-way hash function to shorten the length of plaintext to improve security and efficiency of digital signature and cipher process. The users not only can access different system, but also can conserve sufficient security of business in card roaming service.