What is a One-Time Password (OTP)?
A One-Time Password is a security credential that is valid for only a single login transaction. Unlike static passwords, which remain active until changed, OTPs protect users against replay attacks, interception, and credential stuffing. If a hacker intercepts a dynamic OTP, the code becomes useless within seconds.
OTPs are the foundation of modern Multi-Factor Authentication (MFA) and Two-Factor Authentication (2FA), implemented on banking portals, corporate networks, and email accounts.
Understanding TOTP vs. HOTP Algorithms
There are two main standard algorithms used to generate one-time passwords:
Time-Based (TOTP - RFC 6238)
Uses the current time as the counter. The code is calculated using the Unix epoch time split into intervals (periods) such as 30 seconds. Codes rotate automatically. This is the standard algorithm used by Google Authenticator, Authy, and Microsoft Authenticator.
Counter-Based (HOTP - RFC 4226)
Uses an incremental counter. The user and the server must synchronize their counter value. A code remains valid until the user uses it to authenticate, after which the counter increments. Common in physical hardware tokens with push buttons.
How Authenticator Apps Sync Codes
Authenticator applications use key agreements to synchronize login validation:
- Secret Exchange: The server generates a random Base32 secret key and shares it with the client via a QR code or alphanumeric text.
- Independent Calculation: Both the authenticator app and the server execute the same cryptographic hashing algorithm (HMAC) using the shared secret and the current counter (or time step).
- Dynamic Truncation: The resulting hash is truncated into a 6 or 8-digit number that is easy for a human to read and type.
- Verification match: The server compares the code submitted by the user with its own local calculation. If they match, entry is approved.
Troubleshooting OTP Authentication Synchronization
If your authenticator codes fail to match the server calculations, verify the following details:
- System Clock Skew: TOTP relies strictly on matching times. Ensure that your phone and server clocks are synchronized with network time servers (NTP).
- Algorithm Configuration: Verify that the HMAC algorithm settings match (SHA-1 vs SHA-256). Mixing them results in different output hashes.
- Counter Drifts (HOTP): If the counter becomes out of sync due to button clicks, servers usually search a look-ahead window of next counter steps to resynchronize automatically.