9. A system for validating an electronic document, the system comprising: one or more processors; an electronic document reader; and a non-transient computer-readable medium having an electronic document stored thereon, the electronic document comprising: a plurality of content items including one or more invariant content items; a set of rules that defines a set of allowed operations which are authorized to be performed on the plurality of content items by the electronic document reader, wherein performing the allowed operations does not result in any changes being made to the one or more invariant content items; a first selective digest that is generated by a document author based on a first digesting of the one or more invariant content items; validation information that results from a comparison of the first selective digest and a second selective digest that is generated by the one or more processors based on a subsequent digesting of the one or more invariant content items, wherein the validation information indicates whether operations have been performed on the electronic document which are not allowed under the set of rules; and instructions configured to cause the electronic document reader to disable the allowed operations when the comparison of the first and second selective digests indicates that operations have been performed on the electronic document which are not allowed under the set of rules.
10. The system of claim 9 , wherein the second selective digest is represented as a hash having a lower layer containing functionality for digesting a basic content item that consists of a string, an intermediate layer containing functionality for digesting a semi-complex content item that consists of an annotation, and an upper layer containing functionality for digesting a complex content item that consists of a page.