Stamper signs and automatically publishes hashes of its history. It is a "block chain" in that sense: If Richardson decided to use the keys to backdate something to two years ago, he would have to fake two years of history, and risk being exposed if even one person came forward with a hash he'd signed contradicting his new fake history.

That is presumably one of the reasons that hasn't happened in the roughly 30 years the service has been ticking along.