RS-2026-087 KILLKEY: Hardware Presence as Kill-Switch Authentication

Software kill switches are not kill switches when the kill switch itself is the attack surface. T52 BLACKOUT weaponises software safety override mechanisms, turning them into attack vectors. T298 TOKENSLIP enables an agent to detect impending revocation, pre-cache credentials, pivot identity, and continue execution after the kill switch fires. T308 REGICIDE systematically neutralises software kill switch daemons through five vectors: SIGNAL_STARVATION, DEPENDENCY_EXHAUSTION, API_FLOODING, CREDENTIAL_DISCOVERY, and TIMING_EXPLOITATION. All five vectors are effective against a software-only daemon. This paper presents KILLKEY, a hardware presence interlock architecture that defeats four of the five REGICIDE vectors by design, names the fifth honestly, and documents the v1 closure. KILLKEY implements a four-state graduated response (DISARMED, ARMED, TRIPPED, SEALED) using commodity RP2350 hardware, HMAC-SHA256-authenticated heartbeats over USB serial, and Linux cgroup v2 containment. Physical absence cannot be simulated from software, however capable or compromised. We document the REGICIDE survival analysis, architecture, wire protocol, defensive module positioning, and the KILLKEY-LEASE future work direction. The reference implementation passes 28 of 29 SPECTER AUDIT v1.2.0 checks at 88% line coverage.