Can you explain what JWT is and how it is used in ASP.NET applications?

JWT (JSON Web Token) is a compact, URL-safe way to transmit information between parties as a JSON object. In ASP.NET applications, JWT is often used for user authentication and authorization.

Working principle:

• When a user logs in, the server creates a JWT containing useful data (e.g., user ID, roles) and signs it with a secret key.
• The client stores this token and sends it in the Authorization header for subsequent requests.
• The server verifies the token's signature, extracts the data, and makes access decisions based on it.

Example of JWT configuration in ASP.NET Core:

services.AddAuthentication(options => {
    options.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
    options.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
})
.AddJwtBearer(options => {
    options.TokenValidationParameters = new TokenValidationParameters {
        ValidateIssuer = true,
        ValidateAudience = true,
        ValidateLifetime = true,
        ValidateIssuerSigningKey = true,
        ValidIssuer = "yourIssuer",
        ValidAudience = "yourAudience",
        IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes("yourSecretKey"))
    };
});

Thus, JWT allows secure and convenient session management without server-side state storage.