Signing key error persists after attempted fix

[Tony1337]

When running sudo apt-get update, I get the following:

W: GPG error: http://us.archive.ubuntu.com precise Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>

I have attempted the following, as suggested by several help sites:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 40976EAF437D05B5

I have run sudo apt-get upgrade as well as rebooted. However, I still get the same signing key error.

[albandy]

Change your repository server:
go to software center

Select edit --> software source (or something like that, I'm translating from spanish)

and in Download from select main server or primary server.

then test again if works.

[Tony1337]

I changed from "Server for United States" to "Main server" and there is no longer an error, thanks. I then changed it back to "Server for United States" and there is still no error. What do you think solved the problem?

[albandy]

Probably keys are updated when you choose other sources.