To verify the downloaded packages in this apt repository you should
import the Intevation File Distribution Key into apt:

  gpg --keyserver hkp://keys.gnupg.net --recv-keys EC70B1B8
  gpg --export EC70B1B8 | apt-key add -

or

  wget -O - https://ssl.intevation.de/Intevation-Distribution-Key.asc | apt-key add -