This is how we should set permissions on private keys in .ssh folder on Linux. Thanks to this guys on source link.

ssh-permissions.md

• .ssh directory: 700 (drwx------)
• public key (.pub file): 644 (-rw-r--r--)
• private key (id_rsa): 600 (-rw-------)
• lastly your home directory should not be writeable by the group or others (at most 755 (drwxr-xr-x)).

source: https://superuser.com/questions/215504/permissions-on-private-key-in-ssh-folder/215506#215506