The difficulty with signatures (both digital and not) is that they can be duplicated, and even beyond that accepting one involves a lot of trust. In essence, a signature is not a way to ensure that a particular document came from someone in particular, but a way to establish a paper trail in case there is ever any doubt or whatever.
When I write a cheque, and sign at the bottom, the teller at the bank who eventually looks at it doesn't know for sure that is my signature. In fact, I knew someone who had her credit card stolen, and the thief used it four times, signing in four completely different ways. So a physical signature is just a way to say "Until someone says otherwise, this document is from Lai."
Thing with digital signatures is, they're digital. Therefore, they can be instantly copied with 100% accuracy given just one example of the original. The fact that in order to make a signature at all useful you need to give it to people, means that even security through obscurity is impossible. You can't copy-protect anything because the act of giving someone a document is copying it.
So, digital signatures can't be used to identify documents as coming from a specific source, unless they hold a reference to a database somewhere (either belonging to the signer or some third party) which keeps track of every document legitimately signed with that signature. Without a way to verify that not only is that signature on that cheque identical to mine, but that I personally made a record of having signed it, the signature is about as secure as the one at the bottom of this post. Then we get into ways of ensuring that the database you're checking is in fact a valid one...