If you encounter this error while `sudo apt-get update` on Debian
W: GPG error: http://ftp.debian.org experimental Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553 NO_PUBKEY 7638D0442B90D010
_Solution_
$ gpg --keyserver pgpkeys.mit.edu --recv-key 8B48AD6246925553
$ gpg -a --export 8B48AD6246925553|sudo apt-key add -
$ gpg --keyserver pgpkeys.mit.edu --recv-key 7638D0442B90D010
$ gpg -a --export 7638D0442B90D010|sudo apt-key add -
Notes: replace the key number with the one in the error message.
Check if the keys are added
$ sudo apt-key list