GPG error: http://ftp.debian.org jessie-backports InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553 NO_PUBKEY 7638D0442B90D010

GPG error: http://ftp.debian.org jessie-backports InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553 NO_PUBKEY 7638D0442B90D010

Running into this issue when trying to install certbot under Debian, the following can be done to fix it. Step 1 has probably been done already

echo 'deb http://ftp.debian.org/debian jessie-backports main' | sudo tee  /etc/apt/sources.list.d/backports.list
sudo -i
wget -O - https://ftp-master.debian.org/keys/archive-key-8.asc | apt-key add -
wget -O - https://ftp-master.debian.org/keys/archive-key-8-security.asc | apt-key add -
apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 8B48AD6246925553
apt-get update

Tip from here.