At the same time, Dorothy Denning at Purdue University was publishing her Ph.D. dissertation, which dealt with "lattice-based information flows" in computer systems. (A mathematical "lattice" is a partially ordered set, characterizable as a directed acyclic graph, in which the relationship between any two vertices is either "dominates," "is dominated by," or neither.) She defined a generalized notion of "labels"—corresponding more or less to the full security markings one encounters on classified military documents, "e.g.", TOP SECRET WNINTEL TK DUMBO—that are attached to entities. Bell and LaPadula integrated Denning's concept into their landmark MITRE technical report—entitled, "Secure Computer System: Unified Exposition and Multics Interpretation"—whereby labels attached to objects represented the sensitivity of data contained within the object (though there can be, and often is, a subtle semantic difference between the sensitivity of the data within the object and the sensitivity of the object itself), while labels attached to subjects represented the trustworthiness of the user executing the subject. The concepts are unified with two properties, the "simple security property" (a subject can only read from an object that it "dominates" ["is greater than" is a close enough—albeit mathematically imprecise—interpretation]) and the "confinement property," or "*-property" (a subject can only write to an object that dominates it). (These properties are loosely referred to as "no-read-up" and "no-write-down," respectively.) Jointly enforced, these properties ensure that information cannot flow "downhill" to a repository whence insufficiently trustworthy recipients may discover it. By extension, assuming that the labels assigned to subjects are truly representative of their trustworthiness, then the no-read-up and no-write-down rules rigidly enforced by the reference monitor are provably sufficient to constrain Trojan horses, one of the most general classes of attack ("sciz.", the popularly reported worms and viruses are specializations of the Trojan horse concept).