API Authentication

API authentication is the first line of defense for any API, establishing the identity of the caller before authorization checks determine what they are permitted to do. There are several common authentication methods, each with different security properties and use cases. API keys provide simple identification but offer limited security since they are static shared secrets. OAuth 2.0 provides delegated authorization with scoped, time-limited tokens. JWTs enable stateless authentication with embedded claims.

More advanced authentication methods include mutual TLS (mTLS), where both the client and server present certificates to authenticate each other, providing strong identity verification for service-to-service communication. HMAC-based authentication signs each request with a shared secret, ensuring request integrity and preventing replay attacks when combined with timestamps and nonces.

Choosing the right authentication method depends on the API's threat model, client types, and infrastructure. Public APIs serving web browsers typically use OAuth 2.0 with PKCE. Internal microservices often use mTLS or JWT with short expiration times. Third-party integrations may use API keys for simplicity with strict IP allowlisting. Regardless of the method chosen, best practices include transmitting credentials only over TLS, implementing brute-force protection on authentication endpoints, logging all authentication failures, supporting credential rotation without downtime, and never embedding authentication secrets in client-side code or version control repositories. Multi-factor authentication should be considered for high-privilege API operations.