Why am I getting authentication errors for packages from an Ubuntu repository?

apt-key utility

apt-key is used to manage the list of keys used by apt to verify packages. Packages which have been verified using these keys will be considered trusted. To update the local keyring with the keyring of Ubuntu archive keys and removes from the keyring the archive keys which are no longer valid.

$ sudo apt-key update
$ sudo apt-get update

try install something...

sudo apt-get update

Was enough to fix it for me.

I had this issue on Ubuntu 12.10 (Quantal Quetzal). The same issue occurred if I tried to use the Synaptic Package Manager GUI.

It turned out that my system was still set to use a proxy server for a network I was no longer connected to and thus could no longer use. Once I disabled the proxy server, apt was able to authenticate the packages properly.

In order to disable the proxy on Ubuntu 12.10 (Quantal Quetzal), follow these steps:

1. Press the Super button on your keyboard (A.K.A. the Windows Logo button)
2. Type Network and press enter
3. Select Network Proxy
4. Change the method to None
5. Click Apply System Wide
6. apt should now be able to authenticate the packages