@Chet
"Now we can't simply inspect a file for an appropriate signature"
SSL Certs have never purported to verify the "goodness" of data. This is a common misunderstanding amongst the plebs but I am shocked, shocked I say, that someone posting on behalf of Sophos would also labour under such a basic misconception.
You should know that digital signatures are no different at all from traditional signatures. All they do is confirm that the signer is who they say they are. They do not have any bearing whatsoever on the quality or veracity of the signed material.
There is no reason whatsoever that Osama Bin Laden himself couldn't sign his latest jihad orders just as George W Bush signed his documents declaring that Waterboarding is AOK and should be actively applied at GitMo.
In neither case do their signatures qualify the ethical validity of the orders, they just confirm that the orders were signed by the person who has the authority to make such orders.
If part of Sophos "security scan" included a "scan for an appropriate signature" then all I can say is I'm glad I don't use Sophos products for security.