Comment 21 for bug 705988

Workaround by Steven Major (semajor) from https://answers.launchpad.net/ubuntu/+source/update-manager/+question/172657:

in a terminal, submit:
      sudo apt-get update

There will be an error like this:
      GPG error: http://software.rc.fas.harvard.edu natty/ Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY ####

where #### is some string of digits and letters.

'####' is the missing key. To provide this key, submit:
      sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys ####

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This way of stopping "untrusted software" is harmful in 3 ways:
1. User is not aware what these keys really mean, so it contributes nothing to security.
2. It stops updating the whole system because of one questionable source.
3. It reminds of ancient mswindows idiocy to dictate what software is good for user and what is not.

A GUI warning for user to think again before installing, is maximum that is still ethical, imho.