What is Authentication in ASP.NET Core Web APIs?

Authentication in ASP.NET Core Web APIs is the process of verifying the identity of a user, application, or service before allowing access to protected API resources. It answers the question, "Who are you?" Modern Web APIs often expose sensitive business data, making authentication one of the most critical security mechanisms in enterprise applications. ASP.NET Core supports multiple authentication methods, including: JWT (JSON Web Token) OAuth 2.0 OpenID Connect Microsoft Entra ID authentication Cookie Authentication Identity Server API Keys (for specific scenarios) The most common approach for REST APIs is JWT Bearer Authentication. The authentication process typically works as follows: A user logs in with valid credentials. The authentication server validates the credentials. A signed JWT token is generated. The client includes the token in the Authorization header of subsequent API requests. ASP.NET Core validates the token before granting access to protected endpoints. Authentication is often combined with Authorization, which determines what an authenticated user is allowed to do based on roles, claims, or policies. Benefits of authentication include: Protects sensitive APIs Prevents unauthorized access Supports secure communication Enables role-based access control Integrates with cloud identity providers Supports Single Sign-On (SSO) Proper authentication is essential for building secure, scalable, and enterprise-grade REST APIs.