Google JWT Authentication with AspNet Core 2.0

Posting my ultimate approach for posterity.

As Tratcher pointed out, the AddGoogle middleware is not actually for a JWT authentication flow. After doing more research, I realized that what I ultimately wanted is what is described here: https://developers.google.com/identity/sign-in/web/backend-auth

So my next problems were

1. I could not rely on the standard dotnet core Jwt auth middleware anymore since I need to delegate the google token validation to google libraries
2. There was no C# google validator listed as one of the external client libraries on that page.

After more digging, I found this that JWT validation support was added to C# here using this class and method: Google.Apis.Auth.Task<GoogleJsonWebSignature.Payload> ValidateAsync(string jwt, GoogleJsonWebSignature.ValidationSettings validationSettings)

Next I needed to figure out how to replace the built in JWT validation. From this SO questions I came up with an approach: ASP.NET Core JWT Bearer Token Custom Validation

Here is my custom GoogleTokenValidator:

public class GoogleTokenValidator : ISecurityTokenValidator
{
    private readonly JwtSecurityTokenHandler _tokenHandler;

    public GoogleTokenValidator()
    {
        _tokenHandler = new JwtSecurityTokenHandler();
    }

    public bool CanValidateToken => true;

    public int MaximumTokenSizeInBytes { get; set; } = TokenValidationParameters.DefaultMaximumTokenSizeInBytes;

    public bool CanReadToken(string securityToken)
    {
        return _tokenHandler.CanReadToken(securityToken);
    }

    public ClaimsPrincipal ValidateToken(string securityToken, TokenValidationParameters validationParameters, out SecurityToken validatedToken)
    {
        validatedToken = null;
        var payload = GoogleJsonWebSignature.ValidateAsync(securityToken, new GoogleJsonWebSignature.ValidationSettings()).Result; // here is where I delegate to Google to validate

        var claims = new List<Claim>
                {
                    new Claim(ClaimTypes.NameIdentifier, payload.Name),
                    new Claim(ClaimTypes.Name, payload.Name),
                    new Claim(JwtRegisteredClaimNames.FamilyName, payload.FamilyName),
                    new Claim(JwtRegisteredClaimNames.GivenName, payload.GivenName),
                    new Claim(JwtRegisteredClaimNames.Email, payload.Email),
                    new Claim(JwtRegisteredClaimNames.Sub, payload.Subject),
                    new Claim(JwtRegisteredClaimNames.Iss, payload.Issuer),
                };

        try
        {
            var principle = new ClaimsPrincipal();
            principle.AddIdentity(new ClaimsIdentity(claims, AuthenticationTypes.Password));
            return principle;
        }
        catch (Exception e)
        {
            Console.WriteLine(e);
            throw;

        }
    }
}

And in Startup.cs, I also needed to clear out the default JWT validation, and add my custom one:

services.AddAuthentication(options =>
            {
                options.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
                options.DefaultScheme = JwtBearerDefaults.AuthenticationScheme;
                options.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;

            })
            .AddJwtBearer(o =>
                {
                    o.SecurityTokenValidators.Clear();
                    o.SecurityTokenValidators.Add(new GoogleTokenValidator());
                }

Maybe there is an easier way, but this is where I landed and it seems to work fine! There was additional work I did that I left out of here for simplicity, for example, checking if there is already a user in my user's DB that matches the claims provided by google, so I apologize if the code above does not 100% work since I may have removed something inadvertently.

I just published a NuGet package to handle validation of Google OpenID Connect tokens.

The package relies on Microsoft's JWT validation and authentication handler from Microsoft.AspNetCore.Authentication.JwtBearer, with some added validation around hosted domains.

It contains a single public extension method, UseGoogle, on JwtBearerOptions that lets you configure the handler to validate Google OpenID Connect tokens, without other dependencies:

services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
    .AddJwtBearer(jwt => jwt.UseGoogle(
        clientId: "<client-id-from-Google-API-console>",
        hostedDomain: "<optional-hosted-domain>"));

If you want to take a look at the source, you can find it here.