How can I configure ASP.NET Core to support multiple JWT token issuers (e.g., Firebase and a custom issuer)?

Supporting Multiple JWT Token Issuers in ASP.NET Core

This guide explains how to configure ASP.NET Core to authenticate JWTs from multiple sources, such as Firebase and a custom issuer. The standard AddJwtBearer method only supports a single authority, making this a common challenge.

The Solution: Multiple Authentication Schemes

The key is to utilize multiple authentication schemes within ASP.NET Core. This allows the application to handle JWTs from different issuers independently.

Implementation (ASP.NET Core 2 and later):

<code class="language-csharp">services
    .AddAuthentication() // No default scheme specified
    .AddJwtBearer("Firebase", options =>
    {
        options.Authority = "https://securetoken.google.com/my-firebase-project";
        options.TokenValidationParameters = new TokenValidationParameters
        {
            ValidateIssuer = true,
            ValidIssuer = "my-firebase-project",
            ValidateAudience = true,
            ValidAudience = "my-firebase-project",
            ValidateLifetime = true
        };
    })
    .AddJwtBearer("Custom", options =>
    {
        // Configure validation parameters for your custom JWT issuer here.
        //  Example:
        options.TokenValidationParameters = new TokenValidationParameters
        {
            ValidateIssuer = true,
            ValidIssuer = "your-custom-issuer",
            ValidateAudience = true,
            ValidAudience = "your-api-audience",
            ValidateLifetime = true,
            IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes("your-secret-key")) // Or your key retrieval method
        };
    });

services
    .AddAuthorization(options =>
    {
        options.DefaultPolicy = new AuthorizationPolicyBuilder()
            .RequireAuthenticatedUser()
            .AddAuthenticationSchemes("Firebase", "Custom")
            .Build();
    });</code>

Key Improvements:

• No Default Authentication Scheme: AddAuthentication() is called without parameters. This prevents automatic authentication with a single scheme, allowing all defined schemes to attempt authentication.
• Named Authentication Schemes: AddJwtBearer is used with a scheme name ("Firebase", "Custom") to register each issuer separately.
• Combined Authorization: The default authorization policy is configured to use both "Firebase" and "Custom" schemes. This ensures that authentication attempts will be made against both issuers.

Handling Errors:

You might encounter IDX10501 errors during authentication failures. This is often due to the authentication middleware checking each scheme sequentially. In many cases, this error can be safely ignored.

ASP.NET Core 6 and Later:

In newer versions, a default authentication scheme is required. Here's an adapted example:

<code class="language-csharp">builder.Services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
    .AddJwtBearer(options =>
    {
        options.Audience = "your-api-audience";
        options.Authority = "your-identity-server-url"; // Or your default JWT issuer
    })
    .AddJwtBearer("AzureAD", options =>
    {
        options.Audience = "your-api-audience";
        options.Authority = "your-azure-ad-authority";
    });

builder.Services.AddAuthorization(options =>
{
    options.DefaultPolicy = new AuthorizationPolicyBuilder(
        JwtBearerDefaults.AuthenticationScheme,
        "AzureAD")
        .RequireAuthenticatedUser()
        .Build();
});</code>

Remember to replace placeholders like "my-firebase-project", "your-custom-issuer", "your-secret-key", "your-api-audience", "your-identity-server-url", and "your-azure-ad-authority" with your actual values. Consider using more robust key management for production environments. Policy-based authorization can provide more fine-grained control for complex scenarios.

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