The OTP (One-Time Password) authenticates a sensitive action: login, payment, changing contact details. Its success rests on a constant trade-off between security and user experience. Here is how to make the call.
1. The right length
Six digits are the standard: enough entropy to resist random guessing, short enough to enter without error. Dropping to four weakens it; going to eight degrades the experience with no real gain given proper attempt limiting.
2. A short validity period
A code valid for too long remains exploitable if intercepted. A window of a few minutes is enough. State the expiry clearly in the message: Your code is 480912. Valid for 5 min.
3. Strictly single use
The code must be invalidated as soon as it is used — or as soon as a new one is generated. Allowing multiple validations with the same code reopens an obvious flaw.
A reusable OTP is no longer an OTP. Immediate invalidation after use is non-negotiable.
4. Attempt limiting
Limit the number of entry attempts and the number of code requests per user and per period. It is the most effective defense against brute force and abuse.
5. Protection against pumping
"SMS pumping" generates OTP requests en masse to premium-rate numbers to divert revenue. Anomaly detection, per-country caps and filtering of suspicious prefixes are essential.
6. A clear, plain message
The message should contain the code, its validity period and, ideally, the service name. Avoid clickable links in an OTP message: they open the door to phishing.
- The code, clearly visible;
- The validity period;
- No links, no attachments.
7. Voice fallback
When the SMS does not arrive — poorly covered area, carrier in difficulty — a voice call dictating the code saves the login. Cross-channel fallback applies to OTP too.
8. Measuring the completion rate
Track the share of users who actually validate their code. A dropping rate signals a deliverability or usability problem, well before the complaints reach support.
With FasterMessage, OTP generation, expiry, validation and anti-fraud are handled for you, with built-in voice fallback. You keep control of the rules; we handle the mechanics.