when I newly install kali linux from kali.org and update it, it occurs:

root@kali:/var/lib/apt# sudo apt-get update
Get:1 http://kali.mirror.garr.it/mirrors/kali kali-rolling InRelease [30.5 kB]
Err:1 http://kali.mirror.garr.it/mirrors/kali kali-rolling InRelease
  The following signatures were invalid: EXPKEYSIG ED444FF07D8D0BF6 Kali Linux Repository <devel@kali.org>
Reading package lists... Done
W: GPG error: http://kali.mirror.garr.it/mirrors/kali kali-rolling InRelease: The following signatures were invalid: EXPKEYSIG ED444FF07D8D0BF6 Kali Linux Repository <devel@kali.org>
E: The repository 'http://http.kali.org/kali kali-rolling InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

After find it from Internet, I find out the solution from 《this post
just run the command:
apt-key adv --keyserver hkp://keys.gnupg.net --recv-key ED444FF07D8D0BF6

executing: /tmp/apt-key-gpghome.gUduUHesu2/gpg.1.sh --keyserver hkp://keys.gnupg.net --recv-key ED444FF07D8D0BF6
key ED444FF07D8D0BF6:
134 signatures not checked due to missing keys
gpg: key ED444FF07D8D0BF6: "Kali Linux Repository <devel@kali.org>" 136 new signatures
gpg: Total number processed: 1
gpg:         new signatures: 136

problem solve.