Imported from jeremylongshore/tons-of-skills-marketplace (plugins/api-development/api-authentication-builder/skills/building-api-authentication/SKILL.md). Install upstream with npx skills add jeremylongshore/tons-of-skills-marketplace --skill building-api-authentication. Copyright stays with the author (MIT).

Building API Authentication

Overview

Build secure API authentication systems supporting JWT Bearer tokens, OAuth 2.0 authorization code and client credentials flows, API key management, and session-based authentication. Implement token issuance, validation, refresh rotation, revocation, and role-based access control (RBAC) with scoped permissions across all API endpoints.

Prerequisites

Cryptographic library: jsonwebtoken (Node.js), PyJWT (Python), or jjwt (Java)
Secure secret storage: environment variables, AWS Secrets Manager, or HashiCorp Vault for JWT signing keys
Database table for user credentials, refresh tokens, and API key storage
Bcrypt or Argon2 for password hashing (never store plaintext passwords)
OAuth 2.0 provider credentials for third-party auth integration (Google, GitHub, Auth0)

Instructions

Examine existing authentication setup using Grep and Read, identifying current auth mechanisms, middleware placement, and any endpoints bypassing authentication.
Implement JWT token issuance on successful login: sign with RS256 (asymmetric) or HS256 (symmetric), including sub (user ID), iat, exp (15-minute access token), roles, and scopes in the payload.
Create authentication middleware that extracts the Bearer token from the Authorization header, verifies the signature and expiration, and injects the decoded user context into the request object.
Implement refresh token rotation: issue a long-lived refresh token (30 days) alongside the access token, store a hash of the refresh token in the database, and rotate on each refresh (invalidating the previous token).
Build role-based access control (RBAC) middleware that checks user.roles against endpoint-required roles, supporting both role-level (admin, user) and scope-level (read:users, write:orders) authorization.
Add API key authentication as an alternative to JWT for machine-to-machine communication: generate cryptographically random keys, store hashed values, and validate against the X-API-Key header.
Implement OAuth 2.0 client credentials flow for service-to-service authentication, with token caching and automatic renewal before expiration.
Add brute-force protection on login endpoints: rate limit to 5 attempts per minute per IP, implement progressive lockout (15 min, 1 hour) after repeated failures, and log all authentication attempts.
Write security tests covering: valid/invalid/expired tokens, refresh token rotation, role enforcement, API key validation, brute-force lockout, and token revocation.

See ${CLAUDE_SKILL_DIR}/references/implementation.md for the full implementation guide.

Output

${CLAUDE_SKILL_DIR}/src/auth/jwt.js - JWT token issuance, verification, and refresh logic
${CLAUDE_SKILL_DIR}/src/auth/middleware.js - Bearer token authentication middleware
${CLAUDE_SKILL_DIR}/src/auth/rbac.js - Role-based and scope-based access control middleware
${CLAUDE_SKILL_DIR}/src/auth/api-keys.js - API key generation, hashing, and validation
${CLAUDE_SKILL_DIR}/src/auth/oauth.js - OAuth 2.0 flow implementations
${CLAUDE_SKILL_DIR}/src/routes/auth.js - Login, register, refresh, and logout endpoints
${CLAUDE_SKILL_DIR}/tests/auth/ - Authentication and authorization security tests

Error Handling

Error	Cause	Solution
401 Token Expired	JWT `exp` claim is in the past	Client should attempt token refresh; if refresh fails, redirect to login
401 Invalid Signature	JWT signed with different key or tampered payload	Verify signing key matches between issuance and validation; check for key rotation issues
403 Insufficient Scope	Authenticated user lacks required role/scope for endpoint	Return required scope in error body; log authorization failure with user and endpoint details
Refresh token reuse	Previously rotated refresh token used (possible token theft)	Invalidate all user sessions immediately; alert user of potential compromise; require re-authentication
API key leaked	API key exposed in client-side code, logs, or version control	Revoke compromised key immediately; issue replacement; scan for exposure source

Refer to ${CLAUDE_SKILL_DIR}/references/errors.md for comprehensive error patterns.

Examples

JWT with refresh rotation: Login returns {accessToken (15min), refreshToken (30d)}; client stores refresh token securely; on 401, client calls POST /auth/refresh with old refresh token, receives new pair, old refresh token is invalidated.

Multi-provider OAuth: Support "Sign in with Google" and "Sign in with GitHub" using OAuth 2.0 authorization code flow, creating local user accounts on first sign-in and linking subsequent provider connections.

API key with scoped permissions: Generate API keys with specific scopes (read:analytics, write:webhooks), stored as SHA-256 hashes, displayed to the user only once at creation, with key rotation support.

See ${CLAUDE_SKILL_DIR}/references/examples.md for additional examples.

Resources

RFC 7519 JSON Web Token (JWT): https://tools.ietf.org/html/rfc7519
RFC 6749 OAuth 2.0 Authorization Framework
OWASP Authentication Cheat Sheet: https://cheatsheetseries.owasp.org/
Auth0 architecture patterns: https://auth0.com/docs/

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