What is TOTP? Time-Based OTP Explained for Developers

If you have ever scanned a QR code into Google Authenticator and watched it spit out a fresh 6-digit code every 30 seconds, you have used TOTP. It is the most widely deployed second factor on the internet, the algorithm that powers virtually every authenticator app, and a deserved alternative to SMS OTP for users who want a faster, free and offline-capable verification step.

This explainer answers what TOTP is, how the algorithm actually generates codes, where it shines compared to SMS, and how to implement it (or layer it on top of an existing SMS-OTP flow).

TOTP — Definition

TOTP (Time-based One-Time Password) is an algorithm that derives a short numeric code from a shared secret and the current time. It is defined in RFC 6238 and is a time-based variant of HOTP (RFC 4226).

The defining properties:

• A shared secret is exchanged once, at enrolment (usually via QR code).
• Both server and client derive the current code by hashing secret + current_time_window.
• The code rotates automatically — typically every 30 seconds — without any further communication.

How TOTP is Generated

The full RFC 6238 derivation, step by step:

1. Take the shared secret K (a base32-encoded byte string, usually 20 bytes).
2. Compute T = floor((current_unix_time - T0) / X), where T0 = 0 and X = 30 (the standard time-step).
3. Compute HMAC = HMAC-SHA1(K, T) — an 8-byte big-endian counter.
4. Take the last 4 bits of the HMAC as an offset; extract 31 bits starting at that offset.
5. Modulo by 10^d (where d is the desired digit count, usually 6) and zero-pad.

// Pseudocode
T = floor(unixTime / 30)
HMAC = HMAC-SHA1(secret, T)
offset = HMAC[19] & 0x0F
truncated = (HMAC[offset] & 0x7F) << 24
          | HMAC[offset+1] << 16
          | HMAC[offset+2] << 8
          | HMAC[offset+3]
code = truncated % 1000000

Real implementations use battle-tested libraries — Python pyotp, Node otplib, Go otp, etc. Roll your own only as a learning exercise.

TOTP vs SMS OTP

When to Choose TOTP

Strong candidates for TOTP:

• Internal admin dashboards.
• B2B SaaS where users are technically inclined.
• High-volume login flows where SMS cost dominates.
• Markets with poor cellular coverage where SMS delivery is unreliable.

Stick with SMS OTP when:

• You serve mass-market consumer users in India.
• Sign-up frequency is low and SMS cost is negligible.
• Your users are unlikely to install an authenticator app or back up recovery codes.

Implementing TOTP

The integration usually has three pieces:

1. Enrolment. Generate a random 20-byte secret, store it server-side, encode it in a otpauth://totp/... URL, render the URL as a QR code, and have the user scan it with Google Authenticator / Authy.
2. Verification. Receive the 6-digit code, recompute the expected codes for time-windows T-1, T, and T+1 (to allow ±30s clock drift), and compare in constant time.
3. Replay protection. Store the highest accepted time-window per user; reject any code from the same or earlier window.

For most teams, the right call is to keep using SMS OTP via StartMessaging for primary enrolment and recovery, and add a TOTP enrolment screen as an optional speed-up. The TOTP code itself is a few lines with a library; the harder problem is always recovery.

Backup Codes and Recovery

Lost phones break TOTP-only setups. The standard mitigations:

• Backup codes. 10 single-use codes generated at enrolment, displayed once, encouraged to print or store in a password manager.
• SMS fallback. Allow users to receive an SMS OTP if they lose their authenticator. This pulls security back to the weakest factor, but is the right UX trade-off for most consumer apps.
• Identity-bound recovery. Manual support flow with KYC verification. Required by many regulators for fintech.

Pitfalls

• Server clock drift. If your servers are off by more than 30s, every code will appear invalid. Run NTP.
• Storing the secret in plaintext. Encrypt at rest using KMS or libsodium.
• No replay protection. A code from window T can be replayed inside the 30s window unless you track the last accepted T.
• Logging the code. Even a TOTP code in your access log is a 30-second window of vulnerability.

FAQ

Want SMS OTP as your primary factor with TOTP as an upgrade later? StartMessaging gets you sending OTPs in five minutes — at Rs 0.25 each, with no DLT registration to deal with.