运行apt update && apt upgrade时提示The following signatures couldn’t be verified because the public key is not available。这种情况是由于某种原因，Debian Backports的GPG密钥不会自动从GPG密钥服务器获取。
InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553 NO_PUBKEY 7638D0442B90D010其中8B48AD6246925553为key1，7638D0442B90D010为key2
gpg --keyserver pgp.mit.edu --recv-keys \
key1 key2然后
gpg --armor --export key1 | apt-key add -gpg --armor --export key2 | apt-key add -最后再次运行apt update && apt upgrade即可。