Software updater warning: requires installation of untrusted packages

This is some problem in yout cache run those commands to solve it:

sudo apt-get clean
sudo mv /var/lib/apt/lists /var/lib/apt/lists.bak
sudo mkdir -p /var/lib/apt/lists/partial
sudo apt-get clean 
sudo apt-get update

source and more info

I had the same problem. This happens due to the missing of some GPG keys.

Go to applications->accessories->terminal or press CTRL+ALT+T.

And, type below commands:-

sudo apt-get update --fix-missing

You will get an error saying that some public key is not available. Note the key string ( after the NO_PUBKEY; eg:3C962022012520A0 )

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com XXXXXX

Replace the XXXXXX with the key string

Ex:- sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 3C962022012520A0

Now try updating your system again.

sudo apt-get update

This will surely solve this issue.

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