# Hotp

> 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.

- Skill: `bytesagain/hotp` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add bytesagain/hotp`
- Raw SKILL.md: https://api.skillmd.com/api/skills/bytesagain/hotp/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Security
- Author: bytesagain (https://skillmd.com/u/bytesagain)
- Updated: 2026-09-21
- Page: https://skillmd.com/skills/bytesagain/hotp

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# HOTP Reference

HMAC-based one-time passwords.

## Commands

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