在 18.04 上运行，尝试时出现sudo do-release-upgrade -d以下错误：
authenticate 'focal.tar.gz' against 'focal.tar.gz.gpg'
Authentication failed
Authenticating the upgrade failed. There may be a problem with the network or with the server.
我看过很多帖子要重新安装 sudo apt-get install --reinstall ubuntu-keyring
我做了，没有结果。
我从http://mirrors.linode.com/ubuntu/dists/focal-proposed/main/dist-upgrader-all/20.04.18/手动下载了focal.tar.gz和focal.tar.gz.gpg
我确实运行了 gpg 检查
gpg --receive-keys 871920D1991BC93C
gpg --receive-keys 3B4FE6ACC0B21F32
gpg --verify focal.tar.gz.gpg focal.tar.gz
看起来成功（有警告，但至少匹配）
gpg: Signature made mer. 08 avril 2020 19:38:12 CEST
gpg: using RSA key 3B4FE6ACC0B21F32
gpg: Good signature from "Ubuntu Archive Automatic Signing Key (2012) <ftpmaster@ubuntu.com>" [unknown]
gpg: …