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Framework Karl Crary Susmit Sarkar De ember, 2003 CMU-CS-03-108 S hool of Computer S ien e Carnegie Mellon University Pittsburgh, PA 15213 Abstra t Foundational erti ed ode systems seek to prove untrusted programs to be safe relative to safety poli ies given in terms of a tual ma hine ar hite tures, thereby improving the systems' exibility and extensibility. Previous e orts have employed a stru ture wherein the proofs are expressed in the same logi used to express the safety poli y. We propose an alternative stru ture wherein safety proofs are expressed in the Twelf metalogi , thereby eliminating from those proofs an extra layer of en oding needed in the previous a ounts. Using this metalogi al approa h, we have onstru ted a omplete, foundational a ount of safety for a fully expressive typed assembly language.