~$ sudo apt-get update 
Hit:1 http://mirror.math.princeton.edu/pub/ubuntu zesty InRelease
Hit:2 http://repo.steampowered.com/steam precise InRelease               
Hit:3 http://mirror.math.princeton.edu/pub/ubuntu zesty-updates InRelease      
Hit:4 http://mirror.math.princeton.edu/pub/ubuntu zesty-backports InRelease    
Hit:5 http://mirror.math.princeton.edu/pub/ubuntu zesty-security InRelease   
Hit:6 http://mirror.math.princeton.edu/pub/ubuntu zesty-proposed InRelease
Hit:7 http://ppa.launchpad.net/appgrid/stable/ubuntu zesty InRelease    
Hit:8 https://repo.skype.com/deb stable InRelease  
Hit:9 https://dl.winehq.org/wine-builds/ubuntu zesty InRelease
Hit:10 http://ppa.launchpad.net/audio-recorder/ppa/ubuntu zesty InRelease
Hit:11 http://ppa.launchpad.net/inkscape.dev/trunk/ubuntu zesty InRelease
Get:12 https://content.runescape.com/downloads/ubuntu trusty InRelease [2,236 B]
Hit:13 http://ppa.launchpad.net/unit193/encryption/ubuntu zesty InRelease      
Ign:12 https://content.runescape.com/downloads/ubuntu trusty InRelease
Fetched 2,236 B in 1s (2,204 B/s)
Reading package lists... Done
W: GPG error: https://content.runescape.com/downloads/ubuntu trusty InRelease: The following signatures were invalid: AAC9264309E4D717441DB9527373B12CE03BEB4B
W: The repository 'https://content.runescape.com/downloads/ubuntu trusty InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.