the-biergarten-app/web/backend/API/API.Core/Authentication/JwtAuthenticationHandler.cs

using System.Security.Claims;
using System.Text.Encodings.Web;
using System.Text.Json;
using Shared.Contracts;
using Infrastructure.Jwt;
using Microsoft.AspNetCore.Authentication;
using Microsoft.Extensions.Options;

namespace API.Core.Authentication;

/// <summary>
/// Custom ASP.NET Core authentication handler that validates bearer JWT access tokens
/// supplied in the <c>Authorization</c> header against the <c>JWT</c> authentication scheme.
/// </summary>
/// <param name="options">The monitored options for the <c>JWT</c> authentication scheme.</param>
/// <param name="logger">The logger factory used by the base <see cref="AuthenticationHandler{TOptions}"/>.</param>
/// <param name="encoder">The URL encoder used by the base <see cref="AuthenticationHandler{TOptions}"/>.</param>
/// <param name="tokenInfrastructure">The infrastructure service used to validate JWT access tokens.</param>
/// <param name="configuration">The application configuration, used as a fallback source for the JWT secret key.</param>
public class JwtAuthenticationHandler(
    IOptionsMonitor<JwtAuthenticationOptions> options,
    ILoggerFactory logger,
    UrlEncoder encoder,
    ITokenInfrastructure tokenInfrastructure,
    IConfiguration configuration
) : AuthenticationHandler<JwtAuthenticationOptions>(options, logger, encoder)
{
    /// <summary>
    /// Validates the incoming request's bearer JWT access token and produces an authentication result.
    /// </summary>
    /// <remarks>
    /// The signing secret is resolved first from the <c>ACCESS_TOKEN_SECRET</c> environment variable, falling
    /// back to the <c>Jwt:SecretKey</c> configuration value, to stay consistent with the secret source used
    /// when tokens are issued. Authentication fails if the secret is not configured, the
    /// <c>Authorization</c> header is missing, the header does not use the <c>Bearer</c> scheme, or the
    /// token fails validation.
    /// </remarks>
    /// <returns>
    /// A successful <see cref="AuthenticateResult"/> containing the validated <see cref="System.Security.Claims.ClaimsPrincipal"/>
    /// on success, or a failure result describing why authentication could not be completed.
    /// </returns>
    protected override async Task<AuthenticateResult> HandleAuthenticateAsync()
    {
        // Use the same access-token secret source as TokenService to avoid mismatched validation.
        var secret = Environment.GetEnvironmentVariable("ACCESS_TOKEN_SECRET");
        if (string.IsNullOrWhiteSpace(secret))
        {
            secret = configuration["Jwt:SecretKey"];
        }

        if (string.IsNullOrWhiteSpace(secret))
        {
            return AuthenticateResult.Fail("JWT secret is not configured");
        }

        // Check if Authorization header exists
        if (
            !Request.Headers.TryGetValue(
                "Authorization",
                out var authHeaderValue
            )
        )
        {
            return AuthenticateResult.Fail("Authorization header is missing");
        }

        var authHeader = authHeaderValue.ToString();
        if (
            !authHeader.StartsWith(
                "Bearer ",
                StringComparison.OrdinalIgnoreCase
            )
        )
        {
            return AuthenticateResult.Fail(
                "Invalid authorization header format"
            );
        }

        var token = authHeader.Substring("Bearer ".Length).Trim();

        try
        {
            var claimsPrincipal = await tokenInfrastructure.ValidateJwtAsync(
                token,
                secret
            );
            var ticket = new AuthenticationTicket(claimsPrincipal, Scheme.Name);
            return AuthenticateResult.Success(ticket);
        }
        catch (Exception ex)
        {
            return AuthenticateResult.Fail(
                $"Token validation failed: {ex.Message}"
            );
        }
    }

    /// <summary>
    /// Writes a JSON 401 Unauthorized response when authentication fails or is required but not supplied.
    /// </summary>
    /// <param name="properties">The authentication properties associated with the challenge.</param>
    /// <returns>A task that completes once the response body has been written.</returns>
    protected override async Task HandleChallengeAsync(AuthenticationProperties properties)
    {
        Response.ContentType = "application/json";
        Response.StatusCode = 401;

        var response = new ResponseBody { Message = "Unauthorized: Invalid or missing authentication token" };
        await Response.WriteAsJsonAsync(response);
    }
}

/// <summary>
/// Options for the <c>JWT</c> authentication scheme handled by <see cref="JwtAuthenticationHandler"/>.
/// </summary>
/// <remarks>No additional options are defined beyond those provided by <see cref="AuthenticationSchemeOptions"/>.</remarks>
public class JwtAuthenticationOptions : AuthenticationSchemeOptions { }