Hmac Based One Time PasswordEdit
HMAC-based One-Time Password (HOTP) is a cryptographic approach to generating short-lived codes that authenticate a user or device without transmitting a password. It rests on a shared secret and a moving factor, producing a numeric code that is valid for a limited window of use. HOTP is a mature standard widely used in hardware tokens and software authenticator apps, and it remains a foundational element in the broader landscape of two-factor authentication. The method is defined in RFC 4226 and is often discussed alongside its time-based cousin, TOTP, which substitutes a clock-driven moving factor for the counter. The protocol is part of the broader effort led by OATH to promote open, interoperable authentication technologies. In practice, HOTP codes are typically six digits, though the length can be extended to seven or eight digits depending on policy and risk tolerance. For a broader context on one-time passwords in general, see one-time password.
HOTP forms a bridge between symmetric secret sharing and practical, low-friction authentication flows. The server and client each hold the same secret key and track a moving factor that increments with each authentication event. When a user or device presents a code, the server recomputes the expected value from the shared secret and the current counter, and validates the match. The reliance on a shared secret means that secure provisioning and protection of that key are central to the security of the entire system. See also HMAC as the underlying cryptographic primitive.
How HOTP works
Inputs and construction: The core inputs are a secret key and a moving factor. The secret key is a shared, opaque value known to both the client (for example, a smartphone app) and the server. The moving factor is typically an 8-byte integer counter that increases with each OTP generated. The standard specifies using an HMAC construction, often with SHA-1 as the hash function, although other hash functions can be used in practice. The basic operation is HMAC(K, C), where K is the secret key and C is the counter, followed by a dynamic truncation step to derive a numeric code.
Dynamic truncation and digit generation: After computing the HMAC, a portion of the result is selected via dynamic truncation to yield a 31-bit value. This value is then reduced modulo 10^digits to produce the human-readable code (commonly six digits). The dynamic truncation step is described in detail in RFC 4226 and is central to producing codes of fixed length that appear random to observers.
Verification process: On the server side, the same secret and the expected counter value are used to recompute the code. If the presented code matches the locally computed value within an acceptable window of counters (to accommodate minor desynchronizations between client and server), authentication succeeds. This process assumes proper protection of the shared secret and careful management of counters to avoid leakage or drift.
Relationship to TOTP: TOTP replaces the counter with a time-based moving factor, changing the synchronization mechanism but preserving the same underlying HOTP generation method. In many deployments, HOTP forms the basis of tokens that can also be used in a time-based mode through the TOTP framework.
Interoperability and standards: The HOTP algorithm is widely implemented in both hardware tokens and software apps. It is designed to be interoperable across platforms and systems that adopt the standard, enabling organizations to deploy a mix of devices and clients without bespoke integrations. See OATH for context on standardization efforts that include HOTP and related technologies, and RFC 4226 for the formal specification. For time-based variants, see RFC 6238.
Security properties and considerations
Secret protection: The security of HOTP hinges on keeping the shared secret confidential. If an attacker gains access to the secret key, they can generate valid codes for any counter value, potentially defeating the protection offered by the factor. Consequently, secure provisioning, storage, and handling of the secret on both client devices and servers are essential. See also Hardware token implementations that emphasize secure key storage.
Counter management and resynchronization: The moving factor (the counter) must be managed carefully to avoid unintentional drift between client and server. Systems typically allow a small window of counters to be checked during verification, enabling recovery from minor synchronization issues. Excessive drift or poor state management can create usability friction or introduce security risks if stale counters are replayed.
Phishing and man-in-the-middle considerations: While HOTP provides a robust second factor against password theft, it does not inherently prevent credential phishing if the attacker can prompt the user to reveal a code and relay it to the attacker’s system. A layered security approach—such as pairing HOTP with phishing-resistant workflow controls, device binding, or phishing-aware UX—strengthens defense in depth.
Brute-force and rate limiting: Because each code has a finite space (digits), attackers may attempt guessing within a short window. Implementations mitigate this risk with server-side rate limiting, lockout policies, or require re-verification after a failed attempt. The relatively small code size is a trade-off between usability and resistance to rapid guessing.
Hash function considerations: While SHA-1 has historically been the default hash in many HOTP deployments, modern practice often favors stronger hash functions (e.g., SHA-256) to align with current security expectations. The HOTP mechanism itself is hash-agnostic in principle, as long as both sides agree on the function used.
Implementation and deployment considerations
Provisioning and lifecycle: HOTP requires secure provisioning of the shared secret to each legitimate client device. This is commonly accomplished via QR code scanning or encrypted key distribution. The same secret then becomes the basis for all future codes. Proper lifecycle management includes revocation and re-issuance in cases of device loss or compromise.
Token types and user experience: Hardware tokens and smartphone apps are the most common carriers of HOTP codes. Hardware tokens provide physical possession as a factor, which can be resistant to malware on the user’s device, while software tokens offer convenience and portability. See Hardware token for more on physical devices and Two-factor authentication for broader context on multi-factor strategies.
Integration with authentication systems: HOTP is designed to plug into existing authentication infrastructures where a server can store user-specific secrets and counters and perform verification on each login attempt. Standards-based approaches enable compatibility with multiple identity providers and access controls, including integration with institutions and enterprises that rely on centralized security policies.
Resilience and operational considerations: Operators should consider backup and recovery processes, scour for drift or clock issues in backup devices, and implement monitoring for abnormal authentication patterns. In environments with high security requirements, organizations often combine HOTP with additional controls such as device attestation, anomaly detection, or risk-based authentication.
Adoption, use cases, and comparative context
Common use cases: HOTP is well suited to environments where a stable, portable second factor is needed and where a counter-based model aligns with the organization’s authentication workflow. It remains a practical choice for legacy systems and mixed ecosystems that require interoperability across vendors.
Comparison with alternatives: Time-based OTP (TOTP) removes the need to track a per-user counter, instead using time synchronization to generate codes. Push-based and SMS-based methods offer alternative user experiences but introduce different risk profiles (for example, SIM-swapping risks or push notification phishing). In many security architectures, HOTP/TOTP exists alongside other methods as part of a layered approach to authentication. See Two-factor authentication and Push notification authentication for related concepts.
Standards and ecosystem: The HOTP standard sits within a broader ecosystem of open, interoperable authentication technologies coordinated by OATH and standards bodies. The distinction between HOTP and TOTP is a matter of moving factor, not of fundamental cryptographic approach, and both share the same goal: providing a stronger, shared-secret-based second factor.