25. A system comprising a computer readable storage device storing a computer program, a display device, and one or more processors operable to interact with the display device and to execute the computer program and perform operations comprising: reading an electronic document, the electronic document comprising content rules, an existing signed state, and existing content items; authenticating the electronic document one or more times, each authenticating comprising: classifying the existing content items of the electronic document into existing invariant content items and existing modifiable content items, according to the content rules; receiving one or more user actions for one or more of the existing one or more content items; determining whether the one or more user actions are permitted by the content rules; reclassifying one or more existing modifiable content items into new invariant content items in response to the one or more user actions; generating an object digest for an aggregation of the existing invariant content items and the new invariant content items by digesting the existing invariant content items and the new invariant content items, wherein the aggregation includes a simple content item, a semi-complex content item and a complex content item, the object digest is generated according to their complexity and generating the object digest includes ignoring one or more existing modifiable content items in the electronic document that are not reclassified into new invariant content items; generating a saved state of the electronic document; and adding a new signed state to the electronic document, the new signed state comprising the object digest, the saved state, and an electronic signature.
25. A system comprising a computer readable storage device storing a computer program, a display device, and one or more processors operable to interact with the display device and to execute the computer program and perform operations comprising: reading an electronic document, the electronic document comprising content rules, an existing signed state, and existing content items; authenticating the electronic document one or more times, each authenticating comprising: classifying the existing content items of the electronic document into existing invariant content items and existing modifiable content items, according to the content rules; receiving one or more user actions for one or more of the existing one or more content items; determining whether the one or more user actions are permitted by the content rules; reclassifying one or more existing modifiable content items into new invariant content items in response to the one or more user actions; generating an object digest for an aggregation of the existing invariant content items and the new invariant content items by digesting the existing invariant content items and the new invariant content items, wherein the aggregation includes a simple content item, a semi-complex content item and a complex content item, the object digest is generated according to their complexity and generating the object digest includes ignoring one or more existing modifiable content items in the electronic document that are not reclassified into new invariant content items; generating a saved state of the electronic document; and adding a new signed state to the electronic document, the new signed state comprising the object digest, the saved state, and an electronic signature. 36. The system of claim 25 , wherein: the content rules state that a first user action of a first signature causes a first subset of the existing modifiable content items to be reclassified into new invariant content items and a user action of a second signature causes a second subset of the existing modifiable content items is reclassified into new invariant content items; the first signature is not a part of the first subset of the existing modifiable content items; the second signature is not a part of the second subset of the existing modifiable content items; and the first subset of the existing modifiable content items does not overlap the second subset of the existing modifiable content items.