Thinktecture Identity server v3 - Facebook Assertion Flow - oauth-2.0

Question

Is there a possibility to configure OAuth2 AssertionFlow with Facebook in Thinktecture Identity Server v3?
There was a post on leastprivilege.com about implementing AssertionFlow for Microsoft OAuth and AuthorizationServer but I need to integrate with Facebook and, furthermore, AuthorizationServer is marked as deprecated and it's not maintained anymore.

Answer 1

In response to #NathanAldenSr's comment, I publish some code of my working solution.
Server side - custom validator:
public class FacebookCustomGrantValidator: ICustomGrantValidator
{
private readonly IUserService userService;
private const string _FACEBOOK_PROVIDER_NAME = "facebook";
// ...
async Task<CustomGrantValidationResult> ICustomGrantValidator.ValidateAsync(ValidatedTokenRequest request)
{
// check assetion type (you can have more than one in your app)
if (request.GrantType != "assertion_fb")
return await Task.FromResult<CustomGrantValidationResult>(null);
// I assume that fb access token has been sent as a response form value (with 'assertion' key)
var fbAccessToken = request.Raw.Get("assertion");
if (string.IsNullOrWhiteSpace(assertion))
return await Task.FromResult<CustomGrantValidationResult>(new CustomGrantValidationResult
{
ErrorMessage = "Missing assertion."
});
AuthenticateResult authebticationResult = null;
// if fb access token is invalid you won't be able to create Facebook client
var client = new Facebook.FacebookClient(fbAccessToken);
dynamic response = client.Get("me", new { fields = "email, first_name, last_name" });
// create idsrv identity for the user
authebticationResult = await userService.AuthenticateExternalAsync(new ExternalIdentity()
{
Provider = _FACEBOOK_PROVIDER_NAME,
ProviderId = response.id,
Claims = new List<Claim>
{
new Claim("Email", response.email),
new Claim("FirstName", response.first_name),
new Claim("LastName", response.last_name)
// ... and so on...
}
},
new SignInMessage());
return new CustomGrantValidationResult
{
Principal = authebticationResult.User
};
}
}
You can easily test it with OAuth2Client that is also provided by Thinktecture (in Thinktexture.IdentityModel Client Library nuget package).
string fbAccessToken = "facebook_access_token_you_aquired_while_logging_in";
string assertionType = "assertion_fb";
var client = new OAuth2Client(
new Uri("your_auth_server_url"),
"idsrv_client_id",
"idsrv_client_secret");
string idsrvAccessToken = client.RequestAssertionAsync(assetionType, fbAccessToken,).Result;

Answer 2

IdentityServer v3 also supports assertion flow. The samples wiki has two samples on that (called "Custom Grants):
https://github.com/thinktecture/Thinktecture.IdentityServer.v3.Samples/tree/master/source