Mobile App OTP: Keep Secrets on the Server (React Native & Flutter)

Framework tutorials like Node.js OTP sends assume a trusted server. Mobile teams sometimes ask whether to call an SMS API straight from React Native or Flutter. The answer affects security and compliance: this post is only about that architecture split—not another language port of the same code samples.

Never Ship SMS Provider Keys in the App

Anything in a mobile binary can be extracted. A leaked key means someone else sends SMS on your wallet—see API key rotation and fraud prevention . Your mobile app should only talk to your backend with your own session or JWT authentication.

Client vs Server Responsibilities

Mobile client: collect phone number, show OTP field, handle autofill , call your REST routes like POST /auth/request-otp and POST /auth/verify-otp.

Backend: validate user identity context, enforce rate limits , generate or delegate OTP storage, call StartMessaging with the server API key, return generic success or error to the app without leaking provider internals.

React Native and Flutter Notes

Use your stack’s standard HTTP client (fetch, Dio, etc.) only for your API. Avoid adding native modules that embed third-party SMS SDKs with secrets. For Expo, follow the same rule: server-side integration, client-side UI only.

Pair With Autofill UX

Once the pipeline is secure, invest in UX: the autofill article in this blog covers platform APIs; your React Native or Flutter screens should expose the right text field types so iOS and Android can suggest codes.

FAQ

See FAQ above.