Software updater warning: requires installation of untrusted packages [duplicate]

I am using Ubuntu 15.04 and when I try update for Software updater I always get this error:

enter image description here

I also tried to not download some packages and download the main ones but the error always stays there.

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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