> ## Documentation Index
> Fetch the complete documentation index at: https://fastapi.codewithsiva.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# Authentication

> Understanding authentication and its role in securing applications.

# Authentication

Authentication is the process of verifying the identity of a user before allowing access to an application.

It answers the question:

> **Who are you?**

For example, when a user logs in using an email and password, the application verifies the credentials. If they are valid, the user is considered authenticated.

## Why Authentication is Important

* Prevents unauthorized access.
* Verifies the identity of users.
* Protects sensitive data and resources.
* Provides a secure foundation for authorization.

## Common Authentication Mechanisms

### Username & Password

The most common authentication method. The user enters their credentials, which are verified by the server.

### Session-Based Authentication

After a successful login, the server creates a session and stores it. The client sends a session ID (usually in a cookie) with every request.

**Commonly used in:** Traditional web applications.

### Token-Based Authentication

After login, the server generates a token. The client includes this token in every request, allowing the server to identify the user.

**Commonly used in:** REST APIs and mobile applications.

### JWT (JSON Web Token)

A widely used token-based authentication mechanism. The server issues a signed JWT after login, and the client sends it in the `Authorization` header for subsequent requests.

**Commonly used in:** Modern REST APIs and microservices.

### OAuth 2.0

Allows users to log in using a third-party provider without sharing their password with the application.

Examples:

* Login with Google
* Login with GitHub
* Login with Microsoft

### Single Sign-On (SSO)

A user logs in once and gains access to multiple related applications without logging in again.

### Multi-Factor Authentication (MFA)

Requires more than one verification factor to authenticate a user.

Examples:

* Password + OTP
* Password + Authenticator App
* Password + Fingerprint

## Authentication Flow

```text theme={null}
User Login
    │
    ▼
Verify Credentials
    │
    ▼
Authentication Successful
    │
    ▼
Issue Session or Token
    │
    ▼
Access Protected Resources
```

## What We Will Implement

In this course, we will implement authentication using:

* Password Hashing
* Username & Password
* JWT (JSON Web Tokens)
* Protected API Endpoints
* Current User Retrieval