| name | rfc4226 |
| description | RFC 4226 - HOTP: An HMAC-Based One-Time Password Algorithm. Defines the standard algorithm for generating one-time password values using HMAC. Use when working with HOTP, one-time passwords, HMAC-based authentication, two-factor authentication systems, or OATH standards. |
RFC 4226: HOTP Algorithm
Title: HOTP: An HMAC-Based One-Time Password Algorithm
Authors: D. M'Raihi, M. Bellare, F. Hoornaert, D. Naccache, O. Ranen
Published: December 2005
Category: Informational
Source: RFC 4226
Abstract
RFC 4226 describes an algorithm to generate one-time password (OTP) values based on Hashed Message Authentication Code (HMAC). It provides a security analysis of the algorithm and discusses important parameters for secure deployment. The algorithm is designed for use across a wide range of network applications including remote VPN access, Wi-Fi network logon, and transaction-oriented web applications.
The HOTP algorithm is a joint effort by the OATH (Open AuTHentication) membership to specify a freely distributable algorithm that facilitates interoperability across commercial and open-source implementations, enabling broader adoption of two-factor authentication.
Quick Reference
| Aspect | Details |
|---|
| Algorithm | HMAC-Based One-Time Password (HOTP) |
| Base Algorithm | HMAC (Hash-based Message Authentication Code) |
| Type | Event-based (counter-based) |
| Purpose | Two-factor authentication |
| Use Cases | VPN access, Wi-Fi logon, web applications |
| OATH Standard | Yes |
Key Concepts
HOTP Algorithm Core
- HOTP Value Generation: Uses HMAC-SHA-1 (or other hash functions) with a secret key and a counter
- Counter-Based: Each OTP is generated using a monotonically increasing counter value
- Digit Length: Typically produces 6-8 digit numeric codes (configurable)
Algorithm Components
- Shared Secret: A secret key known only to the token and validation server
- Counter: A moving factor (event-based counter) that changes with each OTP generation
- HMAC: Hash-based Message Authentication Code that combines secret and counter
- Dynamic Truncation: Converts HMAC output to a numeric OTP value
Security Properties
- Resistant to replay attacks (when properly implemented with counter synchronization)
- No plaintext password transmission
- Limited window for valid OTP usage
- Resilient against brute-force attacks (with proper digit length)
Algorithm Description
The HOTP algorithm generates one-time passwords as follows:
HOTP(K, C) = Truncate(HMAC-SHA-1(K, C))
Where:
K = Shared secret keyC = Counter value (8-byte string in network byte order)Truncate = Dynamic truncation function that extracts a 31-bit string from HMAC output- Final OTP = Truncated value modulo 10^Digits (where Digits is typically 6)
Security Considerations
Validation Requirements
- Server MUST validate OTP values against the expected counter window
- Server SHOULD implement throttling to prevent brute-force attacks
- Server SHOULD support counter resynchronization for token drift
Resynchronization
The RFC defines methods for handling counter drift between the token and server:
- Simple Resynchronization: Accept OTP values within a look-ahead window
- Counter-Based Resynchronization: Use a special resynchronization OTP
Threat Model
- Brute Force: Attacks are feasible with insufficient digit length; 6 digits provides ~1 million possibilities
- Replay Attacks: Mitigated by counter-based system and validation window
- Man-in-the-Middle: OTP alone doesn't prevent MITM; requires secure channel
- Shared Secret Compromise: Entire system security depends on secret key protection
Extensions
Composite Shared Secrets
Allows combining multiple shared secrets for enhanced security or migration scenarios.
Bi-Directional Authentication
Enables mutual authentication where both client and server authenticate to each other.
Variable Parameters
- Number of Digits: Can be configured from 6 to 8+ digits
- Alphanumeric Values: Can produce alphanumeric OTPs instead of numeric
- Hash Function: Can use SHA-256, SHA-512 in addition to SHA-1
When to Use This Skill
Use this skill when:
- Implementing or working with HOTP-based authentication systems
- Designing two-factor authentication solutions
- Integrating with OATH-compliant hardware or software tokens
- Evaluating security of one-time password implementations
- Studying authentication algorithm design and security analysis
- Working with RFC 4226 directly or its implementations
Reference Implementation
Appendix C of RFC 4226 includes reference implementations in pseudocode and test values for verification in Appendix D.
Related Standards
- OATH: Open AuTHentication initiative - industry alliance behind HOTP
- RFC 2119: Key words for use in RFCs to Indicate Requirement Levels
- RFC 3979: IETF Intellectual Property Rights
- RFC 6238: TOTP (Time-based One-Time Password) - builds on HOTP concepts
Full RFC Text
See references/RFC4226.txt for the complete document.
