Difference Between OTP, TOTP, and HOTP

OTP (One-Time Password), TOTP (Time-Based One-Time Password), and HOTP (HMAC-Based One-Time Password) are authentication mechanisms that generate unique codes for user verification. While they share similarities, their differences lie in how and when the codes are generated and validated.

What is OTP?

OTP (One-Time Password) is a generic term for a password that is valid for only one authentication session or transaction. OTPs are commonly used in multi-factor authentication systems.

Example: A code sent via SMS during login.
Usage: Protects against password reuse and some phishing attacks.

What is TOTP?

TOTP (Time-Based One-Time Password) is a type of OTP that is generated based on the current time.

Key Features of TOTP:

Time Dependency:
- TOTP codes are valid for a short period (usually 30–60 seconds).
Algorithm:
- Uses the current timestamp and a shared secret key to generate the code.
Example:
- Google Authenticator or Microsoft Authenticator codes.

Advantages:

No need for server-side storage of codes.
More secure than SMS-based OTPs.

Limitations:

Requires synchronized clocks between the server and the client device.

What is HOTP?

HOTP (HMAC-Based One-Time Password) is a type of OTP generated using a counter.

Key Features of HOTP:

Counter Dependency:
- Codes are based on an event counter that increments with each authentication request.
Algorithm:
- Uses a shared secret key and a counter value in a hashing function (HMAC).
Example:
- Used in some hardware tokens for secure authentication.

Advantages:

Does not rely on time synchronization.
Ideal for systems with predictable event triggers.

Limitations:

May lead to desynchronization if the server and client counters become out of sync.

Key Differences Between TOTP and HOTP

Feature	TOTP	HOTP
Dependency	Time-based	Event (counter) based
Expiration	Valid for a set time (e.g., 30s)	Valid until used or counter changes
Use Case	Mobile apps like Google Authenticator	Hardware tokens or software tokens
Synchronization	Requires synchronized clocks	Requires counter synchronization

When to Use Each

OTP: General term applicable to any single-use password mechanism.
TOTP: Ideal for mobile app-based two-factor authentication where time synchronization is feasible.
HOTP: Suitable for hardware tokens or scenarios where counters can be managed effectively.

Conclusion

While OTP serves as a broad category, TOTP and HOTP are specific implementations. TOTP offers time-based dynamic codes, suitable for fast-paced environments, while HOTP provides counter-based authentication for more controlled use cases. Both methods are widely used for securing sensitive systems and enhancing authentication processes.