API Authentication Methods
APIs require authentication to ensure that only authorized clients can access resources. Choosing the correct authentication mechanism is critical for security, scalability, and maintainability.
This page explains the most commonly used API authentication methods, how they work internally, where they are used in production, and their advantages and limitations.
1. API Key Authentication
API key authentication uses a simple static token that identifies the client. The key is sent with every request, usually as a header or query parameter.
GET /api/data
X-API-Key: abc123xyz
Common Use Cases
- Internal microservices
- Low-risk public APIs
- Rate-limited access to services
Advantages
- Very easy to implement
- Low overhead
Disadvantages
- No user identity
- No fine-grained permissions
- Keys are often long-lived and hard to rotate
Security Insight: API keys should always be transmitted over HTTPS and rotated periodically.
2. Basic Authentication
Basic Authentication sends a username and password encoded in Base64 with each request.
Authorization: Basic dXNlcjpwYXNzd29yZA==
Base64 is not encryption; it is only encoding. Anyone intercepting the request can decode the credentials.
Common Use Cases
- Legacy systems
- Internal admin APIs
Advantages
- Simple and widely supported
Disadvantages
- Credentials sent on every request
- No session or token expiration
- Unsafe without HTTPS
Security Insight: Basic Auth must never be used without HTTPS.
3. JWT (JSON Web Token) Authentication
JWT authentication uses a signed token that contains user claims. The token is generated after login and sent with every request.
Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
How It Works
- User authenticates once
- Server issues a signed JWT
- Client sends JWT with each request
- Server verifies signature (no session storage)
Common Use Cases
- REST APIs
- Microservices
- Mobile and SPA applications
Advantages
- Stateless and scalable
- No server-side session storage
Disadvantages
- Hard to revoke tokens
- Token leakage risk
- Improper storage can cause XSS attacks
Security Insight: Use short-lived access tokens and refresh tokens. Store JWTs in HttpOnly cookies when possible.
4. OAuth 2.0
OAuth 2.0 is an authorization framework that allows applications to access user resources without sharing passwords.
OAuth is commonly used for:
- Login with Google / GitHub
- Third-party API access
- Enterprise SSO
How OAuth Works (Simplified)
- User authenticates with Identity Provider
- Client receives access token
- Token is used to access APIs
Advantages
- Industry standard
- Supports scopes and permissions
- No password sharing
Disadvantages
- Complex implementation
- Multiple moving parts
Security Insight: OAuth should be combined with PKCE for public clients.
5. Mutual TLS (mTLS)
Mutual TLS authenticates both client and server using X.509 certificates. Each client must present a valid certificate signed by a trusted CA.
Common Use Cases
- Banking systems
- Internal microservices
- High-security enterprise environments
Advantages
- Strong cryptographic authentication
- No shared secrets
Disadvantages
- Operational complexity
- Certificate lifecycle management
Security Insight: mTLS is often combined with OAuth for identity and authorization.
Comparison Summary
| Method | Security | Scalability | Complexity |
|---|---|---|---|
| API Key | Low | High | Low |
| Basic Auth | Low | Low | Low |
| JWT | Medium | High | Medium |
| OAuth 2.0 | High | High | High |
| mTLS | Very High | Medium | High |
Which One Should You Use?
- Internal services: API keys or mTLS
- Public APIs: OAuth 2.0 or JWT
- Enterprise systems: OAuth + mTLS
- Simple prototypes: API keys (with HTTPS)
Final Thoughts
There is no one-size-fits-all solution for API authentication. The correct choice depends on your system architecture, security requirements, and operational maturity.
Understanding the trade-offs of each method is essential for designing secure, scalable APIs.