---
name: hotp
version: 1.0.0
description: >
  HOTP (RFC 4226) HMAC-based one-time password reference. Counter-
  based OTP for hardware tokens. Covers algorithm step-by-step,
  Python implementation from scratch, pyotp/otpauth libraries,
  counter synchronization, resync protocol, look-ahead window,
  and security considerations vs TOTP.
author: BytesAgain
homepage: https://bytesagain.com
source: https://github.com/bytesagain/ai-skills
tags: [hotp, otp, 2fa, authentication, security, rfc4226]
---

# HOTP Reference

HMAC-based one-time passwords.

## Commands

| Command | Description |
|---------|-------------|
| `intro` | Algorithm, HOTP vs TOTP |
| `implement` | Python/Node.js code |
| `security` | Counter sync, resync |