rfc4226

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

1. Shared Secret: A secret key known only to the token and validation server
2. Counter: A moving factor (event-based counter) that changes with each OTP generation
3. HMAC: Hash-based Message Authentication Code that combines secret and counter
4. 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 key
• C = 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.