BADSIG 40976EAF437D05B5

When running apt-get update today, I got this error message:

The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key

Strange. And it was no good retrying, the same error occurred over and over again. apt-get stopped complaining after running update with this option:

apt-get update -o Acquire::http::No-Cache=True

and this:

apt-get update -o Acquire::BrokenProxy=true

I suspect my company's Barracuda Spam & Virus Firewall is to blame, as it has been blamed for arbitrarily corrupting downloads before by my co-workers. How can I know for certain?