1. A method comprising: receiving at a signing device an electronic document provided by a source device; preventing alterations to the electronic document on the signing device; generating a signature document by calculating a one-way hash for the electronic document and embedding a digital watermark representing the one-way hash in an arbitrary digital representation; altering a characteristic of the signature document based on a user input; and associating the electronic document with the signature document, wherein the electronic document and the signature document together comprise a signed electronic document.