zyga@pi ~ $ sudo apt-get update
Hit:1 http://archive.raspbian.org/raspbian jessie InRelease                                                              
Hit:2 http://archive.raspbian.org/raspbian wheezy InRelease                                                              
Hit:3 http://archive.raspbian.org/raspbian stretch InRelease      
Hit:4 http://raspberrypi.collabora.com wheezy InRelease           
Hit:5 http://archive.raspberrypi.org/debian wheezy InRelease      
Err:4 http://raspberrypi.collabora.com wheezy InRelease           
  The following signatures were invalid: FF9FD9E73808E2376CB76F0AED4BF9140C50B1C5
Reading package lists... Done
E: The method driver /usr/lib/apt/methods/https could not be found.
N: Is the package apt-transport-https installed?
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://raspberrypi.collabora.com wheezy InRelease: The following signatures were invalid: FF9FD9E73808E2376CB76F0AED4BF9140C50B1C5
E: Failed to fetch https://mm.gravedo.de/raspbian/dists/jessie/InRelease  
W: Failed to fetch http://raspberrypi.collabora.com/dists/wheezy/InRelease  The following signatures were invalid: FF9FD9E73808E2376CB76F0AED4BF9140C50B1C5
W: Some index files failed to download. They have been ignored, or old ones used instead.