API Authentication Explained: JWT vs OAuth 2.0 vs API Keys

API Authentication Explained: JWT vs OAuth 2.0 vs API Keys

Authentication is the gateway to your API. Get it wrong, and you risk exposing sensitive data or locking out legitimate users. Get it right, and you create a secure, scalable foundation for your platform. This guide demystifies the three most common API authentication strategies.

The Core Question: Who Is Calling My API?

Before choosing an auth strategy, clarify who your API consumers are:

• Machine-to-machine (a cron job, microservice, or backend script)
• End users (humans logging into an app)
• Third-party developers (integrating with your platform)

Each use case maps to a different authentication approach.

API Keys: Simple but Limited

API keys are opaque strings passed in a request header or query parameter. They're the easiest to implement and understand.

How They Work

1. Developer registers and receives a unique key (e.g., sk_live_abc123)
2. Developer includes the key in every request: Authorization: Bearer sk_live_abc123
3. Your API validates the key against your database

Best For

• Server-to-server integrations
• Public APIs where user identity isn't required
• Rapid prototyping and internal tooling

Limitations

• No built-in expiry — a leaked key is valid until manually revoked
• No user context — you know the app, but not the individual user
• Requires secure storage on the client side

JWT (JSON Web Tokens): Stateless and Self-Contained

JWTs encode claims (user ID, roles, expiry) directly in the token, signed with a secret or private key. Your API can validate the token without hitting a database.

JWT Structure

A JWT has three Base64-encoded parts separated by dots: header.payload.signature

Best For

• Authenticated user sessions in web and mobile apps
• Stateless APIs that need to scale horizontally
• Microservices that pass user identity downstream

Key Considerations

• Keep tokens short-lived (15–60 minutes) and use refresh tokens
• Store tokens in httpOnly cookies, not localStorage, to prevent XSS
• Never store sensitive data in the JWT payload — it's encoded, not encrypted
• Implement a token revocation strategy (blocklist or short expiry)

OAuth 2.0: Delegated Authorization

OAuth 2.0 is a framework that allows users to grant third-party applications access to their resources without sharing their credentials. It's the foundation behind "Login with Google" and similar flows.

Key Roles

• Resource Owner — the end user
• Client — the third-party app requesting access
• Authorization Server — issues tokens (e.g., Auth0, Okta, Google)
• Resource Server — your API

Common Grant Types

• Authorization Code — for web apps with a backend (most secure)
• Client Credentials — for machine-to-machine (no user involved)
• PKCE — for SPAs and mobile apps (Authorization Code with extra protection)

Best For

• Allowing third parties to integrate with your platform
• Any scenario involving user consent and delegated access
• Enterprise SSO and identity federation

Decision Guide

Scenario	Recommended Approach
Internal tool, server-to-server	API Key or OAuth Client Credentials
User login for your own app	JWT (with refresh tokens)
Third-party access on behalf of user	OAuth 2.0 Authorization Code + PKCE
Public API for developers	API Keys + OAuth 2.0

The Takeaway

These approaches aren't mutually exclusive. Most production APIs use a combination: OAuth 2.0 for the authorization flow, JWTs as the token format, and API keys for developer-facing integrations. Start simple, add complexity only when your use case demands it.