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.

2 Answers

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

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.