OTP
A single-use password or code that expires after one login session or short time period, used as a second factor in MFA.
What is OTP?
OTP (One-Time Password) is a password that is valid for only one login session or transaction. Once used, it becomes invalid and cannot be reused. OTPs are a core component of multi-factor authentication (MFA), providing a second factor that proves the user has access to something (their phone, email, or hardware token).
Two main types of OTP:
- TOTP (Time-Based OTP): Codes based on time (RFC 6238) - changes every 30 seconds (Google Authenticator)
- HOTP (HMAC-Based OTP): Codes based on counters (RFC 4226) - changes after each use (RSA SecurID)
OTPs can be delivered via:
- SMS: Text message with code (convenient but vulnerable to SIM swapping)
- Email: Email with code (accessible but can be intercepted)
- TOTP Apps: Google Authenticator, Authy (most secure, works offline)
- Hardware Tokens: YubiKey, RSA SecurID (most secure, physical device)
Analogy
Think of an OTP like a disposable camera - you use it once to take a picture (log in), and then the film is used up. If someone finds your used camera (stolen OTP), they can't take another picture with it because the film is already exposed.
Types and Use Cases
- Two-Factor Authentication: Send OTP via SMS/Email/TOTP as second factor after password login
- Transaction Authorization: Require OTP for high-value transfers or sensitive account changes
- Password Reset: Send OTP to verify ownership of email/phone during password recovery
- New Device Login: Require OTP when login comes from unrecognized device or location
How it Works
// OTP Generation and Validation (Simplified)
// Generate OTP (6-digit numeric)
function generateOTP() {
return Math.floor(100000 + Math.random() * 900000).toString();
}
// Send OTP via SMS (using Twilio example)
await twilioClient.messages.create({
body: `Your verification code is: ${otp}. Valid for 5 minutes.`,
to: userPhone,
from: '+1234567890'
});
// Validate OTP
function validateOTP(userInput, storedOTP, expiryTime) {
if (Date.now() > expiryTime) return { valid: false, reason: 'expired' };
if (userInput !== storedOTP) return { valid: false, reason: 'invalid' };
return { valid: true };
}OTP vs Static Passwords
OTP
Static Passwords
OTP is single-use and time-limited
Static Password is reusable and permanent until changed
OTP provides something you have (phone, token)
Password is something you know
OTP is phishing-resistant (expires quickly)
Passwords can be reused by attackers if stolen
Best Practices for OTP
- Prefer TOTP over SMS: SMS OTP is vulnerable to SIM swapping; TOTP (Google Authenticator) is more secure
- Short expiry times: Set OTP expiry to 5-10 minutes maximum to limit attack window
- Rate limit attempts: Limit OTP verification attempts to 3-5 tries to prevent brute-force attacks
How LoginRadius Powers OTP
LoginRadius CIAM platform provides comprehensive OTP authentication support across multiple channels: SMS (Twilio, MessageBird), Email, TOTP (Google Authenticator, Authy), and push notifications. Our APIs handle OTP generation, delivery, validation, rate limiting, and automatic expiry. LoginRadius allows you to configure OTP policies (length, expiry time, max attempts) per application or user group. We also provide fallback mechanisms, brute-force protection for OTP endpoints, and detailed audit logs for compliance reporting.
FAQs
SMS OTP provides basic security but has significant vulnerabilities: (1) SIM Swapping - attackers can transfer your phone number to their SIM, (2) SMS Interception - SS7 protocol vulnerabilities allow interception, (3) Phishing - users can be tricked into sharing SMS codes. For better security, use TOTP apps (Google Authenticator) or FIDO2 hardware keys (YubiKey). NIST no longer recommends SMS OTP for AAL2+ without restrictions.
TOTP (Time-Based OTP) generates codes based on current time (changes every 30 seconds) - used by Google Authenticator, Authy. HOTP (HMAC-Based OTP) generates codes based on a counter (changes after each use) - used by RSA SecurID. TOTP is more common today because it doesn't require the server and client to stay in sync (both just use current time). HOTP requires counter synchronization, which can cause issues if tokens are generated but not used.
LoginRadius supports multiple OTP methods: SMS OTP (via Twilio, MessageBird, and other providers), Email OTP, TOTP (Google Authenticator, Authy), and push notification OTP. Our platform provides OTP generation, delivery, validation, rate limiting, and expiry management. LoginRadius also offers configurable OTP policies (length, expiry, attempts), fallback options, and detailed logs for compliance. For highest security, we recommend TOTP or FIDO2 over SMS OTP.