WARNING: The following packages cannot be authenticated!

# apt-get update
# apt-get upgrade
・・・
WARNING: The following packages cannot be authenticated!
  libc6 libc6-dev libgmp-dev libgcc1-armhf-cross
Install these packages without verification [y/N]? ^C

• apt-get: WARNING: The following packages cannot be authenticated! | hdknrinthespace

を参考に鍵をインストールしてやりなおし

# apt-get install debian-archive-keyring

したけど、また同じエラーが出る。