解决方案：
1. 先创建用户，参考如下命令
# useradd test
2. 切换到新用户：
# sudo su - test
3. 在新用户中用命令ssh-keygen生成密钥。
$ ssh-keygen
Generating public/private rsa key pair.
Enter file in which to save the key (/home/test/.ssh/id_rsa):
Created directory '/home/test/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/test/.ssh/id_rsa.
Your public key has been saved in /home/test/.ssh/id_rsa.pub.
The key fingerprint is:
8d:d2:8f:5e:b8:9d:2e:69:59:1a:b6:a8:1f:ed:b8:ae test@jacobliu2018
The key's randomart image is:
+--[ RSA 2048]----+
| |
| |
| |
| . o |
| . S . |
| oo+. |
| .o+Bo |
| .=B+ . |
| E==o+o+ |
+-----------------+
4. 键入以下命令，在服务器上安装公钥并修改文件authorized_keys权限为只有本用户可以读写：
$ cd .ssh
$ cat id_rsa.pub >> authorized_keys
$ chmod 600 authorized_keys
5. 将文件id_rsa拷贝出来到windows主机，参考如下步骤：
a. 切换到root 用户： sudo su -
b. 复制到ec2-user用户目录下: cp /home/test/.ssh/id_rsa /home/ec2-user/
c. 增加文件/home/ec2-user/.ssh/id_rsa的其他用户读写权限： chmod +r /home/ec2-user/.ssh/id_rsa
d. 通过winscp将文件/home/ec2-user/.ssh/id_rsa拷贝windows主机
e. 在windows主机中将文件id_rsa转化为putty支持的格式。
f. 之后就可以用putty加密钥登陆linux主机了。