Authentication

Authentication & Authorization

Core Principles

1. Use ASP.NET Identity for user management — Don't build your own user store. Identity handles password hashing, lockout, two-factor, and email confirmation.
2. JWT for APIs, cookies for web apps — APIs use Bearer token authentication; Blazor/MVC apps use cookie authentication.
3. Policy-based authorization over roles — Policies are testable, composable, and more expressive than [Authorize(Roles = "Admin")].
4. Never store secrets in code — Use user secrets in development, Azure Key Vault / environment variables in production.

Patterns

JWT Bearer Authentication

// Program.cs
builder.Services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
    .AddJwtBearer(options =>
    {
        options.TokenValidationParameters = new TokenValidationParameters
        {
            ValidateIssuer = true,
            ValidateAudience = true,
            ValidateLifetime = true,
            ValidateIssuerSigningKey = true,
            ValidIssuer = builder.Configuration["Jwt:Issuer"],
            ValidAudience = builder.Configuration["Jwt:Audience"],
            IssuerSigningKey = new SymmetricSecurityKey(
                Encoding.UTF8.GetBytes(builder.Configuration["Jwt:Key"]!)),
            ClockSkew = TimeSpan.Zero
        };
    });

builder.Services.AddAuthorization();

Token Generation

public class TokenService(IConfiguration config, TimeProvider clock)
{
    public string GenerateToken(User user, IEnumerable<string> roles)
    {
        var claims = new List<Claim>
        {
            new(ClaimTypes.NameIdentifier, user.Id),
            new(ClaimTypes.Email, user.Email!),
            new(ClaimTypes.Name, user.UserName!)
        };

        claims.AddRange(roles.Select(role => new Claim(ClaimTypes.Role, role)));

        var key = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(config["Jwt:Key"]!));
        var credentials = new SigningCredentials(key, SecurityAlgorithms.HmacSha256);

        var token = new JwtSecurityToken(
            issuer: config["Jwt:Issuer"],
            audience: config["Jwt:Audience"],
            claims: claims,
            expires: clock.GetUtcNow().AddHours(1).DateTime,
            signingCredentials: credentials);

        return new JwtSecurityTokenHandler().WriteToken(token);
    }
}

Policy-Based Authorization

// Define policies
builder.Services.AddAuthorizationBuilder()
    .AddPolicy("AdminOnly", policy => policy.RequireRole("Admin"))
    .AddPolicy("CanManageOrders", policy => policy
        .RequireAuthenticatedUser()
        .RequireClaim("permission", "orders:write"))
    .AddPolicy("MinimumAge", policy => policy
        .AddRequirements(new MinimumAgeRequirement(18)));

// Custom requirement + handler
public class MinimumAgeRequirement(int minimumAge) : IAuthorizationRequirement
{
    public int MinimumAge => minimumAge;
}

public class MinimumAgeHandler(TimeProvider clock) : AuthorizationHandler<MinimumAgeRequirement>
{
    protected override Task HandleRequirementAsync(
        AuthorizationHandlerContext context,
        MinimumAgeRequirement requirement)
    {
        var dateOfBirthClaim = context.User.FindFirst("date_of_birth");
        if (dateOfBirthClaim is not null &&
            DateOnly.TryParse(dateOfBirthClaim.Value, out var dob) &&
            dob.AddYears(requirement.MinimumAge) <= DateOnly.FromDateTime(clock.GetUtcNow().DateTime))
        {
            context.Succeed(requirement);
        }
        return Task.CompletedTask;
    }
}

Protecting Endpoints

// Protect an entire group
app.MapGroup("/api/admin")
    .WithTags("Admin")
    .RequireAuthorization("AdminOnly")
    .MapAdminEndpoints();

// Protect individual endpoints
group.MapPost("/", CreateOrder)
    .RequireAuthorization("CanManageOrders");

// Allow anonymous on a protected group
group.MapGet("/public-info", GetPublicInfo)
    .AllowAnonymous();

OpenID Connect (External Identity Provider)

builder.Services.AddAuthentication(options =>
{
    options.DefaultScheme = CookieAuthenticationDefaults.AuthenticationScheme;
    options.DefaultChallengeScheme = OpenIdConnectDefaults.AuthenticationScheme;
})
.AddCookie()
.AddOpenIdConnect(options =>
{
    options.Authority = builder.Configuration["Oidc:Authority"];
    options.ClientId = builder.Configuration["Oidc:ClientId"];
    options.ClientSecret = builder.Configuration["Oidc:ClientSecret"];
    options.ResponseType = "code";
    options.SaveTokens = true;
    options.Scope.Add("openid");
    options.Scope.Add("profile");
    options.Scope.Add("email");
});

Accessing Current User

// In minimal API handlers — inject ClaimsPrincipal or HttpContext
group.MapGet("/me", (ClaimsPrincipal user) =>
{
    var userId = user.FindFirstValue(ClaimTypes.NameIdentifier);
    var email = user.FindFirstValue(ClaimTypes.Email);
    return TypedResults.Ok(new { userId, email });
}).RequireAuthorization();

Anti-patterns

Don't Use Role Strings Everywhere

// BAD — magic strings, hard to refactor, not testable
[Authorize(Roles = "Admin,SuperAdmin,Manager")]
public class AdminController { }

// GOOD — policy-based
builder.Services.AddAuthorizationBuilder()
    .AddPolicy("AdminAccess", p => p.RequireRole("Admin", "SuperAdmin", "Manager"));

group.MapGet("/", Handler).RequireAuthorization("AdminAccess");

Don't Store Secrets in appsettings.json

// BAD — committed to source control
{
  "Jwt": {
    "Key": "super-secret-key-12345"
  }
}

# GOOD — use user secrets in development
dotnet user-secrets set "Jwt:Key" "super-secret-key-12345"

Don't Skip Token Validation

// BAD — disabling validation
options.TokenValidationParameters = new TokenValidationParameters
{
    ValidateIssuer = false,      // DON'T
    ValidateAudience = false,    // DON'T
    ValidateLifetime = false,    // DEFINITELY DON'T
};

// GOOD — validate everything (see JWT Bearer Authentication pattern above for full setup)

Decision Guide

Scenario	Recommendation
REST API	JWT Bearer authentication
Blazor Server / MVC	Cookie authentication
External identity provider	OpenID Connect
User registration / login	ASP.NET Identity
Permission checking	Policy-based authorization
Multi-tenant API	Claims-based with tenant claim
API-to-API communication	Client credentials (OAuth 2.0)
Simple API keys	Custom AuthenticationHandler<T>