HOTP
HOTP (HMAC-based One-Time Password) is a cryptographic algorithm that generates one-time passwords (OTPs) for authentication purposes, based on a shared secret key and a counter value. It is standardized in RFC 4226 and is commonly used in two-factor authentication (2FA) systems to enhance security by providing time-insensitive codes that are valid for a single login attempt or transaction. The algorithm uses HMAC (Hash-based Message Authentication Code) with SHA-1 to produce a numeric OTP, typically 6-8 digits long.
Developers should learn HOTP when implementing or integrating authentication systems that require robust security against replay attacks, such as in banking apps, VPN access, or enterprise software. It is particularly useful in scenarios where network connectivity is unreliable, as HOTP codes do not rely on time synchronization like TOTP, making it suitable for offline or low-latency environments. Understanding HOTP helps in building secure 2FA mechanisms that comply with standards like OATH (Initiative for Open Authentication).