Signatures are important at all points in the process. During workflow, they authenticate agreement or disagreement with the action being worked. As part of document and records management, they assure readers that the document is authentic. In the electronic world, digital signatures can give us a higher degree of reliability and integrity than pen and ink signatures on paper. However, they may also complicate some processes if we're not aware of how they work.
Letters are a fairly simple model.
They are normally only signed by one person, so authenticating them can be as simple as using the author's private key in the signature, which can be verified by everyone who receives the letter using that person's corresponding public key . This can apply to either e-mail or documents authenticated by a single authority. Documents with more than one signature are more of a challenge, particularly when the signatures are applied sequentially as part of a coordination process. For example, minute changes to a document, like correcting a spelling error, can invalidate previously applied signatures. Authentication algorithms can't, at present, differentiate between changing content or fixing a typo.