ssh "permissions are too open" error

The keys need to be read-writable only by you:

chmod 600 ~/.ssh/id_rsa

Alternatively, the keys can be only readable by you (this also blocks your write access):

chmod 400 ~/.ssh/id_rsa

600 appears to be better in most cases, because you don't need to change file permissions later to edit it. (See the comments for more nuances)

The relevant portion from the manpage (man ssh)

 ~/.ssh/id_rsa
         Contains the private key for authentication.  These files contain sensitive 
         data and should be readable by the user but not
         accessible by others (read/write/execute).  ssh will simply ignore a private 
         key file if it is              
         accessible by others.  It is possible to specify a
         passphrase when generating the key which will be used to encrypt the sensitive 
         part of this file using 3DES.

 ~/.ssh/identity.pub
 ~/.ssh/id_dsa.pub
 ~/.ssh/id_ecdsa.pub
 ~/.ssh/id_rsa.pub
         Contains the public key for authentication.  These files are not sensitive and 
         can (but need not) be readable by anyone.

Using Cygwin in Windows 8.1, there is a command need to be run:

chgrp Users ~/.ssh/id_rsa

Then the solution posted here can be applied, 400 or 600 is OK.

chmod 600 ~/.ssh/id_rsa

Reference here

I've got the error in my windows 10 so I set permission as the following and it works.

In details, remove other users/groups until it has only 'SYSTEM' and 'Administrators'. Then add your windows login into it with Read permission only.

Note the id_rsa file is under the c:\users\<username> folder.

The locale-independent solution that works on Windows 8.1 is:

chgrp 545 ~/.ssh/id_rsa
chmod 600 ~/.ssh/id_rsa

GID 545 is a special ID that always refers to the 'Users' group, even if you locale uses a different word for Users.