Behavior
The following signatures were invalid: EXPKEYSIG ED444FF07D8D0BF6 Kali Linux
Solultion
wget https://http.kali.org/kali/pool/main/k/kali-archive-keyring/kali-archive-keyring_2018.1_all.deb
apt install ./kali-archive-keyring_2018.1_all.deb
Reference
https://forums.kali.org/showthread.php?38858-apt-get-update-GPG-error-key-expired