I was looking for how to/docs on how to use Slack as an external identity provider using OpenID Connect within FusionAuth but was unable to find one.

After trial and error I did manage to have a start and would like a second opinion on how legit I did it. Here are all the repro steps :

1. I created an app on Slack : https://api.slack.com/apps. All left to default value except tab "OAuth & Permissions", I add redirect URL for my FusionAuth test server (like http://localhost:9011/oauth2/callback).

2. I created an OpenID Connect Identity Providers via FusionAuth Admin interface (Home/Settings/Identity Providers/Add Provider) with those values :

• Client ID : from https://api.slack.com/apps
• Client authentication method : Request body (client_secret_post)
• Client secret : from https://api.slack.com/apps
• Authorization endpoint : https://slack.com/oauth/authorize (I tried hard make it work with v2 but was unable to succeed, something with the user_scope vs scope that slack's api v2 is asking to manage bot) took from here https://api.slack.com/methods/oauth.access
• Token endpoint : https://slack.com/api/oauth.access took from https://api.slack.com/specs/openapi/v2/slack_web.json field "tokenUrl" corresponding to the oauth/authorize endpoint
• Userinfo endpoint : https://slack.com/api/users.profile.get took from https://api.slack.com/methods/users.profile.get because it seemed to give the email with the oauth scope I was able to pass.
• Use POST Method : nope
• Reconcile Lambda : Custom one, back to it at step 3
• Scope : users.profile:read took from https://api.slack.com/legacy/oauth-scopes (it took me a while to understand the difference between slack's scope https://api.slack.com/scopes from the oauth-scopes, but even with the current scope I can only give one scope at a time)
• Email claim : email (didn't manage to get it work, I used a lambda to reconcile)
• Managed domains : empty
• Debug enabler : BIG yes, so usefull in dev
• Applications : Create Registration & Enabled both to yes

3. I created a Lambda for OpenID Connect Reconcile :

function reconcile(user, registration, jwt) {

  user.fullName = jwt.profile.real_name_normalized;
  user.imageUrl = jwt.profile.image_192;
  user.email = jwt.profile.email;

  registration.username = jwt.profile.real_name_normalized;
}

4. Of course update the Identity Provider to use that reconcile lambda.

5. Now time to use it in a test aspnet app based from https://github.com/FusionAuth/fusionauth-example-asp-netcore
   I change the AddOpenIdConnect call in Startup.cs to :

.AddOpenIdConnect("oidc", options =>
	{
		options.Authority = Configuration["SampleApp:Authority"];
		options.ClientId = Configuration["SampleApp:ClientId"];
		options.ClientSecret = "SUCH SECRET";
		
		options.TokenValidationParameters = new TokenValidationParameters
		{
			IssuerSigningKeyResolver = (token, securityToken, kid, parameters) =>
			{
				var client = new HttpClient();
				var response = client.GetAsync("http://localhost:9011/.well-known/jwks.json").Result;
				var responseString = response.Content.ReadAsStringAsync().Result;
				var keys = JsonConvert.DeserializeObject<JwkList>(responseString);

				return keys.Keys;
			},
			ValidIssuers = new List<string>
			{
				"acme.com"
			}
		};

		options.ResponseType = "code";
		options.RequireHttpsMetadata = false;
	});

6. I changed the RequirePermission in Startup.cs, didn't manage to get applicationId in my claims (default permission)

	services.AddAuthorization(options =>
	{
		options.AddPolicy("Registered", policy => policy.RequireAssertion(c =>
		{
			var result = c.User.Claims.Any();
			return result;
		}));
	});

After that I'm able to authenticate on slack, to give permission to get my identity and then to login in my test aspnet

sub : 9bc2f6ae-23d1-4d12-97c9-db3bd1885918
jti : 6b163068-9bd6-4e58-ada5-922991f3f1ef
authenticationType : OPENID_CONNECT
email : much@mail.com
email_verified : true
sid : 4730abf3-ff80-4b23-b83d-bcc16fb60fb7

First did I miss a good doc/post somewhere explaining how to use slack as an Identity Provider ?
Second what I could have done wrong, how to correct it ?
Then does someone manage to get it work with slack's oauth v2 api ?
Last why do I have to give permission again & again when I login ?