Imagine an ‘Oh, yeah?’ button on your browser. There you are looking at a fantastic deal that can be yours just for the entry of a credit card number and the click of a button. "Oh, yeah?", you think. You press the ‘Oh, yeah?’ button. You are asking your browser why you should believe it. It, in turn, can challenge the server to provide some credentials: perhaps, a signature for the document or a list of documents that expresses what that key is good for. Those documents will be signed. Your ...