13. A computer readable storage device storing a computer program which, when executed by one or more computer processors, causes the one or more computer processors to perform operations comprising: receiving, by a computing device processor, an electronic document including a plurality of content items, a rule, a first digest, and a digital signature of a document author, wherein the rule identifies a subset of the content items that are invariant to operations authorized by the rule; generating a second digest for the electronic document by digesting the subset of content items that are invariant to operations authorized by the rule, wherein content items that are modifiable pursuant to the rule are not considered in generating the second digest; invalidating the digital signature of the document author in response to detecting a discrepancy between the first and second digests and restricting access to the electronic document in response to detecting the invalidated digital signature.
13. A computer readable storage device storing a computer program which, when executed by one or more computer processors, causes the one or more computer processors to perform operations comprising: receiving, by a computing device processor, an electronic document including a plurality of content items, a rule, a first digest, and a digital signature of a document author, wherein the rule identifies a subset of the content items that are invariant to operations authorized by the rule; generating a second digest for the electronic document by digesting the subset of content items that are invariant to operations authorized by the rule, wherein content items that are modifiable pursuant to the rule are not considered in generating the second digest; invalidating the digital signature of the document author in response to detecting a discrepancy between the first and second digests and restricting access to the electronic document in response to detecting the invalidated digital signature. 15. The computer readable storage device of claim 13 , wherein the operations further comprise receiving input that creates a new state of the electronic document, wherein the second digest corresponds to the new state of the electronic document.