13. A computer readable storage device storing a computer program which, when executed by a computer processor, causes the computer processor to 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 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, wherein the aggregation includes a simple content item, a semi-complex content item and a complex content item, and the object digest is generated by digesting the existing invariant content items and the new invariant content items 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.
13. A computer readable storage device storing a computer program which, when executed by a computer processor, causes the computer processor to 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 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, wherein the aggregation includes a simple content item, a semi-complex content item and a complex content item, and the object digest is generated by digesting the existing invariant content items and the new invariant content items 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. 14. The storage device of claim 13 , wherein the digesting is recursive on layers of content items in the electronic document.