API Authentication Methods

Authentication vs Authorization

API authentication is the process by which an API checks who is making a request. The API needs to know whether the caller is a trusted application, a user or nobody in particular. It is often confused with authorization, which answers a different question: once the caller's identity is known, authorization decides what that caller may do, such as reading one customer's invoices but not anyone else's.

Most APIs first authenticate each request using a credential sent with it, then apply authorization rules before returning data. Choosing an authentication method is therefore a decision about how credentials are issued, sent, stored and revoked.

Common API Authentication Methods

Several approaches are common, and each suits different situations:

• Basic authentication sends a username and password, encoded in base64, in a request header. Base64 is an encoding, not encryption, so this method is only acceptable over HTTPS and is rarely a good choice for new APIs.
• API keys are long random strings that identify a project or application. They are simple to use, but anyone who holds the key can use it.
• Bearer tokens are credentials placed in the Authorization header, prefixed with the word Bearer. The token can be a random string or a signed JWT, and the server decides whether it is valid.
• OAuth 2.0 lets a user approve an application to access their data without sharing a password. It issues tokens with limited scope and lifetime.
• Mutual TLS (mutual Transport Layer Security) uses client certificates so that both sides prove their identity during the encrypted connection setup. It is common in high-security and internal networks.

Choosing a Method for Your Use Case

The right method depends on who is calling. A server-to-server integration between two businesses might use API keys or a client credentials flow from OAuth. A mobile app acting for a logged-in user usually needs OAuth, so that the user approves access and the app never sees their password. A browser-based app should not hold long-lived secrets in its front-end code, because anyone can read that code.

Whatever the method, the same habits apply: send credentials only over HTTPS, store them in a secrets manager or environment variables rather than in source code, rotate them when people or systems change, and log failed attempts so that unusual activity can be spotted. The articles on API keys, OAuth and JWT explain the most common methods in more depth.