The following signatures were invalid: EXPKEYSIG

If you see such a message

Err:4 home:/ailin_nemui:/irssi-an  InRelease
  The following signatures were invalid: EXPKEYSIG EDB7AED941EEDB57 home:ailin_nemui OBS Project <home:ailin_nemui@build.opensuse.org>

this means that the automatic signing key used by the Open Build Service expired. OBS renews the key periodically, but Debian does not support this.

Fix

Download the key again, following the regular OBS instructions. If it still does not work, verify if there is a second expired copy of the key in /etc/apt/trusted.gpg.d or in apt-key list and remove it as well.