update Kimsufi, problème signature

Bonjour,

Pour aider une petite asso, je fais quelques maintenances sur leur
Kimsufi. L’année dernière, j’ai fait une migration de Debian 8 vers 9,
puis 9 vers 10.

Je n’ai pas fait attention si ce problème faisait suite Í  la mise Í 
jour, mais aujourd’hui que je remets la main dessus :

# apt-get update
Get:1 http://debian.mirrors.ovh.net/debian buster-updates InRelease
[51.9 kB]
Get:2 http://debian.mirrors.ovh.net/debian buster-backports InRelease
[46.7 kB]
Err:1 http://debian.mirrors.ovh.net/debian buster-updates InRelease
The following signatures couldn't be verified because the public key
is not available: NO_PUBKEY 04EE7237B7D453EC NO_PUBKEY
648ACFD622F3D138
Err:2 http://debian.mirrors.ovh.net/debian buster-backports InRelease
The following signatures couldn't be verified because the public key
is not available: NO_PUBKEY 04EE7237B7D453EC NO_PUBKEY
648ACFD622F3D138
Get:3 http://security.debian.org buster/updates InRelease [65.4 kB]
Err:3 http://security.debian.org buster/updates InRelease
The following signatures couldn't be verified because the public
key is not available: NO_PUBKEY AA8E81B4331F7F50 NO_PUBKEY
112695A0E562B32A
Get:4 http://ftp-stud.hs-esslingen.de/debian buster InRelease [121
kB]
Err:4 http://ftp-stud.hs-esslingen.de/debian buster InRelease
The following signatures couldn't be verified because the public
key is not available: NO_PUBKEY 04EE7237B7D453EC NO_PUBKEY
648ACFD622F3D138 NO_PUBKEY DCC9EFBF77E11517
Reading package lists Done
W: GPG error: http://debian.mirrors.ovh.net/debian
buster-updates InRelease: The following signatures couldn't be
verified because the public key is not available: NO_PUBKEY
04EE7237B7D453EC NO_PUBKEY 648ACFD622F3D138
E: The repository 'http://debian.mirrors.ovh.net/debian
buster-updates InRelease' is not signed.
N: Updating from such a repository can't be done securely, and
is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user
configuration details.
W: GPG error: http://debian.mirrors.ovh.net/debian
buster-backports InRelease: The following signatures couldn't be
verified because the public key is not available: NO_PUBKEY
04EE7237B7D453EC NO_PUBKEY 648ACFD622F3D138
E: The repository 'http://debian.mirrors.ovh.net/debian
buster-backports InRelease' is not signed.
N: Updating from such a repository can't be done securely, and
is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user
configuration details.
W: GPG error: http://security.debian.org buster/updates
InRelease: The following signatures couldn't be verified because
the public key is not available.

Essayé avec d’autres dépÍ´ts, même chose.

J’ai vérifié les empreintes des clés de /etc/apt/trusted.gpg.d, ça
semble conforme avec :
https://ftp-master.debian.org/keys.html

Merci,
--
Raphaël
www.leclavierquibave.fr