执行sudo update时遭遇GPG密钥错误，无法完成软件源更新
看起来你碰到了个Linux系统里挺常见的源更新小问题——ProtonVPN的软件源因为缺失对应GPG密钥，导致系统没法验证它的合法性，所以更新时就报错卡住了。先看看你遇到的具体错误信息：
7,898 kB were fetched in 2 s (3,370 kB/s).
Reading package lists… Done
Building dependency tree… Done
Status information is being read in… Done
Update available for 6 packages. Run "apt list --upgradable" to view them.
W: An error occurred while verifying the signature. The depot has not been updated and the previous index files are used.
GPG Error: https://repo.protonvpn.com/debian stable InRelease: The following signatures could not be verified because their public key is not available: NO_PUBKEY 4EDE055B645F044F
W: Failed to get from https://repo.protonvpn.com/debian/dists/stable/InRelease The following signatures could not be verified because their public key is not available: NO_PUBKEY 4EDE055B645F044F
W: Some index files could not be downloaded. They were ignored or old ones were used in their place.
别慌，我之前也碰到过一模一样的情况，咱们把缺失的密钥导入系统就行，给你两种靠谱的解决方法：
方法一：快速导入密钥（适合大多数Debian/Ubuntu系系统）
直接用apt-key命令把缺失的密钥从公共密钥服务器导入到系统可信库：
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 4EDE055B645F044F
导入完成后，重新跑更新命令：
sudo apt update
方法二：符合新系统规范的导入方式（比如Ubuntu 22.04+、Debian 11+）
如果你的系统版本比较新，推荐用密钥环的方式导入，更贴合现在的安全规范：
- 先从密钥服务器获取密钥：
gpg --keyserver keyserver.ubuntu.com --recv-keys 4EDE055B645F044F
- 把密钥导出到系统的可信密钥环目录：
gpg --export 4EDE055B645F044F | sudo tee /usr/share/keyrings/protonvpn-archive-keyring.gpg > /dev/null
- 最后重新执行更新：
sudo apt update
一般来说，跑完上面任意一种方法，GPG错误就会消失，源也能正常更新了。至于为什么之前没问题现在突然报错，大概率是ProtonVPN更新了源的签名密钥，或者你的系统密钥库意外丢失了这个密钥，重新导入就好啦。
备注：内容来源于stack exchange，提问作者Srecko Mitrovic
