You have enabled checking of packages via GPG keys. This is a good
thing.
However, you do not have any GPG public keys installed. You need to
download
the keys for packages you wish to install and install them.
You can do that by running the command:
rpm --import public.gpg.key
For more information contact your distribution or package provider.
[root@localhost ~]# rpm --import public.gpg.key
エラー: public.gpg.key: import read failed.
# yum update
Setting up Update Process
Setting up Repo: base
repomd.xml 100% |=========================| 1.1 kB 00:00
Setting up Repo: updates-released
repomd.xml 100% |=========================| 951 B 00:00