Debian / Ubuntu Linux 若使用外部套件(Apt sources.list 有額外增加),apt update 常常會遇到下述訊息:
由於無法取得它們的公鑰,以下簽章無法進行驗證: NO_PUBKEY 9334A25F8507EFA5
或
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9334A25F8507EFA5
W: GPG error: http://repo.percona.com/apt stretch InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9334A25F8507EFA5
要怎麼解決呢?