加密是对信息进行编码和解码的技术，它通过一定的算法（密钥）将原本能被直接阅读的明文信息转换成密文形式。
密钥即是密文的钥匙，有私钥和公钥之分。
在传输数据时，如果担心被他人监听或截获，就可以在传输前先使用公钥对数据加密处理，然后再进行传送。这样，只有掌握私钥的用户才能解密这段数据，除此之外的其他人即便截获了数据，一般也很难将其破译为明文信息。
1、在需要免登陆的节点生成“密钥对”。（192.168.10.20）
[root@Client ~]# ssh-keygen //默认算法是rsa，也可使用-t指定算法 Generating public/private rsa key pair. Enter file in which to save the key (/root/.ssh/id_rsa): 回车 //按回车键或设置密钥的存储路径 Enter passphrase (empty for no passphrase): 回车 //直接按回车键或设置密钥的密码 Enter same passphrase again: 回车 //再次按回车键或设置密钥的密码 Your identification has been saved in /root/.ssh/id_rsa. Your public key has been saved in /root/.ssh/id_rsa.pub. //此时将自动生成.ssh目录和id_rsa id_rsa.pub两个文件 The key fingerprint is: SHA256:kHa7B8V0nk63evABRrfZhxUpLM5Hx0I6gb7isNG9Hkg root@linuxprobe.com The key's randomart image is: +---[RSA 2048]----+ | o.=.o.+| | . + =oB X | | + o =oO O o| | . o + *.+ ..| | .ES . + o | | o.o.= + . | | =.o.o . o | | . . o. . | | .. | +----[SHA256]-----+
[root@Client.ssh]# ll /root/.ssh/
total 12
-rw-------. 1 root root 1831 Dec 1 19:12 id_rsa
-rw-r--r--. 1 root root 401 Dec 1 19:12 id_rsa.pub //本机公钥
-rw-r--r--. 1 root root 175 Dec 1 19:16 known_hosts
2、将生成的公钥文件（id_ras.pub）传送到需要免登陆的目标主机。
[root@Client ~]# ssh-copy-id 192.168.10.10 //此命令会自动在目标主机（192.168.10.10）创建/root/.ssh/authorized_keys文件且将id_rsa.pub写入其中，如果目标主机已经存在authorized_keys文件，会将id_rsa.pub追加到文件中。
/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
/usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys
root@192.168.10.10's password: 此处输入服务器管理员密码
Number of key(s) added: 1
Now try logging into the machine, with: "ssh '192.168.10.10'"
and check to make sure that only the key(s) you wanted were added.
---------------也可使用scp传输--------------
[root@Client ~]#scp /root/.ssh/id_rsa.pub root@192.168.10.10:/root/.ssh/ //将本地文件推送到目标主机
[root@Server ~]#cat /root/.ssh/id_rsa.pub >> /root/.ssh/authorized_keys //在目标主机上将公钥文件写入authorized_keys
---------------也可使用scp传输--------------
[root@server ~ ]# ls -l /root/.ssh/ //此时目标主机authorized_keys文件中已经存储了id_rsa.pub
total 8
-rw-------. 1 root root 802 Dec 3 18:48 authorized_keys
-rw-r--r--. 1 root root 525 Dec 3 18:53 known_hosts
此时192.168.10.20即可免密登录到192.168.10.10（其基本原理是.10主机authorized_keys文件中存储了.20的id_rsa.pub，.20机器才能免密登录到.10）
注：如果多台机器之间都需要免密登录的话，就需要在authorized_keys文件中相互存储id_rsa.pub。