OAuth 2.0 and JWT Authorization in C#: A Beginner's Guide

Introduction: Why OAuth 2.0 and JWT are the Standard for Modern Authorization?

In modern web development, secure data transfer between client and server is a critical task. Two key protocols that help solve this task are OAuth 2.0 (open authorization protocol) and JWT (JSON Web Token).

OAuth 2.0 allows applications to obtain limited access to user resources without transmitting login and password. JWT, in turn, is a compact and self-contained token format that contains all the necessary information for user identification.

In this article, we will break down how to implement OAuth 2.0 and JWT authorization in C# (.NET). You will learn how to configure middleware, generate a token, and protect API endpoints. The material is designed for beginner developers familiar with the basics of ASP.NET Core.

1. Basics of OAuth 2.0 and JWT

Before writing code, let's understand the key concepts.

What is OAuth 2.0?

OAuth 2.0 is an authorization protocol that allows third-party applications to access user resources (e.g., profile data) without transmitting their credentials. Instead, an access token is used.

The main roles in OAuth 2.0 are:

• Resource Owner — the user who owns the resources.
• Client — the application requesting access.
• Authorization Server — the server that issues tokens after user authentication.
• Resource Server — the server that provides protected resources (e.g., API).

What is JWT?

JWT (JSON Web Token) is a token format consisting of three parts separated by dots:

• Header — contains the token type (JWT) and the signing algorithm (e.g., HS256).
• Payload — contains data (claims) such as user ID, role, and expiration time.
• Signature — a signature that guarantees the integrity of the token.

A JWT token looks like this: eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c

JWT is self-contained: the server does not need to store sessions in a database; all the information is already encoded in the token.

2. Setting up an ASP.NET Core Project for JWT Authorization

Let's create a new ASP.NET Core Web API project and configure JWT authorization.

Step 1: Installing NuGet Packages

To work with JWT, we need the Microsoft.AspNetCore.Authentication.JwtBearer package. Install it via NuGet Package Manager or the console:

dotnet add package Microsoft.AspNetCore.Authentication.JwtBearer

Step 2: Configuring appsettings.json

Add a section to store JWT parameters (secret key, issuer, audience):

{  "Jwt": {    "Key": "YourSuperSecretKey123!@#$%^&*()",    "Issuer": "YourApp",    "Audience": "YourAppUsers",    "ExpireMinutes": 60  }}

Step 3: Configuring Program.cs

In the Program.cs file (or Startup.cs for older versions), add authentication services and middleware:

using Microsoft.AspNetCore.Authentication.JwtBearer;using Microsoft.IdentityModel.Tokens;using System.Text;

var builder = WebApplication.CreateBuilder(args);

// Configure JWT authenticationbuilder.Services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)    .AddJwtBearer(options =>    {        options.TokenValidationParameters = new TokenValidationParameters        {            ValidateIssuer = true,            ValidateAudience = true,            ValidateLifetime = true,            ValidateIssuerSigningKey = true,