AASPAS: Proximity-Initiated, Cryptographically Verified Patient Check-In for Outpatient Clinics — Design, Implementation, and Limits

Outpatient departments in high-volume Indian hospitals routinely ask patients to repeat their clinical history at every visit. Sehat Saathi, a prototype built by Team Medicoders, prepares a structured pre-consultation summary so the doctor does not start from zero. AASPAS is its arrival subsystem: a proximity-initiated, cryptographically verified check-in that links a patient's physical arrival to their clinical record. A laptop in the doctor's room emits a continuous 19.2 kHz ultrasonic tone; the patient's phone detects it with an 8192-point FFT detector (SNR >= 12 dB, five confirmation frames) and completes an HMAC-SHA256 challenge-response with the backend over the clinic LAN. Challenges carry 128-bit nonces, expire in 120 seconds, and are consumed atomically so each can be used exactly once. A server-side initiation state machine - added after an independent audit found fifteen defects - binds every check-in to an active doctor session and a real appointment. The full test suite, 99 tests all passing in a virtual-machine environment, covers replay, race, expiry, rate-limit, and authorization cases. We report the limits plainly: relay attacks are not mitigated, no acoustic range or reliability figures were measured on real hardware, the design still requires the patient's phone, and nothing here is production-ready. Future work targets phone-free check-in using radio-frequency body-reflection sensing.