试述SSH的基本概念，以及和SSL的不同。
参考答案：
安全外壳SSH是一种介于传输层和应用之间的加密通道协议，它原本是设计用于替代rlogin、rsh这类不安全的r-协议集的，现在也可以Telnet、POP3、FTP、X11等这些传统上并不安全的应用协议。SSH提供一种安全
的身份认证及数据加密机制，还可以对传输的数据进行压缩，能够有效地防止网络嗅探、IP欺骗、DNS欺骗等攻击行为。SSH1最大好处是对最终用户和商业用户都是完全免费的，而SSH2则要遵循商用许可协议，但它的基本功能强得多。
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| We are lucky to live in a rich country where we have many things. However, most of us usually 1 some things. Electricity is one thing 2 we waste most. Lights, TVs, 3 and other things are often kept on even when no one is using them. We should turn them off when we do not need them. It 4 less than a second to do it. Anyhow, not wasting electricity save money, because we do not have to 5 the electricity we do not use. The second thing that we often waste is water. Fresh water is precious*, 6 it shouldn't be wasted. It is only a simple matter of turning off a tap*. I am sure 7 can do this easily if he wants to. 8 big waste is food. My mother 9 cook so much food that much of it was thrown away. I told her we had wasted so much 10 . She thought I was right and now cooks 11 what we can eat. It is not 12 to be wasteful. I find there is a simple 13 of not wasting things. That is to use only what we 14 . Do not touch what we do not need. 15 this way we can stop being wasteful. | ||||
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