Explain how a digital certificate attached to an email may help to provide author origin integrity (that is, the receiver of such an email can be sure that the particular email sent from A was indeed authored by A and not anybody else). Hint: Devise an emailing protocol that provides author origin integrity by using digital certificates. Note: Cite the source of information used in your answer, unless you are the one that have developed that protocol.

Explain what additional mechanism(s) will be needed in order for the protocol you devised above to provide sender origin integrity. Present the complete protocol.