REST API Authentication: JWT, OAuth 2.0, and API Keys Explained

REST API Authentication: Which Method Should You Use?

Authentication is the backbone of any secure API. Whether you're building a public API or an internal service, choosing the wrong authentication strategy can lead to security vulnerabilities, poor developer experience, or unnecessary complexity. This guide walks through the three most common approaches: API Keys, JSON Web Tokens (JWT), and OAuth 2.0.

API Keys: Simple but Limited

API keys are the most straightforward authentication mechanism. A unique string is generated and passed with each request — typically in a header or query parameter.

• Best for: Server-to-server communication, internal tools, simple public APIs
• Pros: Easy to implement, easy to revoke, minimal overhead
• Cons: No built-in expiry, no user-level permissions, risky if exposed

A typical API key header looks like this:

Authorization: ApiKey your-secret-key-here

Always store API keys server-side and never expose them in client-side JavaScript or public repositories.

JWT (JSON Web Tokens): Stateless and Scalable

JWTs are self-contained tokens that encode claims about the user or client. They are signed (and optionally encrypted), allowing servers to verify authenticity without consulting a database on every request.

• Best for: Stateless APIs, microservices, mobile apps, single-page applications
• Pros: No session storage needed, includes expiry, carries user claims
• Cons: Tokens can't be invalidated before expiry without extra infrastructure, payload size grows with claims

A JWT has three parts separated by dots: header.payload.signature. Always validate the signature on the server and check the exp claim to prevent replay attacks.

OAuth 2.0: Delegated Authorization at Scale

OAuth 2.0 is the industry standard for delegated authorization. It allows users to grant third-party applications access to their data without sharing credentials. It's what powers "Sign in with Google" and similar flows.

• Best for: Third-party integrations, consumer-facing APIs, platforms with user accounts
• Pros: Fine-grained scopes, access tokens + refresh tokens, widely understood
• Cons: Complex to implement correctly, multiple grant types can be confusing

OAuth 2.0 Grant Types at a Glance

Grant Type	Use Case
Authorization Code	Web apps with a backend server
PKCE	Single-page apps and mobile apps
Client Credentials	Machine-to-machine (no user)
Device Code	Smart TVs, CLI tools

How to Choose the Right Approach

1. Simple internal or server-to-server API? Start with API keys.
2. Need stateless, scalable auth for your own users? Use JWTs, ideally short-lived ones paired with refresh tokens.
3. Building a platform where third parties need to act on behalf of users? Implement OAuth 2.0.

Security Best Practices Across All Methods

• Always use HTTPS — never transmit credentials over plain HTTP.
• Rotate credentials regularly and provide an easy revocation mechanism.
• Apply the principle of least privilege: grant only the permissions needed.
• Log authentication events and alert on anomalies.
• Never log tokens or API keys in plaintext.

No single method is universally "best" — the right choice depends on your use case, your users, and your infrastructure. Understanding the trade-offs puts you in control of building APIs that are both secure and developer-friendly.