What is HOTP (HMAC-based One-Time Password)?
HOTP is a one-time password algorithm used to authenticate users across a range of security applications. It generates a unique numeric or alphanumeric code for each login or transaction, combining a shared secret key with an incrementing counter processed through HMAC (Hash-based Message Authentication Code) cryptographic functions.
HOTP is event-driven: a new password generates only when a specific event occurs, such as a user pressing a button on a hardware token or initiating a login attempt. Passwords are not time-limited and remain valid until the next event increments the counter. This distinguishes HOTP from TOTP (Time-Based One-Time Password), which uses the current time as its moving factor rather than a counter.
How HOTP works
Initialization: The server and HOTP device (a hardware token or authentication app) agree on a shared secret key and a starting counter value of zero. The secret key is randomly generated and securely exchanged between both parties.
Generation: When an OTP is needed, the device combines the secret key and current counter value and passes them through HMAC-SHA1, producing a unique hash.
Truncation: The hash is truncated into a 6 to 8 digit number, which becomes the one-time password.
Increment: After the OTP is used, both the server and device increment their counters by one, preparing for the next generation cycle.
Authentication: The user submits the OTP to the system. The server independently generates an OTP using its stored secret key and counter, then checks whether it matches what the user provided. A match grants access.
Synchronization: If the server and device counters fall out of sync due to unused OTP generations, the server can validate OTPs within a look-ahead window to re-establish synchronization.
Unused HOTPs remain valid until the counter increments through a successful authentication event. This is a meaningful distinction from TOTP, where passwords expire on a fixed time schedule.
Strengths
Uniqueness: Each password is generated fresh for every event, eliminating the risk of password reuse.
No time synchronization required: Unlike TOTP, HOTP does not depend on clock alignment between server and client, which benefits systems where time synchronization is unreliable.
Offline generation: A sequence of HOTPs can be generated in advance for use without network connectivity, which TOTP cannot support due to its time dependency.
Replay attack resistance: Each OTP is valid only once, so intercepted passwords cannot be reused by an attacker.
Interoperability: HOTP is standardized under RFC 4226, enabling compatibility between hardware and software from different vendors.
Versatility: HOTP works across authentication scenarios for both digital and physical access control.
Weaknesses
Counter desynchronization: If OTPs are generated but not used, the server and device counters can drift out of sync, causing authentication failures that require manual resynchronization.
Phishing exposure: An attacker who tricks a user into submitting their OTP on a fake site can capture and use it before it expires.
Man-in-the-middle risk: If an attacker intercepts communication between client and server, they can capture a valid OTP and use it to gain access.
Device dependency: A lost, stolen, or malfunctioning token prevents authentication until a replacement device is provisioned.
No local confirmation: Without a challenge-response implementation, the user receives no confirmation that their OTP was actually consumed.
Brute-force vulnerability: Without rate limiting or lockout policies on the server, an attacker could cycle through possible OTP values until one succeeds.
Insecure key exchange: If the initial secret key and counter are not shared securely, the foundation of the HOTP system is compromised before any authentication occurs.
OTP vs. HOTP vs. TOTP
OTP (One-Time Password) is the base concept: a password valid for a single login session or transaction. It cannot be reused after its intended use. OTP is the foundation on which both HOTP and TOTP are built.
HOTP (HMAC-Based One-Time Password) generates passwords using a shared secret key and an incrementing counter. Both server and device maintain the counter. An HOTP remains valid until it is used or until the next password is generated, with no time limit imposed.
TOTP (Time-Based One-Time Password) is a variant of HOTP that replaces the counter with the current time as its moving factor. TOTP passwords are valid for a short window, typically 30 to 60 seconds, after which a new password generates automatically. The time-based expiry adds a layer of security that HOTP lacks.
