最近，当我尝试在笔记本电脑（Ubuntu 12.04）上检查更新时，检查失败。
sudo apt-get update：
W: GPG error: http://us.archive.ubuntu.com precise Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/main/i18n/Index No Hash entry in Release file /var/lib/apt/lists/partial/us.archive.ubuntu.com_ubuntu_dists_precise_main_i18n_Index
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/multiverse/i18n/Index No Hash entry in Release file /var/lib/apt/lists/partial/us.archive.ubuntu.com_ubuntu_dists_precise_multiverse_i18n_Index
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/restricted/i18n/Index No Hash entry in Release file /var/lib/apt/lists/partial/us.archive.ubuntu.com_ubuntu_dists_precise_restricted_i18n_Index
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/universe/i18n/Index No Hash entry in Release file /var/lib/apt/lists/partial/us.archive.ubuntu.com_ubuntu_dists_precise_universe_i18n_Index
E: Some index files failed to download. …