root says:
This is the same problem that arose with the original idea of seals once the skills of metalworking became commen enough. By the 1500's it was nearly impossible to keep a seal confidential more than a few weeks until somebody got a impression and built a copy.
Seals were duplicatable from the start. You just needed clay and a seal made with the oritinal if you wanted to forge them -- fairly common stuff. Signatures have been duplicatable from the start, too. Signatures and seals are NOT truly authenticating technologies. They are just a legal mechanism for an entity to demonstrate that it has read and agreed to the terms on a document. It was always assumed that someone might forge a signature, which is why if you had a document where you cared that people might disclaim their signature, you got people who could testify to the signature to witness the signing. Digital "signatures" are the first real unforgeable authentication technology mankind has developed. Perry