A partir dos comentários: o problema é resolvido com a atualização de pacotes:
sudo apt-get update && sudo apt-get dist-upgrade
Isso é o que acontece quando eu faço apt-get update :
Get:1 http://security.debian.org stretch/updates InRelease [68.2 kB]
Ign:1 http://security.debian.org stretch/updates InRelease
Get:2 http://ftp.bme.hu/debian stretch InRelease [175 kB]
Get:3 http://ftp.cz.debian.org/debian stretch-updates InRelease [88.5 kB]
Ign:2 http://ftp.bme.hu/debian stretch InRelease
Ign:3 http://ftp.cz.debian.org/debian stretch-updates InRelease
Fetched 332 kB in 1s (175 kB/s)
Reading package lists... Done
W: GPG error: http://security.debian.org stretch/updates InRelease: At least one invalid signature was encountered.
W: The repository 'http://security.debian.org stretch/updates InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://ftp.bme.hu/debian stretch InRelease: At least one invalid signature was encountered.
W: The repository 'http://httpredir.debian.org/debian stretch InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://ftp.cz.debian.org/debian stretch-updates InRelease: At least one invalid signature was encountered.
W: The repository 'http://httpredir.debian.org/debian stretch-updates InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
Aqui está o meu sources.list :
deb http://httpredir.debian.org/debian/ stretch main non-free contrib
deb-src http://httpredir.debian.org/debian/ stretch main non-free contrib
deb http://security.debian.org/ stretch/updates main non-free contrib
deb http://httpredir.debian.org/debian/ stretch-updates main non-free contrib
Como faço para corrigir isso? Eu já tentei as sugestões de aqui.
O Debian implementou novas chaves para assinar seus pacotes e listas, para resolver o erro você precisa da versão mais recente do "debian-archive-keyring". atualizar esse pacote.
# apt-get install debian-archive-keyring