我之前问过这个问题，但被关闭了，因为“重复”。我可以向你保证，我的情况是原创的，因为它不涉及第三方 PPA。我在系统更新时遇到了 GPG 问题。它说没有可用的公钥，我还被警告说我的更新信息已过时。这是终端文本：
W: A error occurred during the signature verification.
The repository is not updated and the previous index files will be used. GPG error:http://extras.ubuntu.com quantal Release:
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 16126D3A3E5C1192
W: GPG error: http:// archive.canonical.com quantal Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
W: GPG error: http:// us.archive.ubuntu.com quantal Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
W: GPG error: http:// us.archive.ubuntu.com quantal-updates Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
W: GPG error: http:// us.archive.ubuntu.com quantal-backports Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
W: GPG error: http:// us.archive.ubuntu.com quantal-security Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
W: Failed to fetch http:// extras.ubuntu.com/ubuntu/dists/quantal/Release
W: Some index files failed to download. They have been ignored, or old ones used instead.
我在 URL 中添加了空格，因为新用户每个问题不允许超过两个超链接。
请不要将其作为重复项关闭。它不是指第三方 PPA，而是指 Canonical 发布的标准更新包。
答案1
在终端中运行以下命令：
sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 40976EAF437D05B5
sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 3B4FE6ACC0B21F32
这将从 Ubuntu GPG 密钥服务器下载丢失的 GPG 密钥（如原始错误消息中所述）。