This error is likely to appear when you added an extra repository to your sources (e.g., ppa.launchpad.net). In my case, it was due to Gnome3.
After running apt-get update, I received the following error message:
Where ≪SIGNATURE≫ was the source signature and consisted of a hexadecimal representation of a large number.
The reason for that error is that I had added the repository, but not its public key. My system couldn't verify it was dealing with the right repository and not a fake one.
To fix it, we have to get the key from some source that we trust. In my case, I got it from Ubuntu. (I am using their system, so I kind of trust them anyway.)
The command to add the key is as follows:
The last command I used just to confirm everything was right. ;)
After running apt-get update, I received the following error message:
W: GPG error: http://ppa.launchpad.net trusty Release: The following signature couldn't be verified because the publix key is not available: NO_PUBKEY ≪SIGNATURE≫
Where ≪SIGNATURE≫ was the source signature and consisted of a hexadecimal representation of a large number.
The reason for that error is that I had added the repository, but not its public key. My system couldn't verify it was dealing with the right repository and not a fake one.
To fix it, we have to get the key from some source that we trust. In my case, I got it from Ubuntu. (I am using their system, so I kind of trust them anyway.)
The command to add the key is as follows:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys ≪SIGNATURE≫
sudo apt-get update
sudo apt-get update
The last command I used just to confirm everything was right. ;)