Cygwin set up ssh

in Windows 8.1 cygwin set up ssh problem in Windows 8.1
In order to directly use Linux environment and hadoop development on Windows 8.1, install cygwin, and set ssh passwordless login.
But after normal ssh-keygen is copied to authorised_keys, the login prompts:
Permissions 0660 for "/home/***/.ssh/id_rsa" are too open. It is required that your private key files are NOT Accessible by others.
meaning that the permissions of the key file are too loose, so I want to narrow the permissions, but no matter how chmod can not receive the rw permission of group and other.
After a look, I found out that it was a problem I encountered on AIX in my work N years ago, that is, the group of files is not, or the reason why the group is displayed as None in cygwin. The solution is to give the group first, simple and rude chgrp Users to group a common user, and then change the permissions is no problem.