So this error has been popping up a lot so I thought I'd resolve it:
W: GPG error: http://cfengine.com precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 535C743689107B44
So I did some research and tried these solutions
gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv 535C743689107B44
and
sudo apt-key adv --keyserver.ubuntu.com --recv-keys 535C743689107B44
both of which terminated with this message
gpgkeys: key 535C743689107B44 not found on keyserver
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0
Any insights are appreciated
You missed the first step in the documentation...
http://cfengine.com/cfengine-linux-distros
To install CFEngine using these repositories, follow these steps: For Debian-based systems (apt):
Download and install the CFEngine GPG key: # wget http://cfengine.com/pub/gpg.key # apt-key add gpg.key # rm gpg.key
Actually I could "fix the issue" for you :) CFengine key is now uploaded to pgp.mit.edu, so if you use that as a keyserver, it should work.