# Authentication

> Use when: implement or review authentication flows, sessions, tokens, and password handling securely.

- Skill: `kimtth/authentication` (Agent Skill)
- Install (CLI): `npx skillmds@latest add kimtth/authentication`
- Raw SKILL.md: https://api.skillmd.com/api/skills/kimtth/authentication/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: kimtth (https://skillmd.com/u/kimtth)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/kimtth/authentication

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Goal: verify identity safely without leaking or mishandling credentials.

Use for:
- login, signup, and session management
- choosing between sessions and tokens
- reviewing password storage and flow security

Workflow:
1. Choose a mechanism: server sessions or signed tokens (JWT).
2. Hash passwords with a strong adaptive function (bcrypt/argon2).
3. Issue, store, and expire credentials securely.
4. Protect against brute force with rate limits and lockouts.
5. Support secure logout and token revocation.
6. Verify flows against common attacks before shipping.

Patterns:
- httpOnly, Secure, SameSite cookies for sessions
- short-lived access tokens with refresh rotation
- MFA for sensitive accounts
- constant-time comparison for secrets

Rules:
- never store passwords in plaintext or with fast hashes
- never log credentials or tokens
- always use TLS for credential transport
- expire and rotate tokens; support revocation