1. A computer-implemented method comprising: receiving, by a computing device processor, an electronic document including content items, a rule and a digital signature, wherein the rule specifies what parts of the electronic document are allowed to change based on user interaction with the electronic document; generating a digest for the electronic document by digesting all of the content items, using multiple functions based upon complexity of the content items, except for at least a first content item that is ignored in the digestion based on the rule; comparing the generated digest with a stored digest that is associated with the electronic document; and invalidating the digital signature if the generated digest indicates a difference in any of the digested content items, wherein if the generated digest indicates no difference in any of the digested content items, the method further comprises: subsequently receiving a user input attempting to create a new state of the received electronic document; determining whether the user input is allowed by the rule; and invalidating the digital signature if the user input is not allowed by the rule, and wherein the rule applies differently to a first author and a second author, such that the user input causes a first digital signature of the first author to be invalidated but does not cause a second digital signature of the second author to be invalidated.