API Authentication Methods Compared: API Keys, OAuth, JWT, HMAC and mTLS

The first question any API asks is "who are you?". Get the answer wrong and you either lock out legitimate users or, worse, let someone else in. The choice of API authentication methods shapes how secure your integration is, how easy it is to use, and how much pain you'll feel when a credential leaks.

There's no single best method. A simple internal service and a public API used by thousands of apps need different things. Here's a practical comparison of the common approaches, with the trade-offs we weigh when designing and consuming APIs.

Authentication vs authorisation#

Quickly, because the terms get mixed up. Authentication proves who is calling. Authorisation decides what they're allowed to do. A valid API key authenticates you; scopes or roles attached to it authorise particular actions. Most methods below handle authentication, and some carry authorisation information too.

API keys#

An API key is a long random string the provider issues to you. You send it with each request, usually in a header like Authorization: Bearer YOUR_KEY or X-API-Key: YOUR_KEY.

Strengths: simple to implement, simple to use, easy to revoke and rotate. Most server-to-server APIs, including messaging APIs, use them.

Weaknesses: a key is a bearer credential. Whoever has it can use it, so a leaked key in a public Git repository or front-end code is an immediate problem. Keys also don't say which user is acting, only which account.

Good practice: send keys in headers, never URLs; keep them in environment variables or a secrets manager; give each integration its own key; scope keys to the minimum permissions; rotate them when people leave or keys might have leaked. Our article on API key security goes deeper.

HTTP Basic authentication#

The client sends a username and password, base64-encoded, in the Authorization header. Some APIs use an account ID and secret this way. It's simple and widely supported, but base64 isn't encryption, so it's only acceptable over HTTPS, and it shares API keys' weakness: whoever has the credentials can use them. Fine for internal tools and some legacy APIs; rarely the first choice for new public ones.

OAuth 2.0#

OAuth 2.0 is the standard for delegated access: letting a third-party app act on a user's behalf without the user handing over their password. "Sign in with Google" and "Connect your Shopify store" are OAuth flows. The user approves specific permissions (scopes), and the app receives an access token, often short-lived, plus a refresh token to get new ones.

Strengths: users never share passwords with third parties, access can be limited by scope and revoked per app, and tokens expire.

Weaknesses: more moving parts. Redirects, state parameters, token storage and refresh logic all need care. For mobile and single-page apps, use the Authorization Code flow with PKCE rather than older implicit flows.

Use it when your API serves third-party apps acting for your users, or when you consume such APIs. For your own backend calling a service with a single account, OAuth's client credentials grant exists, but a plain API key is often simpler.

JWT bearer tokens#

A JSON Web Token (JWT, defined in RFC 7519) is a signed token containing claims: who the user is, when the token expires, what they can access. The server verifies the signature and trusts the claims without a database lookup.

Strengths: stateless verification, which suits distributed systems; built-in expiry; claims can carry roles and scopes.

Weaknesses: hard to revoke before expiry without extra infrastructure, easy to misconfigure (accepting the "none" algorithm, or not checking expiry and audience), and tokens can grow large. Keep them short-lived and verify signature, issuer, audience and expiry every time.

JWTs are a token format, not a complete system. They're often the access tokens inside an OAuth flow.

HMAC request signing#

With HMAC signing, the client and server share a secret. For each request, the client computes a signature, typically HMAC-SHA256 over the method, path, timestamp and body, and sends it in a header. The server recomputes it and compares.

Strengths: the secret itself never travels with the request, tampering with the body breaks the signature, and timestamps prevent replaying old requests.

Weaknesses: more complex for API consumers, and signature mismatches are frustrating to debug (whitespace, encoding and header order all matter).

HMAC is especially common for webhooks, where the provider signs events it sends you. We covered that in webhook signature verification, and the basics of webhooks in what is a webhook.

Mutual TLS (mTLS)#

In normal HTTPS, only the server proves its identity with a certificate. With mutual TLS, the client presents a certificate too, and the server only accepts connections from known clients.

Strengths: very strong authentication at the connection level, common in banking, healthcare and internal service meshes.

Weaknesses: certificate issuance, rotation and distribution add real operational work. It's overkill for most public APIs and small teams.

A quick decision guide#

• Your server calling a service with one account: API key over HTTPS.
• Third-party apps acting for your users: OAuth 2.0, with PKCE for mobile and browser apps.
• Your own front end calling your own API: session cookies or short-lived tokens issued at login.
• Receiving webhooks: verify HMAC signatures and timestamps.
• High-security machine-to-machine traffic: consider mTLS, often alongside another method.

Mistakes that cause real breaches#

1. Secret keys shipped in mobile apps or website JavaScript. They will be extracted.
2. Keys committed to Git, even private repositories that later become public.
3. Tokens that never expire and can't be revoked.
4. Skipping signature or expiry checks on JWTs.
5. One shared key for every integration, so revoking it breaks everything.
6. Logging full request headers, including credentials.

How Tekservix handles it#

For our messaging APIs we use API keys, because most customers call them from their own servers, and keys are the simplest thing that works safely there. You create and revoke keys in the developer area, and incoming events like received SMS reach your server as signed webhooks. If you're building a mobile app on top, keep the key on your server and have the app call your backend. That one rule prevents most of the incidents we hear about.