REST API Authentication Methods Explained: API Keys, OAuth 2.0, and JWTs

Why Authentication Matters in APIs

Every time your application calls an external API — or exposes its own — authentication is what controls who can access what. Picking the wrong method creates security vulnerabilities; picking an overcomplicated one wastes development time. This guide explains the three most common approaches and when to reach for each one.

Method 1: API Keys

An API key is a simple, long random string passed with each request to identify the calling application.

GET https://api.example.com/data
Headers:
  X-API-Key: sk_live_abc123xyz789

How it works

1. You register for an API and receive a key.
2. You attach that key to every request (header, query param, or body).
3. The server validates the key against its database and grants or denies access.

Pros and Cons

• Pro: Dead simple to implement on both client and server.
• Pro: Great for server-to-server communication where the key can be kept secret.
• Con: Keys don't expire by default — a leaked key is a serious risk.
• Con: No built-in concept of user identity or permissions scope.

Best for: Internal APIs, public data APIs (maps, weather), and server-side integrations where there's no user context needed.

Method 2: OAuth 2.0

OAuth 2.0 is an authorization framework, not a single protocol. It allows users to grant your application limited access to their account on another service — without sharing their password. This is the "Login with Google" and "Connect to Stripe" flow you see everywhere.

The Authorization Code Flow (most common)

1. Your app redirects the user to the provider's authorization URL.
2. The user logs in and approves the requested permissions (scopes).
3. The provider redirects back to your app with a short-lived authorization code.
4. Your server exchanges that code for an access token (and optionally a refresh token).
5. You use the access token in API requests on behalf of the user.

Key Concepts

Term	Meaning
Access Token	Short-lived credential used to make API calls
Refresh Token	Long-lived token used to get new access tokens
Scope	What permissions the token grants (e.g., read:email)
Client ID / Secret	Your app's credentials with the OAuth provider

Best for: Any flow where users authorize your app to act on their behalf — social logins, third-party integrations (Slack, GitHub, Stripe).

Method 3: JSON Web Tokens (JWTs)

A JWT is a self-contained token that encodes a payload (claims) and is cryptographically signed. Unlike API keys, the server doesn't need to look up the token in a database — it just verifies the signature.

// JWT structure: header.payload.signature
eyJhbGciOiJIUzI1NiJ9.eyJ1c2VySWQiOiI0MiIsInJvbGUiOiJhZG1pbiIsImV4cCI6MTcxMjAwMDAwMH0.SIG

How it works

1. User logs in; server generates a signed JWT containing user ID, roles, and expiry.
2. Client stores the JWT (typically in memory or an HttpOnly cookie).
3. Client sends Authorization: Bearer <token> with each request.
4. Server validates the signature — no DB lookup needed.

Important caveats

• JWTs cannot be revoked before expiry without a denylist — keep expiry times short (15–60 minutes).
• Never store sensitive data in the payload — it's Base64-encoded, not encrypted by default.
• Use RS256 (asymmetric) instead of HS256 for multi-service architectures.

Best for: Stateless authentication in your own APIs, microservices, and single-page applications.

Choosing the Right Method

Scenario	Recommended Method
Calling a third-party API from your server	API Key
Users logging in to your own app	JWT
Users granting access to another service	OAuth 2.0
Mobile app accessing your own API	OAuth 2.0 (PKCE flow) or JWT