2台のスマホを安全につなぐために工夫したこと — Baby Monitor Timmy

The key design priorities for Timmy were: enabling two smartphones on hand to be used as a baby monitor without purchasing dedicated cameras, and securely connecting them using the same approach whether pairing Android-to-Android, iPhone-to-iPhone, or Android-to-iPhone combinations. When Timmy is already installed on both Android devices, a 4-character meeting code is passed to the second device via QR code or invitation link. The second device advances to the security confirmation screen, but the connection is not complete there. The same 2-digit number appears on both devices, and pairing is confirmed only after the user verifies it on both ends. When combining Android and iPhone, pairing also occurs within the app, followed by the same 2-digit verification. The 4-character code itself is not used as an encryption key. A new ECDH P-256 key exchange is performed for each device, from which the connection key and security confirmation number are derived. The pairing key is not sent to the backend; signaling is protected with AES-256-GCM, and live audio and video are protected via WebRTC. After connection, users can access live audio, one-way video as needed, talkback, and camera control from the parent device. The design does not include continuous cloud recording of video or audio. No personal accounts, advertisements, or tracking SDKs are used. When switching between Wi-Fi and mobile networks, automatic reconnection is attempted. If disconnection persists during reconnection, a clear warning is displayed on the parent device. However, Timmy does not guarantee recovery within a fixed time period or medical-level detection, and cannot serve as a substitute for in-person supervision. Japanese product information and store link: