sudo apt update
Ign:1
http://deb.debian.org/debian stretch InRelease
Hit:2
http://security.debian.org/debian-security stretch/updates InRelease
Get:3
http://deb.debian.org/debian stretch-updates InRelease [90.3 kB]
Err:2
http://security.debian.org/debian-security stretch/updates InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9D6D8F6BC857C906 NO_PUBKEY 8B48AD6246925553
Err:3
http://deb.debian.org/debian stretch-updates InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010
Hit:4
http://deb.debian.org/debian stretch Release
Err:5
http://deb.debian.org/debian stretch Release.gpg
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553 NO_PUBKEY 7638D0442B90D010 NO_PUBKEY EF0F382A1A7B6500
Fetched 90.3 kB in 1s (78.1 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error:
http://security.debian.org/debian-security stretch/updates InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9D6D8F6BC857C906 NO_PUBKEY 8B48AD6246925553
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error:
http://deb.debian.org/debian stretch-updates InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error:
http://deb.debian.org/debian stretch Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553 NO_PUBKEY 7638D0442B90D010 NO_PUBKEY EF0F382A1A7B6500
W: Failed to fetch
http://security.debian.org/debian-security/dists/stretch/updates/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9D6D8F6BC857C906 NO_PUBKEY 8B48AD6246925553
W: Failed to fetch
http://deb.debian.org/debian/dists/stretch-updates/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010
W: Failed to fetch
http://deb.debian.org/debian/dists/stretch/Release.gpg The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553 NO_PUBKEY 7638D0442B90D010 NO_PUBKEY EF0F382A1A7B6500
W: Some index files failed to download. They have been ignored, or old ones used instead.