前言早在一千多年前,人们就认识到在通信中有必要对消息进行保护以及消息的发送者和接收者彼此需要了解对方的身份。直到现代的计算机数据通信中,人们仍然把这两条原始的要求当作最基本的要求之一。随着计算技术和通信技术的发展,尤其是计算机向微小型化方面发展,数据通信也出现了一些新的趋向,计算机用户及电报用户的大量增加,使得通信网的规模变得更加庞大和复杂,而且逐步从专用网发展成公用网。这样,信息的安全与否直接关系到整个网络能否正常运转。因此,在设计一个通信网时,必须把信息保护问题考虑进去,同时也应当采取一些措施, As early as more than a thousand years ago, people realized that in communications, it is necessary to protect the information and the sender and receiver of the information need to know each other’s identity. Until modern computer data communications, people still regard these two original requirements as one of the most basic requirements. With the development of computing technology and communication technology, especially the miniaturization of computers, there have been some new trends in data communications. The proliferation of computer users and telegraph users has made communications networks more and more complex , And gradually developed from a private network into a public network. In this way, the safety of the information directly related to the entire network can operate normally. Therefore, when designing a communication network, information protection must be taken into account, and at the same time, some measures should be taken,