The Ubuntu 871920D1991BC93C signature is seen as invalid on Ubuntu 24.04.1 but not in Ubuntu 24.04 :
$ sudo apt-get update >/dev/null
W: GPG error: http://archive.ubuntu.com/ubuntu noble-backports InRelease: The following signatures were invalid: BADSIG 871920D1991BC93C Ubuntu Archive Automatic Signing Key (2018) <ftpmaster@ubuntu.com>
E: The repository 'http://archive.ubuntu.com/ubuntu noble-backports InRelease' is not signed.
$ sudo apt-key adv --keyserver-options http-proxy=$http_proxy --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 871920D1991BC93C
Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)).
Executing: /tmp/apt-key-gpghome.RnGednz4Kq/gpg.1.sh --keyserver-options http-proxy=http://a.b.c.d:80 --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 871920D1991BC93C
gpg: key 871920D1991BC93C: "Ubuntu Archive Automatic Signing Key (2018) <ftpmaster@ubuntu.com>" not changed
gpg: Total number processed: 1
gpg: unchanged: 1
$ sudo apt-get update >/dev/null
W: GPG error: http://archive.ubuntu.com/ubuntu noble-backports InRelease: The following signatures were invalid: BADSIG 871920D1991BC93C Ubuntu Archive Automatic Signing Key (2018) <ftpmaster@ubuntu.com>
E: The repository 'http://archive.ubuntu.com/ubuntu noble-backports InRelease' is not signed.
$
So I tried according to this solution :
$ sudo apt-get update -o Acquire::http::No-Cache=True >/dev/null
W: GPG error: http://archive.ubuntu.com/ubuntu noble-backports InRelease: The following signatures were invalid: BADSIG 871920D1991BC93C Ubuntu Archive Automatic Signing Key (2018) <ftpmaster@ubuntu.com>
E: The repository 'http://archive.ubuntu.com/ubuntu noble-backports InRelease' is not signed.
$ sudo apt-get update -o Acquire::BrokenProxy=true >/dev/null
W: GPG error: http://archive.ubuntu.com/ubuntu noble-backports InRelease: The following signatures were invalid: BADSIG 871920D1991BC93C Ubuntu Archive Automatic Signing Key (2018) <ftpmaster@ubuntu.com>
E: The repository 'http://archive.ubuntu.com/ubuntu noble-backports InRelease' is not signed.
$
I also tried this solution :
$ sudo -i
# apt-get clean
# cd /var/lib/apt
# mv lists lists.old
# mkdir -p lists/partial
# apt-get clean
# apt-get update >/dev/null
W: GPG error: http://archive.ubuntu.com/ubuntu noble-backports InRelease: The following signatures were invalid: BADSIG 871920D1991BC93C Ubuntu Archive Automatic Signing Key (2018) <ftpmaster@ubuntu.com>
E: The repository 'http://archive.ubuntu.com/ubuntu noble-backports InRelease' is not signed.
#
But none of them worked.
How can I fix this ?
