One-Time Passwords (OTPs) sent via phone numbers are a widely used method for verifying users during sign-up, login, or transactions. OTPs are temporary codes, usually numeric, that provide an extra layer of security. Sending OTPs reliably and securely involves several steps and components. Here’s how it generally works in apps.
1. User Initiates OTP Request
When a user enters their phone number to register or log in, the app triggers an OTP generation process. This usually happens via a “Send OTP” button after the user inputs their number.
2. Generating the OTP
The backend server or an authentication service generates a random numeric code, typically 4 to 6 digits. The OTP should be:
Random and unpredictable to prevent guessing.
Short-lived, usually valid for a few minutes.
Unique per session to avoid reuse.
3. Sending the OTP via SMS
To send the OTP, the app integrates with an SMS gateway or a recent mobile phone number data third-party communication API like Twilio, Nexmo (Vonage), MessageBird, or AWS SNS. The process includes:
Formatting the SMS message: Usually something like “Your verification code is 123456.”
Calling the SMS API: The server sends a request to the SMS service with the phone number and message content.
Carrier delivery: The SMS provider routes the message through mobile carriers to the user’s phone.
4. Handling Delivery and Errors
The app’s backend typically receives delivery status updates (e.g., sent, delivered, failed) via callbacks or API responses from the SMS gateway. This helps:
Retry sending if delivery fails.
Inform users if their number is invalid or unreachable.
5. User Enters OTP
Once the user receives the OTP via SMS, they enter it into the app’s verification field. The app then sends this input to the backend for validation.
6. Validating the OTP
The backend compares the user-entered OTP with the one it generated and sent. Validation checks include:
Matching the code exactly.
Checking expiration time to ensure the OTP is still valid.
Limiting attempts to prevent brute-force guessing.
If the OTP is correct and valid, the user is authenticated or verified.
7. Security Considerations
Rate Limiting: Prevent attackers from requesting OTPs repeatedly or brute forcing codes.
Expiration: OTPs should expire quickly (usually 5 minutes) to minimize risk.
Unique OTP per request: Never reuse OTPs.
Secure transmission: Use HTTPS for API calls and secure storage for generated OTPs.
Prevent Replay Attacks: Invalidate OTPs after successful use.
8. Alternative OTP Delivery Methods
While SMS is most common, apps may also send OTPs via:
Voice calls: The OTP is read out by an automated voice.
Email: For users who register with email addresses.
Authenticator apps: Generate time-based codes without SMS.
9. User Experience Enhancements
Auto-read SMS: On mobile platforms like Android, apps can request permission to auto-read the OTP SMS, speeding up verification.
Resend OTP option: Users can request a new OTP if they don’t receive the first one, typically with cooldown timers to prevent abuse.
Input masks: Help users enter OTPs in the correct format easily.
Conclusion
Sending OTPs via phone numbers involves generating secure, random codes and delivering them through SMS gateways integrated into the app’s backend. Proper validation, security practices, and user-friendly design ensure OTP verification is both effective and convenient. This method remains a reliable choice for two-factor authentication and identity verification in modern apps.