What Is an API Key? A Clear Explanation

An API key is a unique string of characters that identifies who is making a request to a piece of software. Think of it as a combination of an ID card and a password: it tells the server which account is calling it, and it proves that the caller is allowed to do so.

When you use a web app, you usually log in with a username and password. When software talks to other software, there's no login screen — instead, each request carries an API key in a header or query parameter. The server checks that key against its records, decides whether it's valid, and then either processes the request or rejects it.

What an API key actually looks like

API keys are just strings, but they follow conventions that make them recognizable. A few real-world patterns:

• OpenAI keys start with sk-
• Stripe keys start with sk_live_ or pk_live_
• SubToAPI keys start with sub_live_

A typical key looks something like this:

sub_live_9f8a2c1e4b7d6f3a0e5c8b1d2f4a6e9c

The prefix (sub_live_, sk_, etc.) helps developers and automated secret-scanners quickly identify what kind of key it is and where it might have leaked. The rest is a long random value that's practically impossible to guess.

Why APIs use keys instead of passwords

Passwords are designed for humans typing them into a login form. API keys are designed for programs, so they need different properties:

• No interactive login. A script or server can attach the key to every request automatically.
• Scoped permissions. A key can be limited to certain actions, endpoints, or rate limits without touching the account's main credentials.
• Easy to revoke. If a key is compromised, you delete that one key instead of forcing a password reset on the whole account.
• Traceable usage. Every request tied to a key can be logged, so you know exactly which key made which call and how often.

This is why almost every developer-facing service — cloud providers, payment processors, AI models, mapping APIs — uses API keys as the primary way to authenticate programmatic access.

How an API key is used in a request

Most modern APIs expect the key in an HTTP header called Authorization. Here's a generic example:

curl https://api.example.com/v1/resource \
  -H "Authorization: Bearer YOUR_API_KEY"

The server reads the header, validates the key, and responds accordingly. If the key is missing, expired, or invalid, you'll typically get a 401 Unauthorized response.

Here's what it looks like with SubToAPI, which turns your existing Claude access into an HTTPS API with its own keys:

curl https://api.subtoapi.app/v1/messages \
  -H "Authorization: Bearer $SUBTOAPI_KEY" \
  -H "content-type: application/json" \
  -d '{
    "model": "claude-3-5-sonnet",
    "max_tokens": 256,
    "messages": [{"role": "user", "content": "Summarize this article."}]
  }'

The key format is sub_live_..., generated in the dashboard after signup, and it's what gives your application access to streaming responses, tool use, and usage metadata — all documented in the quickstart guide.

API keys vs. other authentication methods

API keys aren't the only way to authenticate, and it helps to know where they fit:

| Method | Typical use case | Complexity | |---|---|---| | API key | Server-to-server calls, single app identity | Low | | OAuth 2.0 | User-delegated access ("log in with Google") | High | | JWT (JSON Web Token) | Session-based auth, often short-lived | Medium | | Basic auth (username/password) | Legacy or internal tools | Low |

API keys are the simplest option and the standard choice when a single application (not an individual end user) needs to call an API repeatedly. OAuth becomes necessary when your app needs to act on behalf of a specific user's account on another platform.

How to keep an API key secure

Because a key is effectively a bearer credential — whoever holds it can use it — protecting it matters as much as generating it correctly.

• Never commit keys to source control. Use .env files and add them to .gitignore.
• Use environment variables in production, not hardcoded strings.
• Rotate keys periodically, and immediately if you suspect a leak.
• Scope keys narrowly. If the platform supports per-team or per-project keys, use separate keys rather than one master key shared everywhere.
• Set usage limits or alerts where available, so an unexpected spike in requests gets flagged early.
• Restrict by IP or origin if the API supports it, especially for server-side keys that never need to be called from a browser.

On platforms with team plans, like SubToAPI's Team and Scale tiers, each seat can get its own key. That way, if one team member leaves or a key is exposed, you revoke that single key instead of rotating credentials for the whole team. Pricing and seat details are on the pricing page.

A quick example: creating and testing a key

Most services follow a similar three-step flow:

1. Sign up for an account on the platform (for SubToAPI, that's the signup page).
2. Generate a key from the dashboard — this is usually a one-click action, and the full key is only shown once.
3. Test it with a simple request, like the curl example above, to confirm it works before wiring it into your application.

From there, the key gets stored securely and referenced in your app's configuration, environment variables, or secrets manager — never pasted directly into client-side code.

questions

Is an API key the same as a password? No. A password authenticates a human logging into an interface, while an API key authenticates a program making automated requests. Keys are usually longer, randomly generated, and scoped to specific permissions rather than a full account login.

Can I use one API key across multiple applications? You can, but it's not recommended. Using separate keys per application or environment (development, staging, production) makes it easier to track usage, revoke access selectively, and limit damage if one key is exposed.

What happens if my API key is leaked? Revoke it immediately from the provider's dashboard and generate a new one. Update any environment variables or config files that reference the old key, and check usage logs for unexpected activity during the exposure window.