我之前问过这个问题，但是被关闭了，说是“重复”。我可以向您保证，我的情况是独特的，因为它不涉及第三方PPA。我在系统更新中遇到了GPG问题。它显示没有可用的公钥，并且我还收到了更新信息已过时的警告。以下是终端文本：
我在URL中加入了空格，因为新用户每个问题只允许有两个超链接。
请不要将此关闭为重复问题。它并不涉及第三方PPA，而是指Canonical发布的标准更新包。
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中加入了空格，因为新用户每个问题只允许有两个超链接。
请不要将此关闭为重复问题。它并不涉及第三方PPA，而是指Canonical发布的标准更新包。