今天运行apt-get update拒绝更新存储库，因为它认为它们没有签名。起初它只是给出了At least one invalid signature was encountered所有存储库的错误，但在尝试运行apt-get clean、清理trusted.gpg.d等之后，它现在说存储库本身没有签名。
我尝试了几个镜像，同样的错误也来自 PPA。（我已将它们从日志中删除）
我该如何解决这个问题？
$ sudo apt update
Get:1 http://security.ubuntu.com/ubuntu xenial-security InRelease [107 kB]
Get:2 http://fi.archive.ubuntu.com/ubuntu xenial InRelease [247 kB]
Err:1 http://security.ubuntu.com/ubuntu xenial-security InRelease
At least one invalid signature was encountered.
Get:3 http://fi.archive.ubuntu.com/ubuntu xenial-updates InRelease [109 kB]
Err:2 http://fi.archive.ubuntu.com/ubuntu xenial InRelease
At least one invalid signature was encountered.
Get:4 http://fi.archive.ubuntu.com/ubuntu xenial-backports InRelease [107 kB]
Err:3 http://fi.archive.ubuntu.com/ubuntu xenial-updates InRelease
At least one invalid signature was encountered.
Err:4 http://fi.archive.ubuntu.com/ubuntu xenial-backports InRelease
At least one invalid signature was encountered.
Reading package lists... Done
W: GPG error: http://security.ubuntu.com/ubuntu xenial-security InRelease: At least one invalid signature was encountered.
E: The repository 'http://security.ubuntu.com/ubuntu xenial-security 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://fi.archive.ubuntu.com/ubuntu xenial InRelease: At least one invalid signature was encountered.
E: The repository 'http://fi.archive.ubuntu.com/ubuntu xenial 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://fi.archive.ubuntu.com/ubuntu xenial-updates InRelease: At least one invalid signature was encountered.
E: The repository 'http://fi.archive.ubuntu.com/ubuntu xenial-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://fi.archive.ubuntu.com/ubuntu xenial-backports InRelease: At least one invalid signature was encountered.
E: The repository 'http://fi.archive.ubuntu.com/ubuntu xenial-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.