\NC3 Interface Boundary Kernel (NC3-IBK): A Technical Architecture for Computable NC3 Interface Specification in Nuclear Weapon System Acquisition The integration surface between the Nuclear Command, Control, and Communications (NC3) enterprise and the LGM-35A Sentinel ICBM — where presidential authority becomes physical action — lacks computational structure. Interface status is reported through documentation review; "interface complete" is a program assertion, not a computed state. The NC3 transport layer (Evolved Strategic SATCOM, SACCS-R, FAB-T) is being fielded concurrently with Sentinel, the threat environment has evolved beyond foundational hardening standards (the EMP Commission recommends 85 V/km E3 HEMP for CONUS versus the 8 V/km NERC benchmark), and a multi-decade dual-stack transition with Minuteman III doubles the integration burden. NC3-IBK proposes a formally structured interface contract — NC3_IF_BOUNDARY_SPEC v2.0 — expressed as the tuple ⟨P(t), E(params), M(stack,epoch), S(lifecycle)⟩. P(t) treats the physical boundary as a lifecycle-managed object requiring Hardness Maintenance/Hardness Surveillance. E(params) parameterizes the E1/E2/E3 HEMP threat envelope rather than hard-coding values that become obsolete. M(stack,epoch) defines nine message families with dual-stack and NC3 transport epoch tagging. S(lifecycle) integrates nuclear surety (DoDI 3150.02, December 2024 reissuance) with cybersecurity requirements including secure update protocols, supply chain risk management (DoDI 5200.44), and Cross Domain Solution fail-state specifications (NCDSMO RAIN principles). Nine acceptance criteria (AC1–AC9) transform interface verification from narrative assertion to computed state. 47 references. All URLs verified as of March 2026. UNCLASSIFIED.