Update fails due to NO_PUBKEY

[Mick Bert <MickBert@posteo.net>]

I am trying to update debian in my iBook G4 since a very long time, but 
the "apt update" command fails due to mising public key for signature 
checking.

My current sources.list is:

# binary default
deb http://ftp.ports.debian.org/debian-ports/ unstable main
deb http://ftp.ports.debian.org/debian-ports/ unreleased main
deb http://incoming.ports.debian.org/buildd/ unstable main

# source
deb-src http://ftp.debian.org/debian unstable main
deb-src http://incoming.debian.org/debian-buildd/ buildd-unstable main

and the debian-version:

buster/sid

It's not the first time I got this error (even though on other systems), 
but I never remember how to get out of it. I allready tried with 
--allow-unauthenticated, with the same result.

--
Mick