Installing packages from not authenicated sources

I faced a strange situation today when I ran the Update Manager in Ubuntu. It showed me a number of updates to be installed and when I clicked on Install Updates it showed me the following error:

The action would require the installation of packages from not authenticated sources

There could be several reason for this error:

• Software is restricted by copyright or legal issues
• The Software is from Canonical Partner and has not been added to Source List
• The application has been installed form its source code

If you do not know which is the root cause of error, then follow the instructions for each of them and then you can find it out. Lets see how to tackle these problem one by one:

(i) Software is restricted by copyright or legal issues

Run the Update Manager. And go to Settings as shown in the pic below:

Now here, go to the Ubuntu Software tab and make sure that Software is restricted by copyright or legal issues field is checked. Close the settings and install updates now.

(ii) The Software is from Canonical Partner and has not been added to Source List

In the Update Manager Settings, go to Ubuntu Software tab and include the Source Code for updates like in the pic below:

(iii) The application has been installed form its source code

In the Update Manger Settings, go to Ubuntu Software tab and check the Source Code option: