WARNING: The following packages cannot be authenticated. `apt-key update` doesn't work

Solution 1:

You have to add the pub key manually.

The command is

sudo apt-key adv --recv-key --keyserver keyserver.ubuntu.com «key number»

where «key number» you must replace it with the missing key.

For example in this error you received

NO_PUBKEY 1F968B3903D886E7

you can do

sudo apt-key adv --recv-key --keyserver keyserver.ubuntu.com 1F968B3903D886E7
sudo apt-get update

That way you will download this key in your keyring from the keyserver (keyserver.ubuntu.com)

About the BADSIG error, check this answer here

Solution 2:

Not the direct answer maybe, but there is a need to update the program: update-manager-core . If any users are getting the error that packages cannot be authenticated when running update manager, then updating this program also updates update managerand provides a really quick answer.

BTW, I used synaptic to do this update, if that is any help. Once the update processed, then update manager worked as it normally does.

see 1204_HWE_EOL in Ubuntu wiki