I'm using Raspbian Jessie but there are a few packages I want that aren't available (but are in Debian Stretch repos). I want to temporarily use the Stretch repo to install them (and any otherwise-unsatisfied dependencies) but without making anything else come from there in the future.I understand things might not work; etc.; I'm just trying something out on a throwaway install :)I tried rigging some files (based on this answer) but I got this.. not sure a) how to fix it and b) whether I'm doing things the right way!W: GPG error: http://ftp.uk.debian.org stretch InRelease: Thefollowing signatures couldn't be verified because the publickey is not available:    NO_PUBKEY 8B48AD6246925553 NO_PUBKEY 7638D0442B90D010
What's the best way to install apt packages from Debian Stretch on Raspbian Jessie?
debian;apt
As backports has letsencrypt, I recommend using jessie-backports as it brings less new/packages dependencies than drinking directly from stretch. To use Jessie backports and install letsencrypt from it:Add to /etc/apt/sources.list:deb http://httpredir.debian.org/debian jessie-backports main contrib non-freeThe run:apt-get updateAs for installing the key, I confirm you can do:gpg --keyserver pgpkeys.mit.edu --recv-key  8B48AD6246925553      gpg -a --export 8B48AD6246925553 | sudo apt-key add -and also with the key 7638D0442B90D010gpg --keyserver pgpkeys.mit.edu --recv-key 7638D0442B90D010     gpg -a --export 7638D0442B90D010 | sudo apt-key add -And finally to install letsencrypt:apt-get install -t jessie-backports letsencrypt