While performing an update on my repository via "apt-get update" I received the error message
The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>
I checked and found a post from 2005 on Ubuntu Forums which you can find here where the user "heyanowut" provided his found fix. For one of the steps in his fix he suggested creating a new /var/lib/apt/lists folder with a sub-folder "partial". The steps he laid out that were most relevant to my issue were:

  • Rename the old lists folder to something like "lists_bu"
  • Create a new lists folder with the same name hs the original i.e. "lists" and make sure you have a sub-directory called "partial"
  • run the clean command
  • run your update command

(First step needs to be performed while in directory /var/lib/apt , as shown of course):
:/var/lib/apt$ sudo mv lists lists_bu
:/var/lib/apt$ sudo mkdir -p lists/partial
:/var/lib/apt$ sudo apt-get clean all
:/var/lib/apt$ sudo apt-get update
Once that is finished and I had no further error messages, I was able to run upgrade tasks as needed.