Abstract: A "signature data object" (SDO) is a machine-readable structured data record which a signer may associate with a parent object, e.g., an electronic document. An SDO comprises three required encoded parts: A signature header structure; a signature data structure; and a signature proof block. An SDO may include data identifying a role in which the signer is acting and a message code encoding a semantic for the parent object/document such as "submitted by," "approved," etc. In one embodiment, the SDO includes at least one descriptor of (1) a subordinate part of the parent object or (2) an attachment of the parent object/document, to whose (i) value or (ii) presence the validity of the signature data object is bound. An SDO may also include data indicating that it supersedes a specified other SDO associated with that parent object.